Biobased Janus molecule for the facile preparation of water solutions of few layer graphene sheets

Article abstract of DOI:10.1039/c5ra11387c

A biobased Janus molecule was used to prepare water solutions of nano-stacks made by few layer graphene. The Janus molecule was 2-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-propanediol (serinol pyrrole, SP), a serinol derivative obtained through the neat reaction of 2-amino-1,3-propandiol with 2,5-hexanedione, with atom efficiency of about 85%. SP contains the pyrrole ring, suitable for the interaction with carbon allotropes and hydroxy groups, that can easily interact with polar surroundings. Adducts, with about 14% by mass of SP, were prepared by reacting SP with a high surface area nano-sized graphite (HSAG), with about 35 graphene layers stacked in crystalline domains. Green methods were adopted, such as ball milling (HSAG-SP-M) and heating. Infrared spectra revealed peaks due to both HSAG and SP and additional peaks that could be attributed to the adduct. Both thermal and mechanical reactions left substantially unaltered the order in the graphitic layers and the interlayer distance, as shown by X-ray diffraction patterns. The relative intensity of the G and D band in the Raman spectrum was not modified by the thermal reaction, whereas enhancement of the D peak was observed after ball milling. Stable water solutions of HSAG-SP-M were prepared in a concentration range from 0.1 to 1 mg mL^-1. Centrifugation allowed isolation of adducts with few stacked graphene layers, as revealed by high resolution transmission electron microscopy. An image of the adduct showed an organic layer tightly adhered to the carbon surface. SP appears a suitable molecule for the easy functionalization of carbon allotropes, such as nano-stacks of graphene layers, without substantially affecting the bulk crystalline organization and promoting the separation of the aggregates into stacks containing a low/very low number of graphene layers.

Full text of DOI:10.1039/c5ra11387c

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PAPER
Biobased Janus molecule for the facile preparation
of water solutions of few layer graphene sheets
Cite this: RSC Adv., 2015, 5, 81142
a
a
a
b
Maurizio Galimberti,* Vincenzina Barbera, Silvia Guerra, Lucia Conzatti,
a
a
a
Chiara Castiglioni, Luigi Brambilla and Andrea Serafini
A biobased Janus molecule was used to prepare water solutions of nano-stacks made by few layer graphene.
The Janus molecule was 2-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-propanediol (serinol pyrrole, SP), a serinol
derivative obtained through the neat reaction of 2-amino-1,3-propandiol with 2,5-hexanedione, with
atom efficiency of about 85%. SP contains the pyrrole ring, suitable for the interaction with carbon
allotropes and hydroxy groups, that can easily interact with polar surroundings. Adducts, with about 14%
by mass of SP, were prepared by reacting SP with a high surface area nano-sized graphite (HSAG), with
about 35 graphene layers stacked in crystalline domains. Green methods were adopted, such as ball
milling (HSAG–SP-M) and heating. Infrared spectra revealed peaks due to both HSAG and SP and
additional peaks that could be attributed to the adduct. Both thermal and mechanical reactions left
substantially unaltered the order in the graphitic layers and the interlayer distance, as shown by X-ray
diffraction patterns. The relative intensity of the G and D band in the Raman spectrum was not modified
by the thermal reaction, whereas enhancement of the D peak was observed after ball milling. Stable water
ꢀ1
solutions of HSAG–SP-M were prepared in a concentration range from 0.1 to 1 mg mL . Centrifugation
allowed isolation of adducts with few stacked graphene layers, as revealed by high resolution transmission
electron microscopy. An image of the adduct showed an organic layer tightly adhered to the carbon
surface. SP appears a suitable molecule for the easy functionalization of carbon allotropes, such as nano-
stacks of graphene layers, without substantially affecting the bulk crystalline organization and promoting
the separation of the aggregates into stacks containing a low/very low number of graphene layers.
Received 15th June 2015
Accepted 10th September 2015
DOI: 10.1039/c5ra11387c
www.rsc.org/advances
39–44
45–49
oxide (GO)
followed by thermal or chemical reduction.
1
. Introduction
Such method gives also the chance of preparing GO. The precise
1
41
Since the discovery of fullerene, nano-sized carbon allotropes structure of GO is still unknown. However, the adopted oxida-
have been the subject of huge amounts of research activity. tion methods lead to extensive modication of the graphene
2
,3
4,5
40
Carbon nanotubes (CNTs), both single and multi-walled,
layers. It was reported that hydroxy and epoxide groups are
6–9
graphene (G), or graphitic nanollers made by few layers of mainly present on the surface of basal planes and carbonyl and
graphene
are studied as they dramatically improve proper- carboxyl groups are bounded to the edges. GO can be dispersed
ties such as electrical and thermal conductivity, mechanical in polar media and can be the platform for further functionali-
10–13
50–62
reinforcement, thermal and ame resistance, of both thermo- zations.
