The Role of Diamines in the Formation of Graphene Aerogels

Abstract

Aliphatic or aromatic diamines undergo nucleophilic attack on the epoxy groups of graphene oxide under hydrothermal conditions resulting in partial functionalization and partial reduction of the graphenic surface. The overall reaction decreases the solubility of graphene oxide and yields a hydrogel that can be dried to a 3D porous structure classified as an aerogel. This article compares the graphene aerogels derived from different aliphatic and aromatic diamines. © 2018 Vrettos, Karouta, Loginos, Donthula, Gournis and Georgakilas.

Full text

ORIGINAL RESEARCH
published: 10 April 2018
doi: 10.3389/fmats.2018.00020
Frontiers in Materials | www.frontiersin.org 1April 2018 | Volume 5 | Article 20
Edited by:
Emilia Morallon,
University of Alicante, Spain
Reviewed by:
Horacio Javier Salavagione,
Consejo Superior de Investigaciones
Científicas (CSIC), Spain
Tatiana S. Perova,
Trinity College, Dublin, Ireland
*Correspondence:
Vasilios Georgakilas
viegeorgaki@upatras.gr
Specialty section:
This article was submitted to
Carbon-Based Materials,
a section of the journal
Frontiers in Materials
Received: 05 February 2018
Accepted: 22 March 2018
Published: 10 April 2018
Citation:
Vrettos K, Karouta N, Loginos P,
Donthula S, Gournis D and
Georgakilas V (2018) The Role of
Diamines in the Formation of
Graphene Aerogels.
Front. Mater. 5:20.
doi: 10.3389/fmats.2018.00020
The Role of Diamines in the
Formation of Graphene Aerogels
Katerina Vrettos 1, Niki Karouta 2, Panagiotis Loginos 1, Suraj Donthula 3, Dimitrios Gournis 2
and Vasilios Georgakilas 1
*
1Material Science Department, University of Patras, Rio Patras, Greece, 2Department of Materials Science and Engineering,
University of Ioannina, Ioannina, Greece, 3Department of Chemistry, Missuri University of Science & Technology, Rolla, MO,
United States
Aliphatic or aromatic diamines undergo nucleophilic attack on the epoxy groups of
graphene oxide under hydrothermal conditions resulting in partial functionalization and
partial reduction of the graphenic surface. The overall reaction decreases the solubility
of graphene oxide and yields a hydrogel that can be dried to a 3D porous structure
classified as an aerogel. This article compares the graphene aerogels derived from
different aliphatic and aromatic diamines.
Keywords: graphene aerogels, diamines, conductive aerogels, carbon superstructures, porous nanostructures
INTRODUCTION
Graphene Oxide (GO) is a graphene derivative with significant scientific interest that has been
expanded enormously after the isolation of single layered graphene nanosheets by Novoselov et al.
(Novoselov et al., 2004; Compton and Nguyen, 2010; Kim et al., 2010; Chen et al., 2012; Zhu
et al., 2012). GO is essentially a graphene monolayer functionalized heavily with oxygen groups,
mainly epoxides, hydroxyls, and carboxylates, although several others have been also proposed
(Hontoria-Lucas et al., 1995; He et al., 1998; Lerf et al., 1998). GO is produced via liquid exfoliation
of graphite oxide by treatment of graphite with strong acids according to established procedures
by Staudenmaier (1898),Hofmann and Konig (1937),Hummers and Offeman (1958) that followed
the first synthesis of GO by Brodie (1859), and its variants (Park and Ruoff, 2009; Mao et al., 2012).
Although the GO core consists of hydrophobic aromatic domains, the presence of surface oxygen
groups adds hydrophilic character that induces a characteristic colloidal stability of GO nanosheets
in water. Successful reductive removal of those oxygen groups, and partial reconstruction of the
aromatic system of GO results to the known-as reduced GO (rGO), which is receiving much
attention as an alternative to pure graphene due to its good electrical properties (Pei and Cheng,
2012; Chua and Pumera, 2014). In addition, removal of oxygen groups reduces the hydrophilic
character of rGO and results in aggregation in water. Thus, under appropriated conditions, the
reductive aggregation of GO may give hydrogels, which can then be transformed to aerogels by
removing pore filling water according to well-established procedures (e.g., freeze drying). Thereby,
graphene aerogels are 3D architectures consisting of a conducting network of rGO nanosheets with
large specific surface areas and micro/nano porosity (Li and Shi, 2012, 2014; Pei and Cheng, 2012;
Zhang et al., 2012; Nardecchia et al., 2013; Li et al., 2014). Graphene aerogels are attracting much
attention for applications in catalysis, environmental remediation, energy storage (supercapacitors,
Li ion batteries) as well as in sensors and biosensors (Chen et al., 2013; Hu et al., 2013; Sun et al.,
2013; Tang et al., 2014). Usually, graphene gelation is assisted by hydrophilic polymers or small
organic molecules, although rGO hydrogels without such promoters have also been reported (Liu
and Seo, 2010). The role of those promoters and their influence in the properties of the final product

Vrettos et al. The Role of Diamines in the Formation of Graphene Aerogels
FIGURE 1 | Hydrogels as prepared by hydrothermal treatment (lower), and after the addition of water (upper).
has not been studied systematically. One particular class of such
promoters, organic diamines, can act effectively as nucleophiles,
attacking the epoxy groups of GO. In that regard, they get
bonded on the surface of GO covalently, effectively bridging
(cross linking) graphene sheets. Furthermore, diamines, being
well-known reducing agents, become responsible for partial
reconstruction of the aromatic system on rGO.
