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Recyclable 3D graphene aerogel with bimodal pore

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Cite this: RSC Adv., 2017, 7, 29722

structure for ultrafast and selective oil sorption
from water†
Open Access Article. Published on 07 June 2017. Downloaded on 08/06/2017 07:49:45.

Muhammad Adil Riaz,a Pejman Hadi,b Irfan H. Abidi,a Abhishek Tyagi,a Xuewu Oua
and Zhengtang Luo *a

Development of next-generation porous sorbents to overcome the challenges, such as low uptake
capacity, slow sorption rate, and non-recyclability, associated with conventional sorbents is of utmost
importance. Herein, we report the synthesis of a highly porous graphene aerogel (GA) with a unique
three-dimensional hierarchical bimodal porous network of macro and meso-pores via a facile
hydrothermal technique; this aerogel has sorption capacity that is more than 5 times that of
conventional commercial sorbents. Fluoroalkyl silane functionalization of the GA surface results in
a significant reduction in its water sorption from 20 g g
1 to 5 g g
1 due to the GA surface becoming
more hydrophobic, which renders it useful in practical application to selectively remove oil from
seawater. Moreover, the sorption rate of the GA for oils and organic solvents has been found to be
extremely fast, and saturation of the GA is completed in a few seconds. This is attributed to its unique
meso–macro bimodal porous structure with large pore channels called macro-pores or voids of various
sizes ranging from 300 nm to over 10 mm, which facilitate mass transport into its inner mesopores of
Received 9th March 2017
Accepted 23rd May 2017
14–18 nm at high rate. Finally, the GA is shown to be a highly recyclable material due to its good
mechanical strength, where the oil- and organic solvent-sorbed GA can be efficiently recovered using
DOI: 10.1039/c7ra02886e
thermal or chemical methods for several sorption–desorption cycles without significant loss in its
rsc.li/rsc-advances capacity, which also makes the process cost effective and environmentally friendly.

sorbents, including low uptake capacity and low sorption rate,

Introduction which are yet to be overcome.6,7
Marine oil spills occur frequently as a result of anthropogenic Graphene, due to its extraordinary properties, has been used
activities, such as oil rig drilling and oil tanker accidents, which for numerous applications such as lithium ion batteries, super-
may cause irreparable ecological impacts and devastating long- capacitors, exible electronics, sensors and composite materials.8
term environmental disasters.1 Various strategies, such as in situ It is also a promising material for sorption applications due to its
burning, are being used for oil spill remediation; however, the high surface area. However, the dispersion of pristine graphene in
gaseous products and particulates emitted through oil combus- water is difficult which precludes its application in aqueous
tion pollute the atmosphere.2 The deployment of booms and media. Therefore, graphene oxide (GO) is an alternative material
skimmers conne the oil to a specic location for collection, but to graphene that has facile aqueous solution processability due to
this method is inefficient in turbulent wavy surfaces, time- the abundance of polar functional groups on its surface.9 For
consuming, laborious and requires a large number of equip- applications, especially those requiring high porosity such as
ment.3,4 Porous sorbent materials have recently attracted great contaminant removal from aqueous solution via sorption
attention owing to their ability to remove all traces of oil from spill processes, the assembly of graphene oxide sheets into a three
sites.5 However, there are several challenges with these porous dimensional structure can be advantageous to form a material
with a high surface area, enhanced porosity and high strength at
very low density.10 This structure has been realized recently via the
one step hydrothermal reduction of graphene oxide under
a
Department of Chemical and Biomolecular Engineering, Hong Kong University of controlled conditions11 which has been used for lithium ion
Science and Technology, Clear Water Bay, Kowloon, Hong Kong. E-mail: keztluo@ batteries and photocatalysis.12–14 Unlike activated carbons and
ust.hk
b
zeolites with micro- and/or mesoporous structures and high
New York Center for Clean Water Technology, Stony Brook University, NY 11794, USA
surface areas,15–17 aerogel sponges are mainly characterized by
† Electronic supplementary information (ESI) available: FTIR analysis and full
range XPS spectra of graphene oxide and graphene aerogel and Table S1–S3 are
their high porosity and large macropores in addition to meso-
provided. See DOI: 10.1039/c7ra02886e pores. Therefore, although activated carbons and zeolites employ