The oxidation–reduction method is however charac-
14
plastic and elastomeric matrices. In recent years, attention has terized by several drawbacks. The oxidation requires strong acids,
been in particular focussed on graphene, in consideration of its explosive oxidation agents, harsh reaction conditions and
3
5–38
huge surface area, exceptional mechanical strength and high long reaction times.
Thermal reduction occurs at high
temperature and under controlled conditions, efficient chemical
Single or few layers of graphene can be obtained through reducing agents could be toxic, hazardous and expensive, such
6–9,15–18
carrier mobility.
6,19
63,64
micromechanical cleavage,
vapour deposition of graphene lms,
tion.
epitaxial growth and chemical as, for instance, those based on hydrazine.
liquid phase exfolia- increasingly acknowledged that the complete restoration of sp
Much interest has been directed, particularly in view of hybridization of graphene carbons can be hardly achieved.
Besides that, it is
2
2
0–27
28–33
34–38
large scale production, to the oxidation of graphite
to graphite
Aim of this work was the introduction, on graphene layers, of
oxygen containing functional groups, in particular of hydroxy
2
a
groups, without substantially affecting the sp hybridization of
Politecnico di Milano, Department of Chemistry, Materials and Chemical Engineering
“
G. Natta”, Via Mancinelli 7, 20131 Milano, Italy. E-mail: maurizio.galimberti@ the carbon atoms, thus keeping structural order inside gra-
polimi.it
phene sheets. The achievement of this goal could allow the easy
preparation of solutions in polar media (e.g. in water) of
b
National Council of Research, Institute for the Study of Macromolecules, Via De
Marini 6, 16149 Genova, Italy
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adducts isolated through centrifugation of supernatant solu-
tions were analyzed through high resolution TEM (HR-TEM).
Electrical conductivity of HSAG–SP adducts was investigated
on sheets deposited on paper.
2
. Experimental
Fig. 1 2-(2,5-Dimethyl-1H-pyrrol-1-yl)-1,3-propanediol (SP).
2.1. Materials
Reagents and solvents commercially available were purchased
and used without further purication: 2,5-hexandione (Merck-
Schuchardt), 2-amino-1,3-propanediol (kindly provided by
Bracco), acetone (Aldrich), high surface area graphite (HSAG)
with Synthetic Graphite 8427® as trademark was purchased from
Asbury Graphite Mills Inc., with a minimum carbon mass % of
graphene layers with almost unchanged thermal and electrical
conductivity, without the need of further chemical treatments.
The peculiar objective of the work was to perform the modi-
cation of graphene layers through simple reactions inspired to
the principles of green chemistry, ideally by using biobased
chemicals.
2
ꢀ1
9
9.5, a surface area of 330 m g , the number of stacked layers in
72
the orthogonal direction to graphitic planes is about 35.
To pursue these objectives, 2-amino-1,3-propanediol, known
as serinol (S), was identied as the starting building block.
Serinol can be directly obtained from renewable sources and is
commercially available, arising from the reaction of easily
available chemicals such as glycerol. The chemoselectivity of its
2
.2. Synthesis of 2-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-
65
propanediol (SP)
2-(2,5-Dimethyl-1H-pyrrol-1-yl)-1,3-propanediol was prepared
67
66,67
following the reaction scheme shown in Scheme 1.
amino and hydroxyl groups was exploited
Knorr reaction
2
to perform Paal–
of S with 2,5-hexanedione (HD) obtaining
-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-propanediol, shown in Fig. 1
68,69
A mixture of hexan-2,5-dione (41.4 g; 0.36 mol) and serinol
30.0 g; 0.33 mol) was poured into a 100 mL round bottomed
(

ask equipped with magnetic stirrer. The mixture was then
stirred, at room temperature, for 6 h. The resulting compound
a,6a-dimethyl-hexahydro-1,4-dioxa-6b-azacyclopenta[cd]penta-
and hereinaer indicated as serinol pyrrole (SP).
The reaction was performed in the absence of solvents and
catalysts, with yield of about 95% and atomic efficiency of about
4
1
lene (HHP) was characterized through H NMR and the yield
was estimated to be 99%. H NMR (400 MHz, DMSO-d6, d in
ppm): 1.28 (s, 6H); 1.77 (m, 2H); 1.93 (m, 2H); 3.60 (m, 4H);
3
85%. SP contains two hydroxyl groups, that promote the
1
compatibility with polar surroundings such as water, eco-friendly
solvents as alcohols, ethers and esters and polar polymer
matrices. SP contains as well as pyrrole ring, that could favour p–
p interaction with the aromatic rings of carbon allotropes. SP
appears as a Janus Bifrons molecule, suitable for the preparation
67,68
.94 (q, 1H).
The product mixture obtained from the synthesis of HHP
ꢁ
was kept under vacuum for 2 h and then heated to 180 C for
50 min. Aer distillation under reduced pressure at 130 C and
ꢁ
66,67,70,71
of solutions of graphitic nanollers in polar media.