In view of the aforementioned, this study describes the
preparation and characterization of a series of graphene
aerogels derived with several aliphatic and aromatic diamines
(aniline derivatives) as shown in Figure 1. Those diamines were
selected based on their solubility in water, their molecular
size and flexibility, their number of nitrogen atoms, their
basicity, and their nucleophilicity. The effectiveness of those
diamines as gelation promoters was investigated via the
morphology and other material properties of the final products
(aerogels).
EXPERIMENTAL SECTION
Materials
1,4-Phenylenediamine (Merck Millipore Schuchard), methylene
dianiline (Chem-Cruz), 3,3-diaminobenzidine (FluoroChem),
triethylenetetramine (Struers), 1,6 hexanediamine (Acros
Organincs Inc.) 1,2- phenylenediamine (Sigma Aldrich), para-
xylylenediamine (Fluka Chemicals) were used as received. Nitric
acid (65%), sulfuric acid (95–97%), potassium chlorate, and
powder graphite (purum) were acquired from Fluka.
Synthesis of Go
GO was prepared according to Staudenmaier’s method
(Staudenmaier, 1898) through oxidation of graphite. In a typical
procedure, graphite (5 g) was added to a cold mixture of 200 mL
H2SO4and 100 mL HNO3, in an ice-water bath. KClO3powder
(100 g) was added to the cold mixture in small portions under
continuous stirring. The reaction was quenched after 20 h by
pouring the mixture into distilled water. The oxidation product
was washed until the pH reached 6.0, and was dried at room
temperature.
Synthesis of Graphene Aerogels
GO (12 mg) was dispersed in alkaline water (4 mL +60 µL of
conc. aq. ammonium hydroxide) using ultrasonication for 1 h.
After addition of diamine (0.1 mmol), the mixture was heated
in a sealed bottle at 95◦C for 24 h. The resulting hydrogels were
washed several times with water to remove unreacted diamine,
and then they were lyophilized for 24 h. The density of final
aerogels was determined from their weight and their physical
dimensions. (Hu et al., 2013; Tang et al., 2014).
Materials Characterization
Thermogravimetric analysis (TGA) was carried out with a TA
Instrument Q500 Thermo Gravimetric Analyzer under ambient
air with a heating rate of 10◦C min−1up to 700◦C. Scanning
electron microscopy (SEM) images was carried out on a Zeiss
EVO-MA10, a Hitachi S-4700 field-emission microscope. FT-
IR spectra were obtained with a ATR technique on a FTS 3000
Excalibur Series Digilab spectrometer and on a Shimadzu FT-IR
8400 spectrometer equipped with a deuterated triglycine sulfate
detector. The samples were in the form of KBr pellets containing
ca. 2% w/w of the material.
XRD was conducted with a D8 Advance Bruker diffractometer
using a CuKa (lD1.5418) radiation source (40 kV, 40 mA) and a
secondary beam graphite monochromator. Diffraction patterns
were recorded in the 2-theta (2’) scale from 2◦to 80◦, in steps
of 0.02◦and with a counting time of 2 s per step. Raman spectra
were collected with a Raman system LabRam HR Evolution RM
(Horiba-Scientific) using a laser excitation line at 532 nm (laser
diode). The laser power was 1.082 mV. All Raman parameters
have carefully controlled to avoid changes in the graphene
materials. Bulk resistance was measured using a Keithley 2401
multimeter with two ITO glasses as electrodes that covered the
upper and lower surface of the aerogels. The conductivity was
estimated taking into consideration the surface of the aerogel as
shown in Table 2 and the thickness l =3 mm.
The porosity of the aerogels was estimated by the following
equation ϕ=Vpore/V where Vpore is the volume of the void-space
and Vis the volume of the bulk material and was estimated by the
dimensions of aerogel. Vpore was estimated via Vpore =V-VrGO
(VrGO =m/drGO where drGO =2 g cm3is the density of rGO and
mis the mass of the bulk material) (Chen et al., 2013).
Frontiers in Materials | www.frontiersin.org 2April 2018 | Volume 5 | Article 20

Vrettos et al. The Role of Diamines in the Formation of Graphene Aerogels
TABLE 1 | Diamines used as gelators for rGO hydrogels.