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electrostatic interaction or p–p interactions to adsorb specic interconnection of the stacked hierarchical macro-sized pores
components, such as phenols and dyes,18–20 graphene aerogels are forms a three dimensional porous network with abundant
well-tailored for the absorption of nonpolar substances, such as voids, which is favorable for the uptake of molecules into the
hydrocarbons and oils. The great potential of graphene aerogels bulk of the material, i.e. absorption. Some literature hypothe-
in oil sorption not only stems from their hydrophobicity, but also size the potential of such materials with well-developed mac-
their large pore sizes which lead to facile oil diffusion into the roporous structures in the adsorption of pollutants from
bulk of the sorbent at high rates. different media.24,25 Nevertheless, we believe that there is no
Some oil sorption studies using graphene based materials evidence of any relationship between the presence of macro-
have been reported. For example, Bi et al. (2012) reported the pores and adsorption rate or intensity. The adsorption
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synthesis of spongy graphene via a 24 hour hydrothermal phenomenon is mainly associated with pores with sizes <50 nm
reaction of a GO ammonia mixture for the adsorption of toxic (micropores and mesopores), and not macropores. Since the
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solvents such as chloroform and toluene.21 Similarly, Zhao et al. detection of micro- and meso-structures is not feasible with
(2012) used thiourea to fabricate graphene sponge for oil SEM, the misconception of higher adsorption capacity by
adsorption.22 Bi et al. (2014) used freeze drying and soot treat- macroporous materials should be refrained. Nevertheless, well-
ment to prepare spongy graphene which improved its sorption developed interlinked hierarchical macropores can decrease the
capacities for different oils and organic solvents.23 diffusion restrictions in the porous channels, resulting in
In this work, an ultrahigh porosity hydrophobic graphene a higher absorption rate and capacity. In this study, the SEM
aerogel (GA) with a bimodal pore size distribution and meso images illustrate that the size of the pores is in the range of 1–10
and macropores is synthesized via a 2 hour hydrothermal mm, which is sufficiently large to diminish the pore blockage by
reaction of only a pure GO solution and its sorption capacity is oil and solvent molecules during the absorption process. The
compared with its commercial counterparts and other porous aforementioned characteristics of the aerogel, as evidenced by
materials in the literature. The rate of sorption is investigated in the SEM micrographs, render it a highly attractive material for
detail with a separate set of experiments for oils and organic applications requiring the absorption of contaminant
solvents. Furthermore, the regeneration of the graphene aerogel molecules.
is examined for multi-cycle operations in order to boost its It is noteworthy that we do not discard the possibility of
economic viability and sustainability. Ultimately, the feasibility adsorption and the absorption phenomenon. We only suggest
of uoroalkyl silane graing on the graphene aerogel surface is that it is not possible to conclude the inuence of the adsorp-
thoroughly examined with the aim of decreasing its surface tion process by only SEM images. Therefore, in order to quan-
energy as a result of the introduction of the C–F moiety so that titatively study the textural characteristics of the prepared
GA can selectively uptake oil spilled on seawater with minimum aerogel, two types of widely applied analyses were adopted:
sorption of water underneath in practical scenarios. mercury intrusion porosimetry for the determination of mac-
ropores (>50 nm) and mesopores (2 nm < dp < 50 nm) and
nitrogen adsorption–desorption for the determination of micro-
Results and discussion (<2 nm) and mesopores. As illustrated in Fig. 2(e), the aerogel
Morphology and chemical characterization possesses a bimodal pore size distribution of macro- and mes-
Fig. 1 demonstrates the SEM image of the synthesized aerogel, opores, where the former has signicantly higher pore volumes,
where graphene sheets are randomly oriented and the which suggests the dominance of macropores in the aerogel

Fig. 1 Graphene aerogel synthesis and its morphology. (a) Schematic showing the experimental steps and synthetic conditions using graphene
oxide as the precursor, (b) sample optical image illustrating the foam-type structure of the GA with good mechanical strength and (c and d) SEM
images showing that graphene sheets are interconnected in a random fashion to form a three dimensional porous structure with pores of various
sizes and shapes.