0.1 mbar, SP was isolated as yellow oil with 96% yield (see Fig. 1
Reaction was performed between SP and a nano-sized
and Scheme 1). The global yield of the two step synthesis was
therefore about 95%. H NMR (400 MHz, DMSO-d6, d in ppm):
graphite with very high surface area (HSAG), higher than
1
2
ꢀ1
3
00 m g , commercially available and obtained through
2
.16 (s, 6H, –CH
CH OH); 3.76 (m, 2H, CH
.73 (t, 2H, CH OH); 5.55 (s, 2H, C-3,4 of pyrrole moiety).
3
at C-2,5 of pyrrole moiety); 3.63 (m, 2H,
milling. HSAG was characterized by a relatively low number of
layers stacked in the direction orthogonal to graphene planes
2
2
OH); 4.10 (quintet, 1H, at C-3 of diol);
6
7,68
4
2
(about 30) and by a high shape anisotropy, dened as the ratio
between the crystallites size in directions orthogonal and
72
parallel to the layers. The interaction of SP and HSAG was 2.3. Synthesis of HSAG–SP adducts
promoted by mechanical and thermal energy. Adducts were
HSAG–SP adducts were prepared by reacting HSAG and SP
with the help of either mechanical (HSAG–SP-M) or thermal
studied by means of elemental, thermogravimetric (TGA),
infrared (IR), wide angle X-ray diffraction (WAXD), transmission
electron microscopy (TEM) analysis and Raman spectroscopy.
Dispersions of HSAG–SP adducts in water were studied through
dynamic light scattering (DLS) and UV spectroscopy. HSAG–SP
(HSAG–SP-T) energy.
2.3.1. Synthesis of HSAG–SP-M adduct. In a 100 mL round
bottomed ask was put in sequence graphite (5 g, 66 mmol) and
acetone (15 mL). The suspension was sonicated for 15 min, using
Scheme 1 Reaction pathway for the preparation of the serinol derivatives HHP and SP.
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a 2 L ultrasonic bath. Aer this time, a solution of SP (5.87 g, law. The Dhk‘ correlation length, in the direction perpendicular
6 mmol) in acetone (15 mL) was added. The resulting suspen- to the hk‘ crystal graphitic planes, was determined applying the
6
sion was sonicated for 15 min. The solvent was removed under Scherrer equation.
reduced pressure. The black powder of graphite/SP was treated
D
hk‘ ¼ Kl/(bhk‘ cos qhk‘
)
(1)
using a planetary ball mill S100 from Retsch, having the grinding
jar moving in a horizontal plane, with a volume of 0.3 L. The
grinding jar was loaded with 6 ceramic balls having a diameter of
where: K is the Scherrer constant, l is the wavelength of the
irradiating beam (1.5419 A, Cu-Ka), b
height, and qhk‘ is the diffraction angle. The instrumental
broadening, b, was determined by obtaining a WAXD pattern of
a standard silicon powder 325 mesh (99%), under the same
˚
is the width at half
hk‘
20 mm. 10.87 g of graphite/SP adduct were put into the jar, that
was allowed to rotate at 300 rpm, at room temperature, for 6 h.
Aer this time, the mixture was placed in a B u¨ chner funnel with
a sintered glass disc. It was repeatedly washed with distilled
7
1
experimental conditions. The width at half height, bhk‘ ¼ (Bhk‘
ꢀ
water (6 ꢂ 100 mL) obtaining 9.8 g of black powder.
ꢁ
b) was corrected, for each observed reection with bhk‘ < 1 , by
2
.3.2. Synthesis of HSAG–SP-T adduct. In a 100 mL round
subtracting the instrumental broadening of the closest silicon
reection from the experimental width at half height, Bhk‘
bottom ask was put in sequence graphite (5 g, 66 mmol) and
acetone (15 mL). The suspension was sonicated for 15 min,
using a 2 L ultrasonic bath. Aer this time, a solution of SP
.
2
.5. Water solutions of HSAG–SP adducts
(
1.116 g, 6.6 mmol) in acetone (5 mL) was added. The resulting
suspension was sonicated for 15 min. The solvent was removed
under reduced pressure. The black powder of graphite/SP different concentrations (1 mg mL ; 0.5 mg mL ; 0.3
2.5.1. Preparation. Water solutions of HSAG–SP adducts at
ꢀ
1
ꢀ1
ꢀ
1
ꢀ1
(
6.10 g) was poured into a 100 mL round bottomed ask mg mL ; 0.1 mg mL ) were prepared. Each solution was
ꢁ
equipped with magnetic stirrer and was heated at 130 C for 6 h. sonicated for 10 min using an ultrasonic bath (260 W) and
Aer this time, the mixture was placed in a B u¨ chner funnel with subsequently UV-vis absorption was measured. The solution (10
a sintered glass disc. It was repeatedly washed with distilled mL) of each sample was put in a Falcon™ 15 mL Conical
water (3 ꢂ 20 mL), obtaining 5.86 g of black powder.
