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Superhydrophobic and Superoleophilic Micro-Wrinkled Reduced Graphene Oxide as a Highly Portable and Recyclable Oil Sorbent Chunfang Feng, Zhifeng Yi, Fenghua She, Weimin Gao, Zheng Peng, Christopher James Garvey, Ludovic Francis Dumée, and Ling Xue Kong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01648 • Publication Date (Web): 04 Apr 2016 Downloaded from http://pubs.acs.org on April 10, 2016

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ACS Applied Materials & Interfaces

Superhydrophobic and Superoleophilic Micro-Wrinkled Reduced Graphene Oxide as a Highly Portable and Recyclable Oil Sorbent

Chunfang Feng,a Zhifeng Yi,a Fenghua She,a Weimin Gao,a Zheng Peng,a,b Christopher J. Garvey,c Ludovic F. Duméea and Lingxue Kong*,a

a

Deakin University, Geelong, Institute for Frontier Materials, Victoria 3216, Australia

b

Agricultural Product Processing Research Institute (APPRI), Chinese Academy of Tropical

Agricultural Sciences (CATAS), Zhanjiang 524001, China

c

Australian Nuclear Science and Technology Organisation (ANSTO), Lucas Heights, New South

Wales 2234, Australia

*

Corresponding author: Lingxue Kong (Tel: +61 3 52272087 and E-mail:

[email protected]).

1 ACS Paragon Plus Environment


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ABSTRACT The potential of superhydrophobic and superoleophilic micro-wrinkled reduced graphene oxide (MWrGO) structures is here demonstrated for oil spill clean-up. The impact of the thickness of MWrGO films on the sorption performance of three different oils was investigated. Water contact angles across the MWrGO surfaces were found to exceed 150º while oil could easily absorbed by the micro-wrinkled structures of MWrGO within seconds after contact. Although the oil surface diffusion rate was not found to be dependent on the thickness of the graphene oxide films, the oil sorption capacity was the largest with the thinner MWrGO films due to the high surface area resulting from their fine surface texture. Furthermore, the composite films can be repeatedly used for at least 20 oil sorption-removal cycles without any notable loss in selectivity and uptake capacity. These MWrGO/elastomer composite films could be applied as a potential candidate material for future oil spill clean-up.

Key words: wrinkled reduced graphene oxide; superhydrophobic; superoleophilic; textured graphene; oil sorption; chemical remediation


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Highlights •

Wrinkled textures of reduced GO induce superhydrophobicity and superoleophilicity for oil sorption.

•

The oil sorption performance, including oil sorption capacity, kinetics and recyclability of micro-wrinkled reduced GO structures is investigated for the first time.

•

A high recyclability of micro-wrinkled reduced GO/elastomer composite films is demonstrated as over 98 % of the oil sorption capacity is maintained after 20 sorptionremoval cycles.



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1. INTRODUCTION Onshore and offshore oil spills during oil exploration and transportation have led to severe economic, environmental and social problems,1-4 requiring the development of robust, quick to setup and effective oil removal technologies to minimize their impact on marine life and ecosystems. Oil absorbents, including activated carbon, expanded graphite, natural fibre products and organoclays,2,

5

have been developed to remediate these environmental disasters. However, these

materials have been proved to possess a relatively low oil sorption capacity and poor recyclability, which result in a high cost and relative low efficiency to clean up oil spills.5-6 New oil absorbers with better chemical stability and physico-mechanical stability upon uptake, high reusability and convenient deployment should be designed to achieve efficient treatment of oil spills. Graphene, an emerging two dimensional material, has high theoretical surface area, hydrophobicity and strong mechanical properties7 and can be easily processed into different unique structures. Various graphene-based superhydrophobic and superoleophilic structures have been designed and fabricated over the past decade, to repel water but absorb oil.1-2, 5, 8-9 Three dimensional (3D) hierarchical graphene structures provide large internal surface area for high oil sorption capacity.3 Graphene and graphene/carbon nanotube (CNT) aerogels with elastic compressive behaviour have been fabricated for selective sorption of organic solvent and oil products with a high oil sorption of up to 320 g/g.3, 5, 10-11 Similarly, spongy graphene materials were constructed through the assembly of graphene oxide (GO) and moulding across a sealed reactor, which achieved a maximum oil sorption of 300 g/g.2, 12 Graphene/polymer sponge composites have been explored as oil spill clean-up materials with a maximum oil sorption capacity ranging from 48 to 112 g/g.9-10, 13 In this system, graphene offers both hydrophobicity and oleophilicity while the polymeric matrix enhances the mechanical properties that improve the recyclability under repeated compression.9 Although these 3D structures offer high oil sorption capacities, challenges still remain to meet the practical requirements for oil absorbents, especially in terms of recyclability and durability.