Aliphatics pKa1Solubility in water
HDA 11.9 800 g L−1
EDA 10.7 Miscible
TETA 9.9 100 g L−1
PXDA 9.2 Miscible
AROMATICS
PPD 6.2 40 g L−1
MDA 4.8 1 g L−1
OPD 4.7 <1 g L−1
DAB 3.6 <1 g L−1
TABLE 2 | Bulk resistance (R), resistivity (ρ), dimensions, and density (d) of the
rGO aerogels.
rGO Mass
(mg)
drGO (mg
cm−3)
S (m2)
(*10−4)
R (k)ρ(m) Porosity %
EDA 20 12.9 3.1 6.0 620 99.4
TETA 20 22.0 1.8 3.6 200 98.9
HDA 21 21.0 2.0 40.0 2600 99.0
PXDA 22 22.0 2.0 1.6 106 98.9
MDA 26 65.0 0.8 2.0 52 96.8
OPD 16 12.8 2.5 0.3 29 99.4
PPD 20 21.0 3.1 1.2 124 99.4
DAB 21 12.0 3.6 0.3 25 99.4
The thickness was measured to 0.5 cm.
RESULTS AND DISCUSSION
The Diamines
Diamines selected for this study are divided in two groups:
aliphatic and aromatic. The first group includes flexible
ethylenediamine (EDA), 1,6 hexanediamine (HDA),
triethylenetetramine (TETA), and a semi-flexible one, para-
xylylenediamine (PXDA). The second group includes rigid
aniline derivatives such as ortho-phenylenediamine (OPD),
para-phenylenediamine (PPD), 3,3′-diaminobenzidine (DAB),
and a semi-flexible one, 4,4′-methylene dianiline (MDA). The
aliphatic diamines are classified as strong bases and strong
nucleophiles, and they were water soluble (see Table 1). On the
other hand, aromatic diamines are weak bases and poorly soluble
in water. EDA (Kim et al., 2013) and PPD (Ma et al., 2012)
FIGURE 2 | The reaction mixture before (a) and after (b) the addition of TETA.
(c) Well shaped hexagonal aerogels from the hydrothermal treatment of TETA
with GO.
have been previously reported in the hydrothermal synthesis of
graphene aerogels.
Hydrogel Formation
During hydrothermal process, after the addition of diamine,
GO nanosheets agglomerated forming monolithic hydrogels as
shown in Figure 1. With the exception of MDA, hydrogels
had almost the same volume as the sol. Hydrogels with MDA
underwent excessive shrinkage (about 50% in linear dimensions).
All hydrogels were washed extensively with water and acetone to
remove unreacted diamines and ammonia, and they were dried
to aerogels using freeze-drying (see Experimental Section).
Introducing diamines as gelation promoters has the dual
advantage of using a single reagent both as a crosslinker of
GO sheets, and as a reducing agent (Herrera-Alonso et al.,
2007; Li et al., 2011; Ma et al., 2012; Kim et al., 2013). In that
regard, nucleophilic attack of diamines to an epoxide group
on the GO surface leads to fuctionalization of graphene with
diamines, which in part is followed by a second nucleophilic
attack, formation of an aziridine ring and elimination of OH as
water; eventually, elimination of the aziridine ring itself leaves
behind a double bond, in analogy to the mechanism suggested
by Stankovich et al. for the reduction of GO by hydrazine
(Stankovich et al., 2007).
Functionalization was evidenced by the presence of the
diamine on the surface. Partial reduction was accompanied by
enhanced conductivity and a drastic decrease of oxygen groups
in the final product. The first remarkable difference between the
aliphatic and aromatic diamines was observed directly after their
addition in the GO dispersion: addition of aliphatic diamines
resulted in immediate formation of GO nanosheet aggregates
(see Figures 2a,b). On the other hand, aniline derivatives were
dispersed slowly in the water phase, and GO aggregates were
formed much later during the hydrothermal heating.
Frontiers in Materials | www.frontiersin.org 3April 2018 | Volume 5 | Article 20

Vrettos et al. The Role of Diamines in the Formation of Graphene Aerogels
FIGURE 3 | SEM images of (a,b) rGOTETA,(c,d) rGOHDA ,(e,f) rGOEDA, and (g,h) rGOPXDA aerogels.
During hydrothermal treatment, diamines function as
crosslinkers leading to agglomeration of functionalized rGO
in well-shaped black monolithic hydrogels. The density of the
final aerogels ranged between 12 and 22 mg cm−3(except for
rGOMDA). Direct aggregation of GO nanosheets via reaction of
epoxy groups with aliphatic diamines, before reduction (which
normally lead to graphitized species) led to the formation of
well-shaped gels and aerogels that kept the dimensions of the
mold as demonstrated in Figure 2c, with a hexagonal-shaped
aerogel. Hydrothermal treatment of GO, under the same
conditions, in the absence of diamine, led to the formation of
rGO aggregates, which after removal of water remained as dense
powders.
Characterization of Graphene Aerogels
The mass of functionalized rGO aerogels ranged between 16
and 26 mg, while the porosity was estimated above 96% for all
materials (see Table 2). The microstructure of final aerogels with
aliphatic diamines consists of pores with diameter between 20
and 40 µm as revealed by SEM (see Figure 3).
Similarly, GO nanosheets with aromatic diamines were partly
reduced and functionalized leading to porous 3D monolithic
Frontiers in Materials | www.frontiersin.org 4April 2018 | Volume 5 | Article 20

Vrettos et al. The Role of Diamines in the Formation of Graphene Aerogels
FIGURE 4 | SEM images of (a,b) rGOPPD ,(c,d) rGOOPD,(e,f) rGODAB , and (g,h) rGOMDA aerogels. (i) magnified image of the crystals that are localized on
graphene surface in rGODAB and (J) rGOMDA aerogels.