71
Centrifuge Tubes and centrifuged at: 2000 rpm for 10 minutes
and at 9000 rpm for 5, 30, 60 and 90 minutes. Supernatants
obtained aer each centrifugation processes were removed and
analyzed. UV-vis absorptions and DLS analysis were measured
2.4. Characterization of HSAG–SP adducts
2
.4.1. Thermogravimetric analysis. TGA tests under owing immediately aer each centrifugation and aer 1 week stored.
ꢀ
1
N
2
(60 mL min ) were performed with a Mettler TGA SDTA/851
2.5.2. UV-vis spectroscopy. The suspensions of adduct (3
instrument according to the standard method ISO9924-1. mL) were placed by pipette Pasteur, in quartz cuvettes of 1 cm
ꢁ
ꢁ
ꢁ
Samples (10 mg) were heated from 30 to 300 C at 10
C
optical path (volume 1 or 3 mL) and analyzed by a spectro-
ꢀ
1
ꢁ
min , kept at 300 C for 10 min, and then heated up to 550 C photometer Hawlett Packard 8452A Diode Array Spectropho-
at 20 C min . Aer being maintained at 550 C for 15 min, tometer. Resets the instrument with pure solvent and has one
they were further heated up to 900 C and kept at 900 C for UV spectrum from 200–340 nm. It was recorded a white the
ꢁ
ꢀ1
ꢁ
ꢁ
ꢁ
ꢀ1
3
0 min under owing air (60 mL min ).
solvent employed. The UV-visible spectrum reported intensity
2.4.2. Elemental analysis. Elemental analysis was per- the absorption as a function of the wavelength of the radiation
formed with a Thermo FlashEA 1112 Series CHNS-O analyzer, between 200 and 750 nm.
ꢁ
aer pretreating samples in an oven at 100 C for 12 h.
2.5.3. Dynamic light scattering (DLS). The nanoparticle
2
.4.3. FT-IR. FT-IR spectra were recorded between 450 and size in HSAG–SP solutions was analyzed with a Zetasizer
ꢀ1
4
000 cm
by using a Perkin-Elmer FT-IR Spectrum One Dynamic Light Scattering system (Malvern Instrument Ltd) at
equipped with Universal ATR Sampling Accessory with dia- room temperature using a l ¼ 632.8 nm He–Ne laser.
mond crystal. Each centrifuged solution, prepared using a concentration of
.4.4. Raman spectroscopy. Raman spectra of powder 1 mg mL , was analyzed by DLS: 3 mL of solution were placed
ꢀ
1
2
samples deposited on a glass slide were recorded by using a using a pipette Pasteur, in a quartz cuvettes of 1 cm optical path
Horiba Jobin Yvon Labram HR800 dispersive Raman spec- and analyzed immediately aer centrifugation and aer 10, 60
trometer equipped with Olympus BX41 microscope and a 50ꢂ and 100 min of storage.
objective. The excitation line at 632.8 nm of a He/Ne laser was
kept at 0.5 mW in order to prevent possible samples degrada- (1 mg mL
2.5.4. Transmission electron microscopy. Water solutions
ꢀ1
)
of adducts were centrifuged using an
tion. The spectra were obtained as the average of four acquisi- ALC-Centrifugette 4206. TEM analysis of supernatants was per-
tions (scan time: 30 s for each acquisition) with a spectral formed with a Zeiss EM900 microscope operating at an acceler-
ꢀ1
resolution of 2 cm
.4.5. Wide angle X-ray diffraction. Wide-angle X-ray of the sample were deposited on a carbon lm supported on a
diffraction (WAXD) patterns were obtained in reection, with standard Cu grid and air-dried for several hours before analysis.
.
ating voltage of 80 kV. Few drops of acetone diluted suspension
2
an automatic Bruker D8 Advance diffractometer, with nickel
2.5.5. High-resolution transmission electron microscopy
ꢁ

ltered Cu-Ka radiation. Patterns were recorded in 10–100 as (HR-TEM). HR-TEM investigations on adducts' samples, iso-
the 2q range, being 2q the peak diffraction angle. Distance lated from the supernatant solutions aer centrifugation, were
between crystallographic planes was calculated from the Bragg carried out with a Philips CM 200 eld emission gun
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microscope operating at an accelerating voltage of 200 kV. Few experimental part. The adopted procedures are summarized in
drops of the water solutions were deposited on 200 mesh lacey the block diagram of Fig. 2.
carbon-coated copper grid and air-dried for several hours before
In brief, HSAG and SP were rst premixed. HSAG–SP mixture
analysis. During acquisition of HR-TEM images, the samples was either ball milled in a jar for 6 h or heated in a ask for 6
ꢁ
did not undergo structural transformation. Low beam current hours at 130 C. The products taken either from the jar or from
densities and short acquisition times were adopted. To estimate the ask were washed several times with water, until SP was
the number of stacked graphene layers and the dimensions of undetectable in the washing water. The HSAG/SP molar amount
the stacks visible in HR-TEM micrographs, the Gatan Digital (in the case of HSAG, the moles of aromatic rings were
Micrograph soware was used.
considered) was explored in the range from 2 : 1 to 10 : 1.