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The retention of absorbed oil across porous architectures is therefore one of the main issues compromising the sorption capacity during the sorption-removal cycles.14-20 Therefore, new graphene-based structures with improved recyclability are required to explore graphene-based materials for the application of advanced oil sorption. The challenge to improve the recyclability of graphene-based oil absorbents is associated with the oil removal methods based on the currently developed 3D porous structures. Oil is typically removed from the absorbents through mechanical compression, burning or distillation, which may affect the reusability of oil absorbents and potentially degrade the valuable oils.5 Although mechanical compression of the absorbents is the most convenient approach to recover the absorbed oil for sponge like absorbents, the recovery of oil may be limited depending on the materials structure and porosity interconnectivity, which in turn decreases the sorption capacity during repeated cycles.5 Pyrolysis may remove most of the absorbed oil,5,

21

but the by-products from

pyrolysis represent a significant pollution hazard to the environment and a waste of valuable oil resources. One of the only sustainable route, distillation, normally requires large amounts of heat/energy to evaporate the oils at boiling points typically higher than 150 ºC.3, 5 In order to tackle these issues, new materials able to offer high recovery and long term stability should therefore be designed. Recently crumpled graphene structures with superhydrophobicity (water contact angle 152º18) have been developed. These materials have the potential for oil sorption with high durability, but the studies on the oil sorption properties of the wrinkled structure have not been reported to date to the best of our knowledge.14-20 In this work, flexible micro-wrinkled reduced graphene oxide (MWrGO) materials were fabricated through a versatile thermo-mechanical shrinking process, followed by the incorporation of polydimethylsiloxane (PDMS) elastomeric matrices.22 The elastomer layer provides both flexibility and mechanical strength to the graphene thin film and is able to intimately interface with the 3D hierarchical architectures to form a light and high specific surface area material. The oil sorption



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capacity, sorption rate and recyclability of GO films with three thicknesses were systematically investigated to reveal the relationship between GO film thickness and oil sorption ability. Three different oils with varying density and viscosity were used to understand the selectivity of MWrGO to oil physical properties. The capillary mechanisms behind oil sorption across the MWrGO films were also discussed. These MWrGO/PDMS composite films have the potential to contribute to oil spill clean-up towards advanced separation processes in the petro-chemical and pharmaceutical industries.


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2. EXPERIMENTAL SECTION

2.1. Materials Graphite powder (particle size 99%) were purchased from Sigma Aldrich (St. Louis, USA). Sulfuric acid (98%) (H2SO4), sodium nitrate (NaNO3), potassium permanganate (KMnO4), and hydrogen peroxide (H2O2) (30%) were ordered from Chem Supply (Gillman, Australia). All chemicals used in this study were of analytical grade. Shrink films (SFs) (PolyShrink™) were purchased from Lucky Squirrel (Belen, USA). Polydimethylsiloxane (PDMS -SYLGARD® 184) was obtained from Dow Corning (Midland, USA). The oils used were: Dynagrade 19 vacuum pump oil (Dynapumps, Belmont, Australia); canola cooking oil (Coles, Hawthorn East, Australia); and engine oil, CastrolTM GTX 20W-50 (Castrol, London, UK). The density and viscosity of these oils are presented in Table S1. These oils are representatives of petroleum products (pump and engine oils) and fats (canola cooking oil) with different density, viscosity and molecular chain length.