Frontiers in Materials | www.frontiersin.org 5April 2018 | Volume 5 | Article 20

Vrettos et al. The Role of Diamines in the Formation of Graphene Aerogels
FIGURE 5 | FT-IR spectra of aerogels with aliphatic diamines.
FIGURE 6 | FT-IR spectra of aerogels with aromatic diamines.
aerogels as shown in Figure 4. Here, pore sizes ranged from
about 50 to 200 µm in rGOPPD , rGOOPD, rGODAB aerogels, while
rGOMDA appeared to be more condensed without visible pores.
The last two aerogels were randomly decorated with crystals of
the diamines that were formed during synthesis, due to the very
low solubility of MDA and DAB in water.
FT-IR spectra of all aerogels showed, in comparison with
the spectrum of GO, a significant decrease of the characteristic
GO bands, and appearance of new bands from the incorporated
diamines (see Figures 5,6). The spectrum of pure GO showed
a broad band at 3,400–3,200 cm−1due to the OH stretching
vibration (carboxyl, hydroxyl groups, or intercalated water), a
shoulder at 1,710 cm−1(carboxylic C=O stretch) and bands at
1,380 cm−1(carboxylic O-H deformation vibration), 1,620 cm−1
(aromatic C=C stretches) (Ma et al., 2012; Hu et al., 2013;
Verma and Dutta, 2015), 1,200 cm−1and 1,046 cm−1[usually
assigned to phenolic C-OH stretches, as well as to epoxide (or
alcoxy) stretches (Guo et al., 2009; Ma et al., 2012; Kellici et al.,
2014)]. After reduction/functionalization, the bands assigned to
oxygen groups at 1,046, 1,200, 1,710, and 3,200–3,400 cm−1were
significantly reduced in the spectra of rGO’s. Covalent grafting
of TETA on graphenic layers was indicated by the appearance
of characteristic peaks at 2,920, 2,850 cm−1(-CH2- stretch),
1,550 cm−1(N-H in plane scissoring vibration or bending),
(Tang et al., 2014) 1,440 cm−1(-CH2- bend), 1,170 cm−1and
1,100 cm−1(C-N stretch). Similar spectroscopic evidence was
recorded in other aerogels with aliphatic diamines as shown in
Figure 5, indicating analogous covalent functionalization.
Regarding the FT-IR spectra of the aerogels with aromatic
diamines a similar behavior is observed. The characteristic peaks
of the oxygen groups were remarkably reduced in comparison
with GO and several new peaks have been appeared indicating
the presence of aromatic diamines in the aerogels. rGOPPD
showed peaks at 3,445 cm−1(OH stretch), 1,624 cm−1(C=C
bending), 1,536 cm−1(bending vibration of NH, indicating –C-
NH-C- formation) (Tang et al., 2014), 1,500 cm−1(phenyl ring
vibration), 1,420 (C-N stretch), 1,100, cm−1(-CO stretch), (Lu
et al., 2014), and 873 cm−1(CH non-planar ring vibrations; see
Figure 6).
rGOMDA showed weak bands at 3,412 cm−1(NH and NH2
stretch), 1,625 cm−1(C=C bending), 1,500 cm−1(phenyl ring
vibration), 1,230, 1,160 cm−1(C-N stretch), and 822 cm−1
(CH non-planar ring vibrations). rGODAB showed peaks at
3,352 cm−1(NH and NH2stretch), 1,625 cm−1(C=C bending
or NH2scissors), 1,573 cm−1(in plane scissoring vibrations or
bending), 1,500, 1,480 cm−1(phenyl ring vibration), 1,425 cm−1
(C-N stretch), 1,140 cm−1(C-O stretch) (see Figure 6).
Raman Spectroscopy
Raman spectra of GO and rGO aerogels are shown in Figure 7.
D (A1g symmetry mode) and G (E2g mode of the sp2carbon
atoms) (Chen et al., 2013) bands in the modified with aliphatic
diamines rGO aerogels appeared as well-defined peaks at ∼1,350
and 1,580 cm−1, respectively. The same bands in the GO
precursor appeared at 1,366 and 1,600 cm−1. And the observed
shift is attributed to the recovery of the hexagonal network of the
carbon atoms with defects. The G band in some spectra appeared
clearly asymmetric due to the contribution of D’ band, while both
D and G bands in rGODAB, rGOOPD aerogels are contaminated
by signals probably due to the presence of aromatic diamines.