Adducts prepared by using either mechanical or thermal energy
were named HSAG–SP-M and HSAG–SP-T, respectively. In the
present work, were characterized HSAG–SP-M and HSAG–SP-T
3. Results and discussion
The synthesis of SP, summarized in Scheme 1, is described in samples having almost the same amount of SP, determined
detail in the experimental part and has been discussed from thermogravimetric and elemental analysis.
66,67
elsewhere.
TGA analysis was performed under nitrogen on HSAG, SP
It is worth underlining the high atomeconomy of the reac- and on the HSAG–SP adducts. Thermographs are shown in
tion (82%), with water as the only co-product of the rst step. Fig. 3. In Table 1, are shown data of mass loss for HSAG and for
The high yield, about 95%, was easily reproduced in all the the adducts.
ꢁ
preparations performed in this research. This led to the atomic
SP achieves its boiling point at about 230 C. Curve of HSAG
efficiency of about 78% for the preparation of the SP sample shows decomposition prole made by two main steps, that can
used in this work.
be attributed to: decomposition of oxygen-containing and
ꢁ
ꢁ
The interaction of HSAG with SP was promoted with the help alkenylic groups for T in the range from 150 C to 700 C and
ꢁ
of mechanical or thermal energy, as described in the combustion with oxygen (see experimental part) at T > 700 C.
Fig. 2 Block diagram for the preparation of HSAG–SP adducts: with the help of mechanical (HSAG–SP-M) or thermal energy (HSAG–SP-T).
Fig. 3 TGA curves of HSAG (a), SP (b), HSAG–SP-T (c), heated HSAG–SP-M (d).
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Table 1 Mass loss for HSAG, HSAG–SP-M and HSAG–SP-T, from TGA
a
analysis
Mass loss [%]
ꢁ
ꢁ
300 C
< T < 500 C
1
50 C
ꢁ
ꢁ
ꢁ
Sample
T < 150 C
< T < 300 C
HSAG
HSAG–SP-M
HSAG–SP-T
0
0.8
0.8
0
2.7
2.7
3.1
7.8
7.7
a
ꢁ
Residual mass loss was above 700 C. Total mass loss was 100%.
Mass loss due to water removal, that could be expected at T <
ꢁ
1
50 C, can not be detected.
Fig. 4 IR spectra of HSAG (a), SP (b), HSAG/SP ¼ 2/1 (c), HSAG–SP-M
ꢁ
Larger mass loss in the temperature range from 150 to 500 C adduct (d), heated HSAG–SP-T adduct (e).
is shown by HSAG–SP-T and HSAG–SP-M. Degradation occurs at
a temperature much higher than the one observed in the SP
curve, indicating that residual SP is not present on HSAG. Both mechanical energy. Spectra of HSAG–SP-T and HSAG–SP-M
the adducts reveal an appreciable mass loss at temperatures adducts are in Fig. 4d and in Fig. 4e, respectively.
ꢁ
below 150 C. Interestingly, the larger amount of mass loss at
In the SP spectrum (Fig. 4b), band in the region from
ꢁ
ꢁ
ꢀ1
ꢀ1
T > 150 C is accompanied by larger mass loss at T < 150 C. It 3370 cm
to 3330 cm
can be reasonably attributed to
could be hypothesized that HSAG–SP adducts that contain hydrogen bonded OH groups, the pyrrole ring is evidenced by
larger amount of oxygenated functional groups hold larger the collective vibration mode of C]C/C–C stretchings, located
ꢀ
1
ꢀ1
amount of water. The onset temperature of the last step at about 1530 cm and by other bands at 1395 and 1490 cm
,
degradation is pretty similar for HSAG and for the adducts, related to vibrational modes involving the pyrrole unit. The
indicating that the reaction with SP did not lead to lower above bands due to pyrrole ring can be taken as ngerprint of
thermal stability of HSAG.
SP. These bands are still clearly detectable in the spectrum of
As reported in the Experimental part, pristine HSAG had the mixture (Fig. 4c) where they do not show frequency shis
9.5% as carbon content. The nitrogen content of HSAG–SP with respect to pristine SP (Fig. 4b). Spectra of both washed
9
adducts was determined by means of elemental analysis. Table adducts (Fig. 4d and e) show bands that cannot be attributed to
3
2
shows the experimental values and the amount of SP, theo- HSAG. In particular, they are: bands in the region of sp CH
retically estimated.