2.2. Preparation of GO Aqueous Suspension GO was synthesized from graphite powder according to a modified Hummers method.23 Briefly, 1 g of graphite was added into 30 mL of cooled concentrated sulphuric acid (98%) and 0.5 g of NaNO3 mixture at vigorous stirring for 1 hour. 5 g KMnO4 was added gradually (1.5 g, 1.5 g and then 2 g) over 2 h while the temperature of the mixture was maintained below 10 °C. The mixture was then heated to 38 °C and stirred until it became light brown. At this point, 50 mL of deionized water was slowly added to the solution while it was heated to 95 °C in a water bath and magnetically stirred for 15 min. An additional 50 mL of deionized water was then added to stop the reaction followed by adding 5 mL of 30% hydrogen peroxide solution until the colour of mixture became bright yellow. The sample was then ﬁltered on a porous anodized aluminium oxide membrane (pore



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size 0.1 µm) (Whatman, Little Chalfont, UK) with a vacuum filtration system and washed with 40 mL of 10 v/v % HCl solution and rinsed with excess deionized water. The solid was subsequently redispersed in deionized water and ultra-sonicated for 30 mins at 200 W with a water bath ultrasonic machine (FS300b, Decon). The aqueous GO suspensions were obtained from the supernatant after centrifuging at 2000 rpm (Eppendorf Centrifuges 5430R). The final concentration of the GO solution (2 mg/mL) was determined gravimetrically.

2.3. Fabrication of MWrGO/PDMS Composite Films GO films of controlled thickness were prepared by vacuum filtering a fixed volume of GO solution (0.5, 1 or 1.5 mL). A uniform GO film was formed on a cellulose filtration membrane (Whatman, United Kingdom, pore size: 0.1 µm) by physical compaction of individual GO sheets. A SF (5×5 cm) was placed onto the GO film while still wet and a weight of about 2 kg was then placed on the SF for 12 h to achieve good adhesion. The filtration membrane was then detached from the GO/SF film. Graphene micro-wrinkles were formed upon shrinking of the SF and subsequent reduction of the GO. In a typical procedure, the GO/SF films were first placed for 2 min in an oven preheated to 175 °C to isotropically shrink the SF to half of its original surface area. The resulting GO/SF films were then placed into a stainless steel autoclave prefilled with hydrazine hydrate (1 mg GO/20 µL of hydrazine hydrate). The autoclave was heated at 90 °C for 10 h in order to reduce the GO to rGO by removing functional groups on the surface of GO sheets.24 PDMS was selected as the flexible support to maintain the MWrGO structure and its thickness can be simply tuned through the mass of added PDMS. In this study, the resultant MWrGO films were coated with 0.4 g / ~7.9 cm2 of PDMS including 10 wt % curing agent in order to intimately bind the MWrGO with the PDMS matrix. The coated PDMS films were cured at 70 ºC for 2 h. The SF substrate was finally dissolved in dichloromethane, and free-standing composite films were obtained by washing with acetone and


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dried at room temperature. The composite films with different GO loadings (GO weight of 1, 2 and 3 mg / ~7.9 cm2) were denoted as MWrGO-1, MWrGO-2 and MWrGO-3.

2.4. Materials Characterization Scanning electron micrographs (SEMs) were acquired on a Supra 55VP (Zeiss, Germany) with an accelerating voltage of 5 kV. The MWrGO surface of all the samples was coated with gold using a BAL-TEC SCD 050 sputter coater (Leica Microsystems, Australia). The thickness of GO film and surface roughness of MWrGO were measured from images obtained from a ContourGT 3D Optical Microscope (Bruker, United States). Six points were measured along a single profile tracing in an area of 937.5 × 1250 µm2 and over 3 tracings per sample. To measure surface roughness, a single profile on an area of 1 mm2 was traced. X ray photoemission spectroscopy (XPS) spectra were acquired at pass energy of 160 eV and 1 eV/step and a chamber pressure of 5×10-9 torr using a Kratos AXIS Nova (Kratos Analytical Ltd, UK). The samples were irradiated with Al Kα radiation (hν = 1486.6 eV) from a monochromatic source operating at 150 W. X-ray powder diffraction (XRD) patterns were recorded in the range of 2θ from 5 to 65° using a Phillips PW-1729 diffractometer (35 kV, 28 mA) (Amsterdam, Netherlands) with CuKα radiation (λ = 0.154 nm). Water or oil contact angles were measured on a KSV CAM 101 (Biolin Scientific, Sweden). The Young–Laplace equation25 was employed to calculate the contact angle from the images of droplet on the films. The time-resolved contact angle changes were conducted in triplicate, to estimate the experimental error, by measuring the contact angle of water or oil droplet as a function of time.