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structure. The macropores have a considerably wide range of surface area of the graphene aerogel based on the BET model
sizes from 300 nm to over 10 mm, which implies the presence of equation was determined to be 49.3 m2 g
1. Zhang et al. ach-
a hierarchical porous structure and is in line with the observa- ieved a BET surface area of 11.8 m2 g
1 for their freeze dried
tions from the SEM images. In contrast with the macropores, graphene aerogel and suggested that employing a supercritical
the mesopores have a much smaller pore size distribution in the uid drying instead of freeze drying would achieve higher
range of 14–18 nm and much smaller pore volumes. The hier- specic surface areas.27 The surface area of graphene aerogels
archical pores can be hypothesized, but not solidly veried, to widely varies using various synthetic methods such as chemical
be in the form of large macropores in the outer surface of the reduction or hydrothermal self-assembly and synthetic condi-
aerogel interconnected with smaller macropores in the bulk of tions such as reaction time, temperature or GO concentration.
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the material, which in turn, are linked to the smaller mesopores We believe that the possible reason for a lower surface area is
as side-branches of the larger pores. This unique hierarchical attributed to the aggregation of GO sheets during self-assembly.
Open Access Article. Published on 07 June 2017. Downloaded on 08/06/2017 07:49:45.

structure with a bimodal pore size distribution provides an Nonetheless, despite the merit of increasing the BET surface
accessible macropore structure which allows the diffusion of area, the presence of macrospores is more favorable than
oils and organics into the bulk of the material at a high rate and mesopores in our specic application since absorption takes
ultimately their retention in the aerogel pore network. Although place by the diffusion of the uid into the large macro-channels
valuable information on the macropore size and distribution in the bulk of the material.
can be obtained through mercury intrusion porosimetry, the The C1s XPS spectra of the graphene oxide and graphene
mesopores and micropores can be more accurately analyzed aerogel are shown in Fig. 2(a) and (b). For graphene oxide, four
using the nitrogen adsorption–desorption method. As shown in distinct peaks located at 284.6 (aromatic C–C and C]C), 286.6
Fig. 2(d), the N2 adsorption–desorption curve follows a type IV (epoxy or alkoxy C–O), 287.8 (carbonyl C]O) and 288.7 eV
isotherm curve according to the IUPAC classication, which is (carboxyl O–C]O) are identied on the surface of the GO.28
characteristic of a mesoporous material.26 The low hysteresis However, the C1s spectrum of the graphene aerogel exhibits
loop area in the isotherm curve suggests the presence of a small markedly different peak intensities, where the aromatic C–C
volume of mesopores in the aerogel structure, which is in close and C]C peak (284.6 eV) is intensied and the peaks for the
agreement with the mercury intrusion porosimetry. The specic oxygen-containing functional groups exhibit signicantly lower

Fig. 2 . Surface, structural and porous properties of the graphene aerogel. (a and b) C1s XPS spectra of the GA and GO samples showing that
graphene oxide has different oxygen containing functional groups bonded to its surface; however, the graphene aerogel does not contain these
functional groups in abundance as shown by the reduced intensity of these groups, (c) XRD results showing that the graphene aerogel has an
amorphous structure in contrast to the crystalline structure of graphene oxide as it shows a sharp peak at around 10 , (d) N2 adsorption–
desorption isotherm of the GA showing the volume of N2 adsorbed at different relative pressures which is used to calculate the textural
characteristics of the GA (inset shows the water contact angle of the graphene aerogel and its surface hydrophobicity) and (e) mercury intrusion
of the graphene aerogel at different pressures (insets show the pore size distribution of the aerogel which suggests its hierarchical structure with
mesopores and macropores of various sizes).