ꢀ
1
stretching at about 2900 cm and features in the region char-
The adducts contain comparable amount of SP, 13% and acteristic of the C–C stretchings of aromatic rings, namely the
ꢀ1
ꢀ1
ꢀ1
1
4% by mass. HSAG–SP-T was obtained by adopting 10/1 as the bands at 1590 and 1470 cm , peaks at 1383 cm and 956 cm
HSAG/SP molar ratio, whereas 2/1 as molar ratio was used for which could be attributable to vibrations involving the diole
the preparation of HSAG–SP-M. Studies have not been per- function and the alkenyl groups of reacted SP molecules. These
formed on the yield of the reaction between HSAG and SP as a ndings conrm what already observed through elemental and
function of the preparation procedure. As mentioned above, TGA analysis, that means the formation of a stable HSAG–SP
samples discussed in this work were selected as they show adduct. Moreover, they also seem to suggest the occurring of a
comparable amount of SP.
reaction or at least of an interaction able to modify the nature
Functional groups present on HSAG–SP adducts were and/or the surrounding of the chemical bonds in the pyrrole
investigated by means of IR spectroscopy. Fig. 4 shows the IR ring of SP. This aspect is under investigation. As working
spectra of pristine HSAG (a), SP (b) and of HSAG/SP mixture hypothesis, Diels–Alder reaction between dangling double
(
HSAG/SP ¼ 2/1 as molar ratio) (c). Such mixture was prepared bonds of graphene layers and the pyrrole rings is taken into
by simply mixing HSAG and SP, without giving either thermal or consideration.
Organization at the solid state of HSAG and of HSAG–SP
adducts was investigated by means of WAXD. Fig. 5 shows
WAXD patterns of HSAG (a), HSAG–SP-M (b), HSAG–SP-T (c).
Assignment of reections due to pristine HSAG is shown in
Fig. 5a. Patterns of HSAG–SP-T and HSAG–SP-M reveal that the
Table 2 Mass% of nitrogen and of SP
Adduct
(002) reection remains at the same 2q value as in the pattern of
Mass% of
HSAG–SP-T
HSAG–SP-M
pristine HSAG. This nding indicates that SP was not interca-
lated in the interlayer space. The number of stacked layers was
calculated by applying the Scherrer equation (see experimental
section) to the (002) reection. Such number was found to be:
35 for HSAG, 24 for HSAG–SP-M, and 29 for HSAG–SP-T. It
a
Nitrogen
SP
1.96
14.03
1.97
13.00
b
a
b
From elemental analysis. Calculated on the basis of the nitrogen
content.
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Fig.
5 WAXD patterns of HSAG (a), milled HSAG–SP-M (b),
HSAG–SP-T (c).
Fig. 6 Raman spectra of HSAG (a) HSAG–SP-T (b), HSAG–SP-M (c).
appears that the functionalization reaction promotes the
reduction of the crystalline order in the direction orthogonal to
the graphene layers. 100 and 110 reections indicate the in with SP. Hence, there are not indications that the reaction of
plane correlation lengths. The in plane order was estimated by HSAG with SP appreciably alter per se the structure of the
applying the Scherrer equation to the 110 reection: the corre- graphitic layers, whereas the milling process appears to play the
lation length was found, for HSAG and for the adducts, in a major role. The interpretation proposed by some of the authors
84
range from 26.5 nm to 28 nm. This result seems to suggest that to comment Raman spectra of ball milled graphite can be
the reaction of HSAG with SP, also via ball milling, does not applied also in the present work. Graphitic samples have a core
substantially alter the in plane order.
sufficiently far from the edge, with electronic and vibrational
Structural characterization of HSAG and HSAG–SP adducts properties that can not be distinguished from those of innite
was performed via Raman spectroscopy, a tool widely employed and ideal graphene layers. A conned crown region close to the
7
3–81
for the study of carbonaceous materials.
trum, two lines, named D and G and located at 1350 cm and tional structures, affected by connement effects, are different
In Raman spec- edge is electronically perturbed and local electronic and vibra-
ꢀ1
ꢀ
1
1
590 cm
respectively, reveal the presence of graphitic from those of the bulk materials. At the very edges, many types
sp -phase. In particular, the G peak is characteristic of bulk of molecular disorder are possible, making the system different
crystalline graphite (graphene), whereas the D peak appears in from the ideal sp carbon layer. In consideration of ndings
2
2
the presence of either disorder or connement (e.g. by edges) of from WAXD analysis, the enhancement of the D band in the
77–82
3
the graphitic layers.
Structural defects can be holes, sp or Raman spectrum of HSAG–SP-M could be prevailingly attrib-
sp carbon atoms, free radicals, distortions from planarity, uted, at least as working hypothesis, to larger amounts of
graed functional groups. Moreover, it is worth reminding that defects in the crown region and at the edges.
graphitic layers have nite dimensions and irregular bound-
On the basis of the results discussed so far and in the light of
aries with, for example, dangling bonds. All these defects give what available in the scientic literature, it seems possible to
rise to Raman scattering in the D region with a frequency that comment that HSAG–SP adducts are nano-stacks of conned
depends on the excitation wavelength. Some of the authors graphene, with peripheral functional groups containing oxygen
performed ball milling of graphitic powders, in order to vary atoms, in particular hydroxyl groups.