The characteristic ID/IGratio (IDand IGwere measured from
the peak height after baseline correction) was increased from 0.78
in pure GO to 0.86–1.19, similarly to other rGO products in the
literature (Stankovich et al., 2007; Pei and Cheng, 2012; Tang
et al., 2014; Abdolhosseinzadeh et al., 2015; Bo et al., 2015; Ji et al.,
2017). That change is usually attributed to the decrease of average
size of sp2domains, together with an increase of the number of
those domains after reduction (Stankovich et al., 2007; Pei and
Cheng, 2012; Chen et al., 2013; Feng et al., 2013). Moreover, the
2D band in all products appeared between 2,700 and 2,730 cm−1,
while it was sharper and more intense in rGOTETA and rGOHDA
Frontiers in Materials | www.frontiersin.org 6April 2018 | Volume 5 | Article 20

Vrettos et al. The Role of Diamines in the Formation of Graphene Aerogels
FIGURE 7 | Raman spectra of aerogels with (A) aliphatic diamines (1:rGOEDA ,
2:rGOHDA, 3:rGOTETA, 4:rGOPXDA ), (B) aromatic diamines (1:rGOPPD ,
2:rGOMDA, 3:rGODAB , 4:rGOOPD).
indicating a higher degree of aromatization in those products,
in accordance with the results of TG analysis as shown below.
Similarly, in rGO aerogels modified with aromatic diamines,
the ID/IGratio ranged between 0.74 and 1. The 2D band
here appeared near 2,730 cm−1(except in rGOOPD where it
appeared at 2,692 cm−1), while it was sharper and more intense
in rGOPPD and rGOOPD indicating an analogous higher degree
of aromatization of those products.
TG Analysis
Thermogravimetric analysis of GO showed a 5% mass loss
up to 100◦C due to removal of water entrapped between GO
nanosheets and a 35% mass loss closer to 200◦C, which is
attributed to pyrolytic removal of oxygen-containing groups
(epoxides, carboxyl, and hydroxyl groups). The remaining
carbon material was removed by oxidation completely by 500◦C
FIGURE 8 | TG analysis of aerogels with aliphatic diamines and GO.
FIGURE 9 | TG analysis of aerogels with aromatic diamines and GO.
(see Figure 8, line 4) (Kuila et al., 2008; Abdolhosseinzadeh et al.,
2015).
Thermographs of rGO aerogels with aliphatic diamines,
as shown in Figure 8, showed a mass loss between 30 and
60%, over 600◦C, attributed to carbon oxidation, indicating
enhanced thermal stability of rGO nanosheets, in comparison
with GO, due to the partial aromatization during hydrogel
formation. Furthermore, rGOEDA showed a mass loss of 45%
between 100 and 200◦C, while the other rGO loose less mass,
gradually between 100 and 550◦C. This is possibly attributed
to the removal of diamine and oxygen containing groups.
HDA appeared to be the most effective in reduction of GO,
since rGOHDA was the most thermally stable (a property that
correlates with aromaticity), in accordance with its superior
basicity/nucleophilicity (see Table 1, pKavalues). At the opposite
Frontiers in Materials | www.frontiersin.org 7April 2018 | Volume 5 | Article 20

Vrettos et al. The Role of Diamines in the Formation of Graphene Aerogels
FIGURE 10 | XRD patterns of rGO aerogels and GO.
PXDA, the weakest base/nucleophile, was the less effective in
reduction of GO, producing less thermally stable rGOPXDA (He
et al., 2011).
The results from TGA of rGO aerogels with aromatic
diamines are shown in Figure 9. rGOPPD, showed a 35% mass
loss below 100◦C (water removal, which appeared in most
aerogels) and almost all the rest material (PPD and carbon mass)
was removed between 500 and 700◦C. rGOOPD and rGOMDA
showed similar behavior and lost about 70% between 500
and 700◦C showing slightly more effective reduction. rGODAB
nanosheets were the most thermally stable and consequently
most aromatic material since a 60% was removed above
700◦C. Comparing thermal stabilities of the four aerogel’s
nanosheets with aromatic diamines, it is observed that DAB
was the best reducing agent, although it was the weakest
base. Then, OPD and MDA followed and finally PPD, the
strongest base (according to the pKa values), was the less
effective reducing agent. That behavior could be attributed,
either to the excess amino groups of DAB, or the flexibility
of MDA due to central aliphatic carbon that could affect the
reduction mechanism, e.g., by facilitating aziridine formation
or the elimination that follows nucleophilic addition of the
diamine.
Figure 10 shows the XRD patterns of GO, and rGO aerogels.
GO showed an intense peak at 2θ=11.4◦, which corresponds
to an interlayer distance of 0.77 nm, due to absorbed water
molecules between graphene layers and the existence of oxygen
functional groups (Ma et al., 2012; Bo et al., 2015). The
XRD patterns of rGO aerogels, showed absence of any intense
characteristic peak, which is an indication of the effective
exfoliation of rGO nanosheets. rGOOPD and rGOPXDA aerogels
showed a weak broad peak at 2θ=26.7◦and 26.4◦respectively,
which attributed to graphitic structure (d =0.34 nm). The other
aerogels showed very weak broad graphite (002) peaks with
2θ=21−24◦, which corresponds to an interlayer spacing of
0.43–0.36 nm and is due to disordered stacking of the reduced
graphene sheets (Bourlinos et al., 2003; Chen et al., 2013; Fan
et al., 2013; Hu et al., 2013; Lan et al., 2016).
Electrical Conductivity
It is known that rGO aerogels are electrically conductive, which
is a property that is recovered only after reduction and partial
aromatization of GO (Ma et al., 2012; Nardecchia et al., 2013; Li
et al., 2014; Jun et al., 2015). However, conductivity is not always
the same and depends basically on the degree of reduction and
restoration of the aromatic system as well as the interconnections
between the rGO nanosheets.