2.5 Oil Sorption and Recyclability Measurement The oil sorption ability of MWrGO/PDMS composite films with different GO thickness was tested against three types of oils. A droplet of oil was dropped on the surface of water in a 9 cm petri



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dish. The composite films were applied with the MWrGO surface facing down on the oil phase. After 3 min, the samples were taken out from the petri dish. After waiting for 10 s to allow the excess oil to drain, the oil sorption capacity (g/g) was determined from the mass gain:

=

ℎ − ℎ ℎ

(1)

The oil sorption was calculated against the mass of MWrGO, as the PDMS with tuneable thickness is used as a support to maintain the MWrGO structure and does not significantly contribute to water and oil sorption. The water sorption of each film was also determined using the same method as above. Recyclability was assessed systematically for different samples. The composite films with absorbed oil were wrapped with a piece of stainless steel mesh and then placed in a 50 mL centrifuge tube with tissue paper at the bottom. Centrifugation was conducted at 9,000 rpm for 5 min to remove the oil. After centrifugation, the dried films were re-used for the next sorption-removal cycle.


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3. RESULTS AND DISCUSSION

3.1. Fabrication of MWrGO/PDMS Composite Films and Characterisation of the GO-coated SF and MWrGO Structures The fabrication process of MWrGO/PDMS composite films is presented in Figure 1. Through controlling the amount of GO in the filtration solution, the thickness of GO films on the surface of SF can be tuned, which was measured to be 0.78, 1.29 and 1.67 µm corresponding to GO densities of 0.08, 0.16 and 0.24 mg/cm2 across the surface of the SFs, respectively (Figure S1). The difference in thickness was also indicated by the variation of transparency of the GO coated SF samples (Figure 2A, C and E). The MWrGO structures were formed through the simple thermo-mechanical shrinking of GO coated SF, followed by a chemical reduction to recover graphene properties. The reduction of GO was confirmed by the absence of the peak at 2θ = 11º on the XRD patterns and from the increased carbon/oxygen ratio from 2.5 to 6.1 calculated from integration of the XPS spectra (Figure S2). Finally, the incorporation of PDMS layer on the MWrGO and etching of SFs provided a mechanical support for MWrGO structure but also offered flexibility to improve the convenience of use compared to MWrGO on rigid SFs. The presence of a micro-wrinkled structure was confirmed by SEM observation (Figure 2 B, D and F), and the surface roughness of the MWrGO/PDMS composite films was measured to be from ~14.7 µm for MWrGO-1, 22.4 µm for MWrGO-2 to ultimately 30.8 µm for MWrGO-3 (Figure S3).

3.2 Wettability Measurement High contact angles, of at least 150º, were measured across the surface of all three MWrGO composite samples. The water contact angle remained unchanged over 60 s highlighting the stability of the system and the absence of dynamic wetting (Figure 3A). The contact angle across the series slightly increased proportionally with the MWrGO film roughness from 152º to 160º between the



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MWrGO-1 and the MWrGO-3. This trend is in a good agreement with the Cassie–Baxter model for the wettability of rough surfaces.26 In addition, the contribution from PDMS to the water contact angle is insignificant as the silane groups from PDMS cannot be detected with XPS on MWrGO side, which suggests a rare chance of contact between water and PDMS. Oil in contact with the surface of MWrGO was found to instantaneously wet the surfaces, therefore exhibiting fast dynamic contact angles for each of the different oils tested. The oil contact angles reached zero within less 30 s regardless of the roughness of the samples (Figure 3B and Figure S4A and B). This result suggests that the MWrGO surface promoted the sorption and wetting of oils as the oil droplets spread almost instantaneously. Additionally the spreading rate for different oils was found to vary between the oils, as shown in Figure 3C and Figure S4C and D. The fastest sorption rate was obtained for the pump oil (55 cSt), followed by canola oil (70 cSt). Full wetting of the surface took the longest time for engine oil (between 30 and 60 s). This may be related to the viscosity of the engine oil which was found to be 8-10 times higher than that of the other oils (550 cSt at 20 ºC (Table S1)). There was no significant difference in oil sorption between samples with different surface roughness. Therefore the difference in viscosity may have led to the different spreading rates of the liquid oils across the MWrGO surface, highlighting the versatility of the materials to absorb different oil chemistries.