84
83
84
the average crystallite size and the amount of defects. Raman
As it should be expected, considering the presence of such
spectrum showed evident D line, whose intensity increased with functional groups, water dispersions of HSAG–SP were easily
the grinding time. Fig. 6 shows Raman spectra of HSAG (Fig. 6a) prepared (as described in the experimental part). Such disper-
HSAG–SP-T (Fig. 6b) and HSAG–SP-M (Fig. 6c). Analysis was sions were analyzed by UV-vis absorption. Dispersions freshly
performed on a good number of spots and spectra can be prepared and aer 30 min centrifugation at 2000 rpm of
considered representative of the investigated samples.
HSAG–SP-M adducts are shown in Fig. 7 (Fig. 7b and c,
In all the spectra, G and D bands are present, with similar respectively), together with a suspension of pristine HSAG
intensity. Interestingly, larger intensity can be observed for the (Fig. 7a). Decantation can not be observed for the dispersions
D band in the spectrum of the adduct prepared through ball based on HSAG–SP adducts.
milling, whereas relative intensity of G and D peak appears
UV-vis absorption analysis was performed on water
substantially unaltered aer the thermal treatment of HSAG dispersions of HSAG–SP-M adduct with 0.1, 0.3, 0.5 and 1
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Values obtained by DLS measurements represent the
hydrodynamic radius of HSAG particles in the dispersions. It
appears that particles of HSAG–SP-M have lower average size
than particles of HSAG. The average size further decreases in
the supernatant suspension. On the contrary, the average size
appears to increase, with respect to HSAG, when the HSAG–SP
adduct is prepared by heating. Also the average size in the
supernatant suspension is larger than in HSAG. It seems thus
that the functional groups can favour the aggregation of HSAG
particles that experience thermal treatment. The average size of
HSAG–SP-M aggregates in solution was investigated at different
times (t ¼ 0, 10, 60, 100 min), aer centrifugation at 9000 rpm
for 60 min. It was found to be about 130 nm at t ¼ 0 and to
remain constant for about 10 nm, achieving a plateau value of
about 200 nm aer 20 min. This nding seems to conrm the
tendency to aggregate of HSAG–SP adducts. The dependence of
Fig. 7 Water dispersions of HSAG (a), HSAG–SP-M: freshly prepared the particles average size on the centrifugation time was
(
b), after 30 min centrifugation at 2000 rpm (c).
determined for the HSAG–SP-M adduct: aer 5, 30, 60 and 90
min of centrifugation, 260 nm, 150 nm, 135 nm and 120 nm
were respectively measured.
ꢀ
1
mg mL as adduct concentration. In Fig. 8a it is shown that
the absorbance monotonously increases with the adduct
concentration. Plot of UV-vis absorbance as a function of the
adduct concentration (not reported in this manuscript)
reveals a linear correlation, suggesting that water solutions of
HSAG–SP-M were prepared. The stability of the solution with
TEM analysis of supernatant suspensions of HSAG,
HSAG–SP-T and HSAG–SP-M adducts, aer 60 min of centrifu-
gation, was performed aer their deposition on a carbon lm.
Analysis was repeated on signicant number of samples.
Representative TEM micrographs are shown in Fig. 9.
HSAG in Fig. 9a is characterized by graphite stacks randomly
arranged, with lateral size that appears lower than that of
HSAG–SP-T (Fig. 9b) and HSAG–SP-M (Fig. 9c). This is probably
due to the fact that only HSAG aggregates of small size can be
ꢀ1
1
mg mL as the adduct concentration was investigated by
taking UV-vis spectra on a samples stored for 1 week and on
samples centrifuged at 9000 rpm for 30 minutes and 1 hour
respectively. In Fig. 8b, it is evident that the same absorbance
was measured for the suspensions freshly prepared and stored
for 1 week, conrming that the adduct did not settle down,
even aer long time. Moreover, it is possible to observe that
only the centrifugation at 9000 rpm led to appreciable
reduction of absorbance, that nonetheless remains at a
remarkable level.
Average size of particles of HSAG and HSAG–SP adducts, in
water dispersions, was determined by means of DLS (as repor-
ted in the experimental part). Table 3 shows data for as prepared HSAG–SP-M
dispersions and for the supernatant dispersions aer 30 min HSAG–SP-T
Table 3 Average size of HSAG, HSAG–SP-T and HSAG–SP-M in water
dispersions, determined through DLS
Average size of particles (nm)
a
Starting dispersion
Supernatant dispersion
HSAG
500
300
1500
194
150
700
centrifugation at 9000 rpm.
a
Aer 30 minutes centrifugation at 9000 rpm.