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intensities. Moreover, the peak area ratios of the oxygen to Sorption capacity and kinetics of the GA
carbon (AO/AC) obtained from the wide-range spectra (see
Drainage time is dened as the duration of time the GA is
Fig. S1†) are 1.38 and 0.37 for GO and GA, respectively. The
retained aer the completion of the sorption in order to drain
signicantly lower AO/AC for GA further conrms the efficient
the oil molecules loosely attached to the outermost GA surface
removal of oxygen-containing functional groups. The lower
by cohesive and adhesive forces. This parameter is unfortu-
concentration of oxygenated surface functional groups is
nately either disregarded or not reported in most literature
further veried by the FTIR analysis (see Fig. S2†). GO displays
which may make the sorption capacities unreasonably high.25,31
a series of absorption bands in the range of 3800 to 1000 cm
1, Therefore, separate experiments were conducted to determine
including a broad band at 3300 cm
1 for the hydroxyl groups,
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the appropriate drainage time of the GA using mineral and

carbonyl bonds in both ketone and carboxylic acid groups at
vegetable oils. The results shown in Fig. 3(a) reveal a very steep
1720 cm
1, aromatic C]C bonds at 1581 cm
1, C–OH stretch-
decrease in the uptake amount in the initial stage of the
ing vibrations at 1404 cm
1 and epoxy or alkoxy C–O groups at
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drainage for both oils. Therefore, the minimum drainage time

1220 and 1080 cm
1. Aer hydrothermal reduction, the inten-
of 1 min is required to ensure the removal of loosely-attached oil
sity of the hydroxyl OH groups decreased signicantly, which
molecules from the GA surface. A drainage time of 2 min was
suggests the removal of oxygen-containing groups from the
considered in all subsequent experiments. Fig. 3(b) displays the
surface of graphene. The reduction in the oxygen containing sorption capacity of the graphene aerogel for several types of
functional groups of the graphene aerogel, as elucidated by the oils and organic solvents. The sorption capacity of the GA
XPS and FTIR techniques, is essential in applications requiring
ranges from 58–70 g g
1 for oils and 44–68 g g
1 for organic
the separation of water insoluble compounds from aqueous
solvents. Some previous studies21–23 reported similar or higher
media, since the diffusion of water molecules into the pores of
sorption capacities; however, GA was synthesized using
the sorbent leads to the occupancy of the pores and thus,
different chemicals or synthetic methods and we reported
a reduction in the uptake capacity of the target water insoluble
a shorter hydrothermal synthetic method (2 hours) using pure
compounds. In order to examine its affinity for water, the
GO. A detailed comparison of the synthesis and performance
wettability of the graphene aerogel surface was evaluated to parameters of the present work with previously reported 3D
around 130 , which indicates the water-repellent hydrophobic graphene based sorbents is given in Table S3.†
nature of the graphene aerogel surface.29 In addition, due to the
The physical and chemical properties of the sorbates are
very high oleophilic nature of the GA, an oil drop could not be
responsible for the difference in the uptake capacity of the
formed on the GA surface for contact angle measurements. The
sorbent material.32 Taking the organic solvent category as an
high hydrophobicity and oleophilicity of GA validate it as an
example, a direct linear relationship between the GA uptake
interesting material for the selective sorption of oil from
capacity and the solvent density is observed (see Fig. S3†). The
aqueous media.
weight-based uptake capacity is enhanced with an increase in
The XRD patterns illustrated in Fig. 2(c) show that the sharp the density of the sorbate, where hexane with the lowest density
peak at around 10 corresponding to the crystalline structure of has the lowest sorption capacity and maximum uptake is ach-
GO30 disappears upon the hydrothermal reaction, which repre-
ieved when DMF with the highest density is used as the sorbate.
sents the amorphous structure of the GA. The presence of
This trend is consistent with that reported in various other
a broad band at 25 is attributed to the irregular structure of the
studies, such as the sorption of solvents by Fe/C nano-
GA and the random overlapping of the graphene sheets to form
composites33 and nanocellulose aerogels.34 If the obtained
irregular network of pores, which is consistent with the struc-
sorption capacities are transformed into the organic solvent
tural observations by SEM.
volume (rather than weight) in unit weight of GA, the sorption

Fig. 3 Sorption capacity results of GA and the effect of drainage time. (a) GA sorption capacity decrease after the initial sorption of vegetable or
mineral oil demonstrates that at least 1 minute of dripping or drainage after the oil sorption is required to represent the true sorption capacity of
the graphene aerogel, (b) sorption capacities of the graphene aerogel for various oils and organic solvents and their comparison with commercial
sorbents. The sorption capacities of the commercial sorbents Corksorb, Pads and SOCs are 6–14 g g
1, 5–7 g g
1 and 10–20 g g
1, respectively,
which are much lower (more than 5 times) than that of the graphene aerogel with capacities of 40–70 g g
1 for various oils and organic solvents.