3.3 Capillary Action Induced Oil Sorption in MWrGO Materials The thermo-mechanical shrinking process created a rough surface from a number of microwrinkles and the roughness was dependent on the thickness of the rGO and the shrinking degree. The curvature created between the neighbouring walls of the wrinkles formed a space that can be imagined to be that between two closely packed plates (Figure 4). The capillary rise of a liquid in this wrinkled structure can be described as27-28


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h=

2

(2)

where h is the height that liquid can rise, is the surface tension of liquid, is the contact angle between liquid and rGO, d is distance between the wrinkles,

is the density of the liquid, and g is

the constant of gravitational acceleration. When water is in contact with the wrinkled surface, the surface tension between water and MWrGO forces the water droplets to curve away from wetting MWrGO (Figure 4A). Due to the water contact angle of flat rGO paper being larger than 90º (Figure S5), according to Equation (2), h is negative, which means that the force (Fwtotal) propels the water away from the MWrGO. The hydrophobicity of rGO also leads to a high surface energy between the water and MWrGO sheets,29 allowing for the water to roll off naturally from the surface (Video Supporting Information). By contrast the low surface energy between oil and the inner wall of graphene wrinkles drives the liquid oil towards the curvature space of wrinkles (Figure 4B). This effect is related to the nature of the oleophilic rGO surface developed from the aromatic carbon rings of the rGO structure recovered during the chemical reduction process of GO, compatible with oil,30 which creates a directional interfacial force between oil and MWrGO. This phenomenon is very similar at the submicrometer scale of the graphene micro-waves to a capillary action in which h is positive (from Equation (2), θ < 90°). Additionally, oil can be stored in the space generated by the wrinkled capillary structure, which endows the oil sorption ability to the MWrGO/PDMS composite film. The different functions of the wrinkles on water and oil provide the composite films with the high oil sorption but water repelling abilities.

3.4 Oil Sorption Capacity and Kinetics Figure 5A presents the sorption of pump oil dyed with oil red floating on the water surface over time. The MWrGO/PDMS composite film was placed on top and can also float on the surface 13 ACS Paragon Plus Environment


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of the water due to its light weight and water repellence. The red-coloured oil gradually vanished over time, once contacting with the film. After reaching saturation, the oil-loaded film was easily removed by lifting it off from the water surface. The sorption capacity of different oils and water was also investigated for MWrGO with different graphene thicknesses (Figure 5B). For sample MWrGO-1, the thinnest sample, the sorption capacity was of 36.3 g/g for pump oil, 41.4 g/g for canola oil and 84.3 g/g for engine oil, and found to be higher than any of the absorption obtained with the thicker graphene micro-wavy samples. Due to the superhydrophobic MWrGO surface, the water uptake for all samples was found to be lower than 0.5 g/g (Table S2). Compared to the oil sorption capacity (> 36 g/g), the water sorption does not have any significant effect on the total measured weight-based oil sorption capacity. Figure 5B also indicates that the highest oil sorption capacity was achieved for the engine oil regardless of the sample roughness. This suggests that the oil uptake capacity is viscosity dependent as the higher the viscosity of the oils (Table S1), the higher the sorption capacity. This trend is in a good agreement with the study for a carbon nanotubes/graphene aerogel where the correlation between viscosity and absorption capacity was investigated.3 In the following section, only sample MWrGO-1 was used for the kinetics calculations since the graphene thickness was not found to have a significant impact on oil spreading rate and since sample MWrGO-1 offered the highest oil sorption capacity. The plot of the oil capacity as a function of the exposure duration was fitted by a second-order sorption rate equation:3, 31

1 1 − = & "#$ − "% "#$

(3)

where t is the contacting time with oil, Qmax is the saturated sorption capacity, Qt is the sorption capacity at time t, and k is the sorption constant. Good fits are obtained with Equation (3) (Figure 5C) with R2 all over 0.95. The fitting results are summarized in Table 1. The calculated sorption 14 ACS Paragon Plus Environment