To have a representative image of the electrical conductivity
of aerogels, bulk electric resistance was measured instead of
sheet resistance, which is preferable mainly in films, thus
avoiding problems with the roughness and the porosity of
the aerogel surfaces. Bulk resistance and resistivity of rGO
aerogels are shown in Table 2. Although the resistivity values
vary, there is a clear trend of aromatic diamines to reduce
aerogel resistivity more efficiently in comparison with aliphatic
diamines (Ma et al., 2012). Considering that electrical resistance
of a graphene superstructure is located in the interconnections
between graphene nanosheets, the reduced resistivity of the
doped with aromatic diamines aerogels could be attributed to a
positive role of the aromatic molecules as conductive pathways in
Frontiers in Materials | www.frontiersin.org 8April 2018 | Volume 5 | Article 20

Vrettos et al. The Role of Diamines in the Formation of Graphene Aerogels
FIGURE 11 | The compression of (a) rGOOPD (b) rGOHDA (c) rGOTETA aerogel cylinders after the placement of a 5 0 g standard precision weight.
the interconnections. Furthermore, aromatic molecules could be
placed as a bridge connecting graphene nanosheets, multiply the
number of interconnections and thus decreasing the resistivity.
In contrast to their ultralight weight, graphene aerogels
showed an outstanding mechanical strength as expressed by their
ability to support loads about 2,500 times heavier than their own
weight, without signs of deformation. As shown in Figure 11a,
the placement of a 50 g standard weight on the top of rGOOPD
aerogel cylinder, resulted in a compression of about 25%. After
the removal of the weight, rGOOPD aerogel cylinder recovered the
initial thickness. Similar elastic compression was observed with
all aerogels with aromatic diamines.
The aerogels with aliphatic diamines, under the same
conditions, showed a lower—about 11%—but inelastic
Frontiers in Materials | www.frontiersin.org 9April 2018 | Volume 5 | Article 20

Vrettos et al. The Role of Diamines in the Formation of Graphene Aerogels
compression, as shown in Figure 11b and rGOHDA. The
other aerogels with aliphatic diamines showed similar behavior,
with the exception of rGOTETA, which was compressed, also
inelastic, about 50%, under the same conditions (see Figure 11c).
CONCLUSION
GO was hydrothermally reduced and subsequently
functionalized with aliphatic and aromatic diamines, forming
well-defined 3D monolithic aerogels, with remarkable
mechanical strength. This study has shown that aromatic
diamines improved the conductivity of aerogels, while porosity
and density were not seriously affected by the diamine structure
or solubility. Amongst aliphatic diamines, the most nucleophilic
one (HDA) was also the most effective in reducing GO, while
among aromatic diamines, a similar correlation was not observed
since DAB, the less nucleophilic one, was the most effective in
reducing GO. Finally, it is concluded that aliphatic and aromatic
diamines are excellent gelation promoters forming stable
well-defined aerogel monoliths.
AUTHOR CONTRIBUTIONS
All authors listed have made a substantial, direct and intellectual
contribution to the work, and approved it for publication.
ACKNOWLEDGMENTS
The authors thank Dr. N. Leventis for his critical reading of the
manuscript and his suggestions for improvement.
REFERENCES
Abdolhosseinzadeh, S., Asgharzadeh, H., and Kim, H. S. (2015). Fast and
fully-scalable synthesis of reduced graphene oxide. Sci. Rep. 5:10160.
doi: 10.1038/srep10160
Bo, Z., Shuai, X., Mao, S., Yang, H., Qian, J., Chen, J., et al. (2015). Green
preparation of reduced graphene oxide for sensing and energy storage
applications. Sci. Rep. 4:4684. doi: 10.1038/srep04684
Bourlinos, A. B., Gournis, D., Petridis, D., Szabo, T., Szeri, A., and Dekany,
I. (2003). Graphite oxide: chemical reduction to graphite and surface
modification with primary aliphatic amines and amino acids. Langmuir
19:6050. doi: 10.1021/la026525h
Brodie, B. C. (1859). On the atomic weight of Graphite. Philos. Trans. R. Soc. Lond.
149, 249–259. doi: 10.1098/rstl.1859.0013
Chen, D., Feng, H., and Li, J. (2012). Graphene oxide: preparation,
functionalization, and electrochemical applications. Chem. Rev. 112,
6027–6053. doi: 10.1021/cr300115g
Chen, M., Zhang, C., Li, X., Zhang, L., Ma, Y., Zhang, L., et al. (2013).
A one-step method for reduction and self-assembling of graphene oxide
into reduced graphene oxide aerogels. J. Mater. Chem. A 1, 2869–2877.
doi: 10.1039/c2ta00820c
Chua, C. K., and Pumera, M. (2014). Chemical reduction of graphene
oxide: a synthetic chemistry viewpoint. Chem. Soc. Rev. 43, 291–312.
doi: 10.1039/C3CS60303B
Compton, O. C., and Nguyen, S. T. (2010). Graphene oxide, highly reduced
graphene oxide, and graphene: versatile building blocks for carbon-based
materials. Small 6, 711–723. doi: 10.1002/smll.200901934
Fan, Z., Tng, D. Z., Nguyen, S. T., Feng, J., Lin, C., Xiao, P., et al. (2013).