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capacity approaches an approximately constant value of 70 mL Although the high uptake capacity of the GA augments its
g
1. This nding is very interesting in the context of the sorp- potential for oil and organic solvent uptake commercially,
tion mechanism. Since the sorption capacity in terms of volume several other characteristics of the GA, including sorption rate,
has an almost constant value, it can be concluded that the recyclability and water affinity (selectivity), also need to be
sorbent molecules are taken up in the bulk of the porous GA examined for its suitability in practical applications. The sorp-
with a large volume of voids, which implies the dominance of tion kinetics of the GA for organic solvents is depicted in
the absorption mechanism rather than adsorption mechanism. Fig. 4(a), where the removal of colored n-hexane by the GA is
This nding revamps the generic misconception that sorption visually monitored. In this qualitative approach, the time-based
using graphene or similar materials, such as synthetic carbon fading of the color reects the uptake rate of n-hexane. Here, the
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nanober lms,24 porous carbon monoliths,35 silica monoliths36 solvent uptake by the GA (only within 3 s) is much faster than
or natural materials such as organoclays and raw cotton,37 the sorption by the CNT sponge (125 s and 50 s for the sorption
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proceeds via an adsorption mechanism. The sorption capacity of octane and ethyl acetate, respectively).38 The high sorption
of the GA is more impressive when comparisons with rate of the GA is attributed to its hierarchical porous structure
commonly-used commercial products under similar experi- which facilitates the diffusion of molecules into its pores.
mental conditions are made. As depicted in Fig. 5(b), the uptake Sorption rate is one of the critical factors in practical pollutant
capacity of the GA is much higher than the commercial sorbents uptake applications, where most of the spreading occurs in the
used. The uptake capacity of the GA ranges from 40 to 70 g g
1, outset of the spillage. Similar experiments were performed for
whereas Corksorb, SPC Pads and SPC SOCS, as the commercial other organic solvents such as toluene and ethanol whose
sorbents, render sorption capacities in the ranges of 6–14 g g
1, results reect very fast sorption rates of a few seconds as well.
5–7 g g
1 and 10–20 g g
1, respectively, for the organic solvents An average uptake rate can be estimated using the sorption
and oils. Also, the GA sorption capacity was compared with that capacity and total time required to remove the colored solvent
reported in literature, as summarized in Table S1 (ESI†). It can to reect the sorption rate for each solvent. Considering the
be seen that only the CNT sponge has relatively comparable sorption capacity of the GA for n-hexane (44 g g
1) and the time
sorption capacities with that of the graphene aerogel. However, required for the sorption process of n-hexane (4 s), the average
the facile preparation of the GA and its lower cost compared sorption rate was determined as 11 g g
1 s
1. Similarly, the
with the CNT sponge renders it a more attractive option for this sorption rates of the ethanol and toluene were determined as
application. 22.9 and 17.7 g g
1 s
1, respectively (see Fig. 4(c)).

Fig. 4 Kinetics of the sorption of graphene aerogel. (a) Experimental system showing the sorption of n-hexane, toluene and ethanol by the
graphene aerogel within 3 seconds, (b) oil uptake experiment using the graphene aerogel shows that the sorption reaches its maximum
saturation level within 1 minute for mineral oil and vegetable oil (inset shows the linear fit of the experimental data to the pseudo second order
kinetic model for both oils) and (c) average sorption rate of the organic solvents.