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constants are 0.106 s-1 for pump oil, 0.058 s-1 for canola oil and 0.022 s-1 for engine oil, suggesting that the fastest sorption rate is achieved in the oil with the lowest viscosity (Table S1). Even though the viscosity of the engine oil is very high (550 cSt), the sorption constant of MWrGO-1 is still comparable with existing studies on high viscosity pump oil.3 The oil sorption capacity, recyclability, removal method, efficiency and convenience of use of the MWrGO films are compared with the existing oil absorbers reported in the literature (Table 2). The oil sorption capacity of MWrGO-1 is higher than the traditional materials (e.g. activated carbon, straw, cotton, and wool fibres, which have an oil sorption capacity less than 23 g/g) and most of the polymer sponges (poly(lactic acid), polyurethane, polyester, and PDMS that can absorb oils between 2.9 - 103 g/g) and comparable to graphene or carbon nanotube based absorbers (aerogels, sponges and foams with an oil sorption capacity of ~13 - 320 g/g).

3.5 Recyclability of MWrGO/PDMS Composites as Oil Absorber The recyclability of the MWrGO/PDMS composite plays an important role in its oil clean up application. After consideration of the oil spreading rate and oil sorption capacity, MWrGO-1 was chosen as the best sample to test the recyclability due to its highest sorption capacity. The recyclability test suggested that the oil sorption capacity can be highly recovered as after 20 absorbing-removing cycles for pump oil, canola oil and engine oil, over 98 % of the original capacity is maintained (Figure 6A, B and C). The absorbed oil can be easily removed from MWrGO samples by centrifugation and collected at the bottom of the centrifuge tube (Figure 6D). The porous stainless steel mesh protected the composite film and separated it from the removed oil by centrifugal force. This indicates that the oil sorption capacity is retained across multiple cycles. Table 2 summarises previously reported materials as oil sorbents. It particularly highlights the oil sorption capacity, recyclability and long term adsorption capacity performance data of these materials, used as a benchmark for our work. On the one hand, polymer-based sorbents offer



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relatively high recyclability, but low sorption capacity (most of them < 25 g/g) and long term sorption capacity over multiple sorption-removal cycles (< 61 % of original capacity.32, 35-38 On the other hand, carbon-based sorbents (such as CNT or graphene sponge, graphene aerogel and graphene/CNT foam) present higher oil sorption capacities (average 115 g/g), but offer poorer recyclability (maximum 10 cycles tested and 50 – 99 % of original capacity remaining).11, 13, 21, 31, 34 Therefore, the development of high performance oil sorbents allowing for high recyclability and high oil sorption capacity is desperately needed. Recently, CNT/polymer and graphene/polymer sponge have been developed for this purpose,9, 12, 22, 39 which can reach maximum 112 g/g sorption capacity and up to 50 sorption-removal cycles but the sorption capacity after cycles is not clear. The oil left in the sorbents greatly decrease the sorption capabilities over repeated usage. In this paper, graphene was combined to an elastomer to fabricate flexible and efficient MWrGO/PDMS oil sorbents whose sorption capacity (up to 84.3 g/g) and recyclability (98 % of original capacity remained after 20 sorption-removal cycles) are not compromised over multiple oil sorption-removal cycles. The MWrGO/PDMS composite films therefore provide a degree of advantageous reusability for the future oil absorbing materials over current materials. The removal of oil using centrifugation is also competitive compared to the reported removal method like mechanical compression, pyrolysis, or distillation. It requires low amount of energy and recovers nearly all spills with no chemical degradation of the adsorbed materials. Apart from the remarkable recyclability and flexibility of the films, the self-supporting nature and simple to use and transport of the materials are also advantageous over existing materials for oil sorption and recovery. Graphene sponge or aerogel structures are the most common structures currently being investigated for oil sorption. Despite their high oil sorption capacity, the large volume of these materials has made them difficult for transportation, which limits their practical application. The flexible MWrGO/PDMS composite fabricated in this work can be rolled and packed easily (Figure 6E and F), potentially saving space towards low-footprint implementation into


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industry of disaster remediation processes, as advanced candidates for the development of the next generation oil spill clean-up materials. The fabrication of large volumes of MWrGO/PDMS materials could also be achieved by using spin- or spray-coating of GO solution onto SF sheets of large surface area.