Morphology effects on electrical and thermal properties of binder less graphene
aerogels. Chem. Phys. Lett. 561–562, 92–96. doi: 10.1016/j.cplett.2013.01.033
Feng, H., Cheng, R., Zhao, X., Duan, X., and Li, J. (2013). A low-temperature
method to produce highly reduced graphene oxide. Nat. Commun. 4,
1537–1539. doi: 10.1038/ncomms2555
Guo, H. L., Wang, X. F., Qian, Q. Y., Wang, F. B., and Xia, X. H. (2009). A Green
approach to the synthesis of graphene nanosheets. ACS Nano 3, 2653–2659.
doi: 10.1021/nn900227d
He, G., Chen, H., Zhu, J., Bei, F., Sun, X., and Wang, X. (2011). Synthesis and
characterization of graphene paper with controllable properties via chemical
reduction. J. Mater. Chem. 21, 14631–14638. doi: 10.1039/c1jm12393a
He, H., Klinowski, J., Forster, M., and Lerf, A. (1998). A new
structural model for graphite oxide. Chem. Phys. Lett. 287, 53–56.
doi: 10.1016/S0009-2614(98)00144-4
Herrera-Alonso, M., Abdala, A. A., McAllister, M. J., Aksay, I. A., and
Prud’homme, R. K. (2007). Intercalation and stitching of graphite oxide with
diaminoalkanes, Langmuir 23, 10644–10649. doi: 10.1021/la0633839
Hofmann, U., and Konig, E. (1937). Untersuchungen über Graphitoxyd. Z. Anorg.
Allg. Chem. 234, 311–336. doi: 10.1002/zaac.19372340405
Hontoria-Lucas, C., López-Peinado, A. J., López-González, J. D., Rojas-Cervantes,
M. L., and Martín-Aranda, R. M. (1995). Study of oxygen-containing groups in
a series of graphite oxides: physical and chemical characterization. Carbon 33,
1585–1592. doi: 10.1016/0008-6223(95)00120-3
Hu, H., Zhao, Z., Wan, W., Gogotsi, Y., and Qiu, J. (2013). Ultralight
and highly compressible graphene aerogels. Adv. Mater. 25, 2219–2223.
doi: 10.1002/adma.201204530
Hummers, W. S., and Offeman, R. E. (1958). Preparation of graphitic oxide. J. Am.
Chem. Soc. 80:1339. doi: 10.1021/ja01539a017
Ji, X., Song, Y., Han, J., Ge, L., Zhao, X., Xu, C., et al. (2017). Preparation
of a stable aqueous suspension of reduced graphene oxide by a green
method for applications in biomaterials. J. Colliod Interface Sci. 497, 317–324.
doi: 10.1016/j.jcis.2016.09.049
Jun, Y. S., Sy, S., Ahn, W., Zarrin, H., Rasen, L., Tjandra, R., et al. (2015). Highly
conductive interconnected graphene foam based polymer composite. Carbon
95, 653–658. doi: 10.1016/j.carbon.2015.08.079
Kellici, S., Acord, J., Ball, J., Reehal, H. S., Morgan, D., and Saha, B. (2014). A
single rapid route for the synthesis of reduced graphene oxide with antibacterial
activities. RSC Adv. 4, 14858–14861. doi: 10.1039/C3RA47573E
Kim, F., Cote, L. J., and Huang, J. (2010). Graphene oxide: surface
activity and two-dimensional assembly. Adv. Mater. 22, 1954–1958.
doi: 10.1002/adma.200903932
Kim, N. H., Kuila, T., and Lee, J. H. (2013). Simultaneous reduction,
functionalization and stitching of graphene oxide with ethylenediamine
for composites application. J. Mater. Chem. A 1, 1349–1358.
doi: 10.1039/C2TA00853J
Kuila, T., Bose, S., Khanra, P., Mishra, A. K., Kim, N. H., and Lee, J. H. (2008). A
green approach for the reduction of graphene oxide by wild carrot root. Carbon
50, 914–921. doi: 10.1016/j.carbon.2011.09.053
Lan, Y., Jin, B., Deng, J., Luo, Y. (2016). Graphene/nickel aerogel: an effective
catalyst for the thermal decomposition of ammonium perchlorate. RSC Adv.
6, 82112–82117. doi: 10.1039/C6RA15661D
Lerf, A., He, H., Forster, M., and Klinowski, J. (1998). Structure of graphite
oxide revisited. J. Phys. Chem. B 102, 4477–4482. doi: 10.1021/jp97
31821
Li, C., and Shi, G. (2012). Three-dimensional graphene architectures. Nanoscale
4:5549. doi: 10.1039/c2nr31467c
Li, C., and Shi, G. (2014). Functional gels based on chemically modified graphenes.
Adv. Mater. 26, 3992–4012. doi: 10.1002/adma.201306104
Li, W., Tang, X. Z., Bin Zhang, H., Jiang, Z. G., Yu, Z. Z., Du, X. S., et al. (2011).