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In contrast to the organic solvent sorption, oil sorption high even aer 8 cycles of sorption–desorption. This is indeed
uptake kinetics can be evaluated qualitatively. As illustrated in very promising since the GA can be used almost innitely in the
Fig. 4(b), the sorption rate of the oils is also very high, where the oil sorption–regeneration process without the need for sorbent
entire uptake process is completed in less than one minute. It is replacement. However, some sorbents lack this advantageous
notable that 90% of the uptake for both oils is achieved in only property. For instance, Deschamps et al. demonstrated that the
20 s. The very fast uptake in the rst few seconds is due to the oil sorption capacity of fresh cotton ber is 20 g g
1, but aer
lack of any diffusion barrier into the interior of the GA owing to only 3 cycles of regeneration, its capacity decreases more than
the large size of its pores. However, aer the macropores are 30% to 14 g g
1.40 Similarly, although the sorption capacity of
lled, a comparatively longer time is required for the diffusion polyurethane sponges was 19 g g
1 for the rst cycle, it
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of the sorbed oil into the bulk of the GA with smaller macro- decreased to 7 g g
1 aer 9 cycles.41 Also, although CNT sponge
pores or even mesopores. It is shown that the experimental had a high sorption capacity of 100 g g
1 for vegetable oil, it
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kinetic data best ts with the linearized form of the pseudo could not retain its high capacity for many cycles, where its
second order model (see inset of Fig. 4(a)) with correlation sorption capacity depleted to 20–40 g g
1 aer 10 cycles of
coefficients as high as 0.99. The rate constants obtained using reuse.38 Unlike the regeneration of oil-sorbed GA using chem-
the pseudo second order model for the mineral oil and vege- ical methods, our regeneration is a thermal process in the case
table oil are 0.00476 and 0.00727 s
1, respectively. The model of the solvent-sorbed GA. Similar to the oil-sorbed GA, the GA
parameters obtained for the three different models are exhibits an excellent recyclability, as shown in Fig. 5(a), where
summarized in Table S2 (ESI†). In previous studies21,22 there are the sorption capacity of the aerogel did not show a signicant
some qualitative tests showing the fast uptake of oil by gra- decreasing trend even aer several cycles of sorption and
phene aerogel, but in this study, the uptake kinetics of oils is desorption and still maintained its high capacity. The amount
quantitatively studied using a separately designed experiment of solvent that can be recovered is above 90% aer each cycle.
and is veried by kinetic models. Moreover, the mechanism of Most of the studies on the uptake of oil by porous materials
oil sorption can be understood by this quantitative kinetic study are carried out in idealistic conditions, where only oil (in the
as it reveals that 90% of oil uptake occurs within 20 s. absence of water) is used as the sorption medium.38,42,43 Never-
theless, in practical scenarios, oil spreads as a thin layer on
water bodies, thus it is important to simulate real conditions
Recyclability and water sorption of GA and determine the uptake efficiency of the sorbent in the
Exploring the fate of the exhausted sorbent is a major challenge presence of water. In seawater, oil slick of few micrometers is
that is disregarded in some literature. Some researchers have formed whose thickness is used to determine the volume of the
proposed to burn oil-sorbed materials to utilize the caloric oil spill. Therefore, to simulate the oil spill sea conditions,
value of the recovered oil.34,38,39 However, this is not an appro- different amounts of the oil was spilt on a xed surface area of
priate method due to the unsustainability of the process and water and the sorption capacity of the GA was evaluated.
lack of any economic justication. Hence, employing a more Fig. 5(b) shows that the GA also takes up water along with oil.
sustainable approach for GA regeneration by removing the Although the water sorption amount of the GA is low, the oil
sorbed molecules from the pores of the GA and its reutilization sorption amount, even in the presence of water, is higher than
is a more attractive and promising option. Fig. 5(a) shows the its commercial counterparts. However, this can be considered
change in the oil sorption capacity of the GA and the material as an undesirable phenomenon as it reduces the sorption
loss for several cycles and it can be seen that no material loss capacity of the GA. The water sorption of the GA is mainly due to
takes place during the regeneration process and that the the presence of some functional groups (although in a low
capacity of the GA does not change and remains appreciably concentration) on the GA surface. As illustrated in Fig. 5(c),

Fig. 5 Multicycle recyclability test and water sorption capacity of the GA. (a) Thermal and chemical regeneration of the organic solvent-sorbed
GA and oil-sorbed GA, respectively, and their re-use for several cycles without much change in sorption capacity, (b) different concentrations of
oil on water and the corresponding uptake of oil and water by the GA and (c) water sorption capacity of GA before and after silane function-
alization showing a drastic reduction in the water uptake.