4 CONCLUSION Flexible oil absorbent materials with tuneable oil uptake capacities were fabricated by controlled incorporation of rough MWrGO onto a PDMS matrix. The thickness of the GO can be controlled by vacuum filtration of fixed amounts of suspended GO followed by a simple and efficient transfer method. Three different thicknesses of GO were characterized in terms of wettability and oil sorption capacity, rate and recyclability. The surperhydrophobicity and superoleophilicity of MWrGO surface were determined by water and oil contact angle. The sample with thinnest GO film (MWrGO-1) was found to have the highest oil sorption capacity of 36.3 g/g for pump oil, 41.4 g/g for canola oil and 84.3 g/g for engine oil. The results from recyclability tests suggested that over 98 % of the oil sorption capacity can be maintained after 20 sorption-removal cycles. These superior oil sorption performance suggests potential application of MWrGO/PDMS composite films for oil sorption, especially to deal with the surface floating oil spills in oceans and rivers. Future work will focus on the enhancement of oil sorption capacity by introducing porous structure into MWrGO layers or constructing MWrGO on both side of PDMS films. Evidence of the impact of materials roughness, correlated to the active specific surface area of the exposed graphitic plans will be also assessed against the sorption performance.

ACKNOWLEDGEMENTS Chunfang Feng and Dr. Zhifeng Yi have been supported by Deakin University DUPRS scholarships. Dr. Ludovic F. Dumée thanks Deakin University for funding through his Alfred Deakin Post-



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doctoral Fellowship (ADPDF). We thank Dr. Anthony Somers (Burwood Campus, Deakin University, Australia) and Dr. Christian Gow (Coherent Scientific, Bruker) for measuring the thickness and roughness, respectively, Dr. Mark Nave and Dr. Andrew Sullivan for discussion on SEM imaging, Dr. Rob Jones (Latrobe University, Australia) for assistance with XPS testing.

ASSOCIATED CONTENT Supporting Information Available: tables showing the density and viscosity of the oil used in this study and water sorption of MWrGO samples and figures showing 2D graphs of GO film with different thickness on SF, XRD pattern and XPS spectra of MWGO and MWrGO, roughness profiles of MWrGO samples, changes of contact angles over time and water contact angle of flat rGO film and engine oil sorption capacity comparison between flat rGO film and MWrGO. This material is available free of charge via the Internet at http://pubs.acs.org.


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Figures

Figure 1. Schematic of the fabrication of MWrGO/PDMS composite films with controllable GO thickness.


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Figure 2. Photographs of GO films with different thickness transferred on the transparent shrinking films (left column), and representative SEM images of MWrGO on PDMS (right column): (A and B) MWrGO-1, (C and D) MWrGO-2 and (E and F) MWrGO-3.



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Figure 3. Wettability of MWrGO/PDMS composite film with different GO thickness. (A) and (B): Changes of contact angles against time on MWrGO-1, MWrGO-2 and MWrGO-3 for water and pump oil droplets, respectively. The results for canola oil and engine oil are shown in Figure S4 A and B. (C) Photographs of water and oil (pump oil, canola oil and engine oil) droplets on the surface of MWrGO with different contact time. The moment when oil droplets just contacted with surface of MWrGO samples was set as 0 s. The time 1 s, 10 s and 60 s were the corresponding oil spreading time after contacting. Sample MWrGO-1 was taken as an example, and the results for other samples (MWrGO-2 and MWrGO-3) are included in Figure S4 C and D in supporting information.


Page 27 of 33

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Figure 4. Schematic of water (A) and oil (B) attachment mechanisms on the surface of MWrGO. The wrinkled structure is simplified in this schematic for a clear view. The angles θw and θo are the water and oil contact angles on the graphene surface, respectively. Fw and Fo are the water and oil interfacial forces created between wrinkles of rGO and liquid droplets, and the Fwtotal and Fototal are the resultant total forces within a curvature for water and oil, respectively.