Simultaneous surface functionalization and reduction of graphene oxide with
octadecylamine for electrically conductive polystyrene composites. Carbon 49,
4724–4730. doi: 10.1016/j.carbon.2011.06.077
Li, Y., Samad, Y. A., Polychronopoulou, K., Alhassan, S. M., and Liao,
K. (2014). Highly electrically conductive nanocomposites based on
polymer-infused graphene sponges. Sci. Rep. 4:4652. doi: 10.1038/srep
04652
Frontiers in Materials | www.frontiersin.org 10 April 2018 | Volume 5 | Article 20

Vrettos et al. The Role of Diamines in the Formation of Graphene Aerogels
Liu, F., and Seo, T. S. (2010). A controllable self-assembly method for large-scale
synthesis of graphene sponges and free-standing graphene films. Adv. Funct.
Mater. 20, 1930–1936, doi: 10.1002/adfm.201000287
Lu, Y., Huang, Y., Zhang, F., Zhang, L., Yang, X., Zhang, T., et al. (2014).
Functionalized graphene oxide based on p-phenylenediamine as spacers and
nitrogen dopants for high performance supercapacitors. Chin. Sci. Bull. 59,
1809–1815. doi: 10.1007/s11434-014-0297-3
Ma, H. L., Zhang, H. B., Hu, Q. H., Li, W. J., Jiang, Z. G., Yu, Z. Z., et al. (2012).
Functionalization and reduction of graphene oxide with p-phenylene diamine
for electrically conductive and thermally stable polystyrene composites. ACS
Appl. Mater. Interfaces 4, 1948–1953. doi: 10.1021/am201654b
Mao, S., Pu, H., and Chen, J. (2012). Graphene oxide and its reduction: modeling
and experimental progress. RSC Adv. 2, 2643–2662. doi: 10.1039/c2ra00663d
Nardecchia, S., Carriazo, D., Ferrer, M. L., Gutierrez, M. C., and del Monte, F.
(2013). Three dimensional macroporous architectures and aerogels built of
carbon nanotubes and/or graphene: synthesis and applications. Chem. Soc. Rev.
42, 794–830. doi: 10.1039/C2CS35353A
Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos, S. V.,
et al. (2004). Electric field effect in atomically thin carbon films. Science 306:666.
doi: 10.1126/science.1102896
Park, S., and Ruoff, R. S. (2009). Chemical methods for the production of
graphenes. Nat. Nanotechnol. 4, 217–224. doi: 10.1038/nnano.2009.58
Pei, S., and Cheng, H. M. (2012). The reduction of graphene oxide. Carbon 50,
3210–3228. doi: 10.1016/j.carbon.2011.11.010
Stankovich, S., Dikin, D. A., Piner, R. D., Kohlhaas, K. A., Kleinhammes,
A., Jia, Y., et al. (2007). Synthesis of graphene-based nanosheets via
chemical reduction of exfoliated graphite oxide. Carbon 45, 1558–1565.
doi: 10.1016/j.carbon.2007.02.034
Staudenmaier, L. (1898). Verfahren zur darstellung der graphitsaure. Ber. Dtsch.
Chem. Ges. 31, 1481 −1487. doi: 10.1002/cber.18980310237
Sun, H., Xu, Z., and Gao, C. (2013). Multifunctional, ultra-flyweight,
synergistically assembled carbon aerogels. Adv. Mater. 25, 2554–2560.
doi: 10.1002/adma.201204576
Tang, G., Jiang, Z. G., Li, X., Zhang, H. B., Dasari, A., and Yu, Z.
Z. (2014). Three-dimensional graphene aerogels and their electrically
conductive composites. Carbon 77, 592–599. doi: 10.1016/j.carbon.2014.
05.063
Verma, S., and Dutta, R. K. (2015). A facile method of synthesizing
ammonia modified graphene oxide for efficient removal of uranyl ions
from aqueous medium. RSC Adv. 5, 77192–77203. doi: 10.1039/C5RA
10555B
Zhang, L., Chen, G., Hedhili, M. N., Zhanga, H., and Wang, P. (2012).
Three-dimensional assemblies of graphene prepared by a novel chemical
reduction-induced self-assembly method Nanoscale 4, 7038–7045.
doi: 10.1039/C2NR32157B
Zhu, Y., Murali, S., Cai, W., Li, X., Suk, J. W., Potts, J. R., et al. (2012). Graphene
and graphene oxide: synthesis, properties, and applications. Adv. Mater. 22,
3906–3924. doi: 10.1002/adma.201001068
Conflict of Interest Statement: The authors declare that the research was
conducted in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Copyright © 2018 Vrettos, Karouta, Loginos, Donthula, Gournis and Georgakilas.
This is an open-access article distributed under the terms of the Creative Commons
Attribution License (CC BY). The use, distribution or reproduction in other forums
is permitted, provided the original author(s) and the copyright owner are credited
and that the original publication in this journal is cited, in accordance with accepted
academic practice. No use, distribution or reproduction is permitted which does not
comply with these terms.
Frontiers in Materials | www.frontiersin.org 11 April 2018 | Volume 5 | Article 20