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although the water uptake of the GA is around 20 g g
1, silane- ltered using a 0.2 micron Nylon Millipore lter and washed
graed GA has a water uptake as low as 5 g g
1. The full range with DI water using a vacuum-assisted ltration setup until
XPS spectra of silane functionalized GA in Fig. S5† shows a pH level of 5.5 was achieved. This acid intercalated sample
prominent uorine peak hence this low water affinity is attrib- was dried in an oven at 60  C overnight. In the next stage, the
uted to the lower surface energy of the GA due to the presence of dried sample was introduced into a 250 mL round bottom ask
C–F groups which can mask the moieties with higher surface containing 150 mL concentrated sulfuric acid and 30 g KMnO4
energies and thus results in the diminution of the water was gradually added it. The mixture was stirred and reacted for
sorption. 4 h at 35  C. Subsequently, the mixture was added to 1 L DI
It should be noted that silane modication of graphene water and stirred for another 2 h. In the last stage, 50 mL 30%
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aerogel does not change its morphological properties such as H2O2 was added dropwise to the mixture. The purple color of
SSA and porous structures. However, silane treatment makes the mixture changed to bright yellow at the end of this step. The
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the surface water repellent as indicated by the contact angle mixture was further stirred for 2 h and allowed to settle for 24 h.
measurements, which makes the oil sorption process more Ultimately, the supernatant was discarded and the product at
effective and efficient. the bottom (graphene oxide, GO) was washed and centrifuged
Overall, it has been demonstrated in our work that this GA with rst with 1 L 10% HCl solution and then with excess DI water
a unique hierarchal bimodal meso–macro porous structure over- until neutrality was achieved.
comes most of the challenges associated with conventional Preparation of graphene aerogel (GA). The graphene aerogel
sorbent materials such as insufficient sorption capacity, low was prepared via a hydrothermal method where 10 mL of gra-
uptake rate, non-recyclability, and high water uptake rendering it phene oxide solution (>2 mg mL
1) was placed inside a Teon-
useful for practical spills of petroleum liquids in seawater. lined autoclave reactor and heated to 160  C for 2 h. The
Moreover, effort has been made to study in depth the kinetics of prepared graphene hydrogel was freeze-dried for 24 h aer
sorption the GA for various oils and organic solvents and to which the porous graphene aerogel (GA) was obtained.
understand mechanism behind very high sorption rate of the GA Fluoroalkyl silane graing onto the GA surface. In the rst
through detailed characterization of its unique porous structure. step, a solution was prepared by dissolving 200 mL
1H,1H,2H,2H-peruorooctyltriethoxysilane (POTS) in 15 mL
hexane in a glass vial. 1 mL water was added aerwards and the
Experimental solution was stirred for 15 min to fully hydrolyze the POTS. In
Chemicals the second step, the graphene aerogel was immersed into the
Expandable graphite akes (GRAFGUARD® 160-50N) with solution and stirred for 12 h to allow the surface functional
a typical expansion volume of 250 cm3 g
1 at 600  C and mean groups of the GA to react with the hydrolyzed POTS. The
particle size of around 350 mm were purchased from Graech uorosilane-treated aerogel was then heated at 80  C overnight
Hong Kong Limited. Microwave heating was used to form and the resultant material was denoted as FS–GA.
expanded graphite before the experiments. All other chemicals
of analytical grade were purchased from Sigma-Aldrich, Hong Characterization
Kong, and were used without further purication. Kaydol White
Mineral Oil was supplied by Sonneborn. Vegetable oil with The surface hydrophobicity of the graphene aerogel was deter-
50.5% pomace olive oil as its main constituent was purchased mined using a DIGIDROP contact angle meter (GBX, France).
from a local market in Hong Kong. The other oils used were Attenuated total reectance Fourier transform infrared (ATR-
engine oil Visco 5000 of British Petroleum (BP) and gear oil FTIR) spectra were recorded on a Bio-Rad FTS 6000 spectrom-
80W-90 of Americo Lubricants Ltd. Three different commercial eter in the wavelength range of 4000–500 cm
1 to determine the
sorbents were used in this research. Thermally-treated hydro- surface functional groups of the prepared materials. X-ray
phobic granular Corksorb product was purchased from Amorim diffraction analysis (XRD, PW1830, Philips) with Cu Ka radia-
Environmental Systems, Portugal and polypropylene SOCs and tion (l ¼ 1.5406 
A) was used to study the crystal structure of the
Pads were obtained from Brady Corporation Hong Kong materials. The morphology of the materials was investigated on
Limited. a scanning electron microscope (SEM, JEOL 6700F). X-ray
photoelectron (XPS) analysis was carried out using an XPS-
PHI5600 system equipped with a monochromatic Al Ka
Material preparation source to understand the chemical bonding states of the surface
Preparation of graphene oxide (GO). Graphene oxide was elements in the materials. The textural properties of the mate-
prepared via a modied Hummers method44 where expansion rials were analyzed on a Beckman Coulter SA3100 surface area
of graphite was carried out prior to the intercalation or oxida- analyzer using the nitrogen adsorption–desorption method at
tion stage by microwave heating for few seconds. Subsequently, 77 K. The Brunauer–Emmett–Teller (BET) and the Barrett–Joy-
1 g microwave-treated expanded graphite, 5 g K2O8 and 5 g P2O5 ner–Halenda (BJH) equations were employed to evaluate the
were added to a round bottom ask containing 30 mL specic surface area and pore size distribution of the materials,
concentrated sulfuric acid. The desired reaction took place by respectively. The samples were outgassed at 120  C prior to the
thoroughly stirring the mixture at 90  C for 4.5 h using an oil analysis in order to remove all the moisture and adsorbed gases
bath as the heating medium. The oxidized product was then on the materials.