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Figure 5. (A) Photographs of oil sorption process for the pump oil floating on the surface of water. The oil was dyed with oil red for a better view of oil removal. (B) Oil sorption capacity of MWrGO with different GO thicknesses for different oil types. The oil viscosity was measured at a temperature of 20 ºC. (C) Plot of oil sorption capacity versus time for the calculation of oil sorption rate for MWrGO-1.


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Figure 6. Recyclability test for different oils: (A) Pump oil, (B) Canola oil, (C) Engine oil. The absorbed oil was removed by centrifugation of composite films. (D) Centrifuge tube for oil collection and the photograph of the composite film before and after centrifugation. Photographs of folded (E) and rolled (F) MWrGO/PDMS composite film, indicating its flexibility and portability.



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Tables

Table 1 Calculated parameters from second-order model in oil sorption rate Samples

Oil Type

k (s-1)

R2

MWrGO-1

Pump Oil

0.106

0.98

MWrGO-2

Canola Oil

0.058

0.98

MWrGO-3

Engine Oil

0.022

0.95


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Table 2 Property summary of existing oil sorption materials for comparison with present work Materials & Structures

Capacity (g/g)

Recyclability (Cycle Numbers)

Removal Methods

Capacity

Oil spill type dealt

Remaining

with

References

Traditional Sorbents (e.g. zeolites, organoclays, activated carbon,

Max. 8-23

11, 33-34

Sorbents

Polymer-based

straw, cotton, wool fibres, etc) Floating oil and

Poly(lactic acid)

Max. 5 (vegetable oil)

5

Low Pressure Distillation

< 61%

Polyurethane

Max. 25 (lubricate oil)

400

Organic Solvent Washing

n.a.

Floating oil

32

Polyurethane

Max. 103 (bean oil)

n.a.

n.a.

n.a.

Floating oil

36

Polyester

Max. 2.9 (crude oil)

n.a.

n.a.

n.a.

Oil/water mixture

37

PDMS

Max. 4.9 (motor oil)

20

Squeezing

n.a.

Floating oil

38

Graphene Capsules


n.a.

n.a.

n.a.

Floating oil

1

Mesoporous Graphene

Max. 30 (kerosene)

10

Heat Evaporation

90%

Oil/water mixture

6

Max. 80 (used engine oil)

n.a.

Burning or Squeezing

n.a.

CNT Sponge Sole Carbon-based Sorbents

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48



10

Burning or Squeezing

40% or 13%


n.a.

n.a.

50% - 93%

21 31 11 Floating oil and oil

Burning off, Distillation or

50-98%

droplet in water

5

Burning off

n.a.

Floating oil and

34

10

Organic Solvent Washing

98%

oil/water mixture

26

Max. 138 (pump oil)

5

Squeezing

70%

Max. 320 (motor oil)

10

High Temperature Evaporation

> 99%

Max. 105 (sesame oil)

6

Max. 140 (n-dodecane)

5

Max. 122 (olive oil)

n.a.

Max. 37 (motor oil)

Squeezing

Graphene Foam

Graphene/CNT Foam

35

Floating oil

Graphene Aerogel

Graphene/CNT Aerogel

oil/water mixture

Organic Solvent Washing and Drying

< 90%

Floating oil

3 36

Floating oil

13



Max. 300 (pump oil)

n.a.

n.a.

n.a.

Floating oil and oil

30

Max. 75 (caster oil)

10

Distillation

99%

droplet in water

2

Exfoliated Graphite

Max. 86 (heavy oil)

n.a.

Squeezing

< 70%

Floating oil

33

CNT/Polymer Sponge

Max. 25 (soybean oil)

n.a.

n.a.

n.a.

Emulsified oil

22


50

Squeezing

n.a.

Max. 48 (diesel)

10

Solvent Extraction

> 95%

39

Max. 99 (soybean oil)

5

Squeezing

20 %

9

20

Centrifugation

> 98%

Composite Sorbents

Spongy Graphene

Carbon-based material/polymer

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 32 of 33

Graphene/Polymer Sponge

Floating oil and oil droplet in water

12

Max. 36.3 (pump oil) MWrGO/PDMS Composite

Max. 41.4 (canola oil)

Floating oil

Present Work

Max. 84.3 (engine oil)


Page 33 of 33

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