Fig. 8 Dependence of UV-vis absorbance on concentration of HSAG–SP-M adduct in water (a) and after centrifugation at 9000 rpm (b).
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Fig. 9 TEM micrographs of water supernatant after 60 min centrifugation at 9000 rpm: HSAG (a), HSAG–SP-T (b) and HSAG–SP-M (c).
suspended in water. Micrographs of both adducts revealed fractionation of HSAG–SP-M adduct, as a function of centrifu-
stacks apparently made by a low number of layers in the case of gation, appears to be due to the different numbers of stacked
HSAG–SP-T (Fig. 9b) and by very few layers in the case of HSAG– layers, rather than to different lateral size, and to the different
SP-M (Fig. 9c). These ndings suggest that the milling proce- amount of SP modier.
dure promoted larger exfoliation of HSAG–SP-M adducts and, as
It was seen in HR-TEM micrograph in Fig. 10b that the gra-
a consequence, the formation of modied stacks of graphene phene nano-stacks are mostly made by about 10 graphene
layers that found it easier to remain dispersed in water, with layers, that means quite a lower number with respect to that
respect to the stacks of HSAG–SP-T adducts. The difference estimated on pristine HSAG via WAXD analysis (about 30).
among the average size of graphene stacks determined by DLS These ndings allow to comment that reaction of HSAG with SP,
and the lateral size of the sheets observed in TEM micrographs through ball milling, and the successive centrifugation of water
seems to suggest the extreme exibility of HSAG–SP adducts.
The structure of HSAG–SP-M adduct was investigated by
performing HR-TEM analysis on samples isolated form the
supernatant solutions, aer centrifugation for 10 min at 2000
rpm and for 5 and 60 min at 9000 rpm. Fig. 10 shows HR-TEM
micrographs at lower and higher magnications.
solutions of the HSAG–SP adduct lead to isolate nano-stacks
Micrographs at lower magnication in Fig. 10a, c and e
reveal that the lateral size of HSAG–SP-M adducts is of the same
order of magnitude in samples isolated aer centrifugations at
different rpm and for different times. This indicates that the
milling step does not cause appreciable breaking of the
graphitic layers. Moreover, it appears that the SP modier is
able to bring into water solution also nano-graphites with pretty
large lateral size. Micrographs at higher magnication allow to
visualize the stacks of graphene layers that are disposed with a
lateral side perpendicular to the beam. Thanks to this disposi-
tion of the nano-stacks, it is possible to estimate the number of
stacked graphene layers. Fig. 10b shows stacks isolated aer
centrifugation for 10 min at 2000 rpm: are visible stacks (indi-
cated in the boxes) of about 3.5–4.2 nm, made by about 10–12
stacked graphene layers. As revealed by the exam of a good
number of micrographs, such stacks are the most abundant
ones in a population of stacks that contain either little larger of
little lower number of layers. Fig. 10d shows a stack (indicated
in the box) isolated aer centrifugation for 5 min at 9000 rpm: is
of about 2.8 nm and contains about 8 stacked graphene layers.
Such a stack was very frequently observed in the analyzed
micrographs aer centrifugation for 5 min. In Fig. 10f it is
shown the adduct isolated aer centrifugation at 9000 rpm for
6
0 min. It seems that a layer of organic substance (indicated by
Fig. 10 Micrographs of HSAG–SP-M adducts isolated from superna-
tant solutions, after centrifugation for 10 min at 2000 rpm (a and b),
the arrow) is tightly adhered to the carbon allotrope: voids can
not be observed between the carbon surface and the organic
substance. It appears that the organic substance, that has to be
5
min at 9000 rpm (c and d) and for 60 min at 9000 rpm. Micrographs
are: low magnification bright field TEM (a, c and e), HR-TEM images (b,
assumed based on SP, is also on the graphene face. The d and f).
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made by few layers of graphene, whose number appears to the separation of graphitic aggregates into stacks containing a
depend on the adopted experimental conditions.
low/very low number of graphene layers. Results here reported
pave the way for the easy functionalization of carbon allotropes.
Moreover, the easy preparation of stable dispersions in polar
media allows to envisage a large variety of applications.
4
. Conclusions
The neat reaction of serinol with 2,5-hexanedione led to the
preparation of 2-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-propanediol,
with yield of about 95% and atomic efficiency of about 78%.
SP, that contains pyrrole ring, was able to form stable
adducts with HSAG, a nano-graphite with high surface area and
high shape anisotropy. Preparation of HSAG–SP adducts was
carried out in the absence of organic solvents or catalysts,
assisted by mechanical energy in a ball milling jar or by thermal
energy, by simply heating the reaction ask. Adducts with
Acknowledgements
Dr Valeria Cipolletti (Politecnico Milano) is gratefully acknowl-
edged for X-ray analysis and “PRIN Research Project 2010–2011”
for the nancial support.
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