29728 | RSC Adv., 2017, 7, 29722–29731 This journal is © The Royal Society of Chemistry 2017
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Sorption experiments above the boiling point of the solvent and the as-dried aerogel
was used in the next sorption cycle. The weight change aer
Sorption capacity. Accurately-weighed graphene aerogel
each cycle reects the regeneration ability of the GA. For the oil-
samples (GA, 5 mg) were immersed in different types of sorbates
sorbed GA, the aerogel was submerged into n-hexane which
(oils and organic solvents) for 2 min (contact time). Then, the
diffused into the pores of the GA and washed the oil away.
samples were gently taken out and allowed to drain for 2 min in
Subsequently, the aerogel was dried in an oven and used for the
order to remove the molecules loosely attached to the outermost
subsequent sorption cycles. The capacities of the GA in each
surface of the sorbent by cohesive and adhesive forces (and not
as a result of diffusion into the bulk of the material). Then, the cycle were determined as described earlier.
Water sorption and selectivity. The water sorption capacities
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GA samples were weighed and the equilibrium sorption

of the GA and silane-modied GA were evaluated by their
capacity of the GA was determined as the amount of sorbate
immersion in pure water and monitoring their weight change.
taken up on a unit mass of dry GA, as follows:
Open Access Article. Published on 07 June 2017. Downloaded on 08/06/2017 07:49:45.

Moreover, the selectivities of the graphene aerogel samples were

Wf
Wi determined by introducing different amounts of oil onto the
Qe ¼ (1)
Wi water surface and carrying out the aforementioned sorption
where, Qe is the sorption capacity of the graphene aerogel and tests.
Wi and Wf are the initial and nal weights of the samples before
and aer the oil sorption, respectively. An identical approach Acknowledgements
was assumed to test the sorption efficiency of the commercial
sorbents. This project is supported by the Research Grant Council of
Sorption kinetics. Two distinct approaches were attempted Hong Kong SAR (project no. 623512 and DAG12EG05). Mr. Riaz
to determine the sorption rate of the organic solvents and oils. appreciates nancial support from Higher Education
For the sorption kinetics of organic solvents, the sorbates were Commission (HEC) of Pakistan. Technical assistance from the
rst stained with an organic dye (Sudan Black B, 1 mg mL
1) Materials Characterization and Preparation Facilities is greatly
and then added on water to create articial seawater conditions appreciated.
in a visible manner. For the sorption kinetics of oil, graphene
aerogel samples (5 mg) were placed in separate containers with References
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