Stable Aqueous Dispersions of Non-Covalently Functionalized Graphene from

Graphite and their Multifunctional High-Performance Applications

Supporting Information

Xiaohong An1,†, Trevor Simmons2,†, Rakesh Shah3, Christopher Wolfe3, Kim Lewis1, Morris

Washington1, Saroj Nayak1, Saikat Talapatra3 and Swastik Kar1,*

1Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, NY

12180 USA

2Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, NY 12180

USA

3Department of Physics, Southern Illinois University Carbondale, Carbondale, IL 62901 USA

† These authors contributed equally.

* Author for correspondence. Email: kars@rpi.edu

1. Production of Graphene solution

1.1 Exfoliation: Figure S1 schematically describes the process of graphene exfoliation via

‚molecular wedging‛ with PCA. Graphene dispersions were prepared by sonicating a mixture

of 100 mg graphite powder (Aldrich, particle size < 45 µm) and 16.5 mg 1-pyrenecarboxilic acid

(Aldrich, 98%) in 50 ml methanol for 45 min using a Branson® 5510 bath sonicator. To this

solution 200 ml of distilled water was mixed (stage 1 of the process, see figure S1a and S1b) and

sonication was continued for 1 day. The dispersion was then allowed to settle overnight (stage

2, figure S1a and S1b). The clear supernatant liquid was poured away, and fresh water was

Figure S1: Exfoliation of graphene from graphite powder using 1-pyrenecarboxylic acid (PCA).

(a) Process flow with different steps (mixing, exfoliation & washing) indicated in colour key.

Sample vials were photographed at some of the representative stages (1)-(3) . The washing step

is repeated 2-3 times to remove excess PCA. (b) Digital photographs of vials containing the

dispersion at stages 1-3 (as indicated in (a)).

(a) (b)

added. The solution was then sonicated for an additional two hours, resulting in a stable

purplish-grey dispersion that does not exhibit any significant precipitation when left on a

laboratory bench top for days or weeks. For purposes of clarity this solution is called the

‚original solution‛(stage 3, figure S1a and S1b).

1.2 Washing: To wash the excess (unbounded) PCA, the original solution was centrifuged at

10,000 rpm for 20 minutes, the supernatant was poured away, the precipitate and remaining

solution is topped off with water, and the mixture sonicated for 3 minutes. This is called the

first wash (figure S1a, also see figure 4 of the main text). This process was repeated several

times to obtain increasingly ‚clean‛ solutions, which refers to solutions of graphene-PCA

complex in water with a minimal quantity of uncombined PCA and methanol. With each wash,

the excess PCA gets removed further, and a limit is reached by the 3rd or 4th wash (as established

by the UV absorbance spectrum and TEM analysis of each wash step, discussed in the main

text). This final wash is made to pass through a filter membrane, with a standard pore size of

~10 µm, to remove any unexfoliated graphite microparticles. The resultant dispersion consists

mostly of flakes of single and few-layer graphene, as well as some flakes of multi-layer

graphene. By vacuum filtering through a nanoporous membrane, and measuring the weight of

the membrane before and after deposition of graphene on it, we find that the yield is about 1 wt.

%, and the solubility of the graphene flakes in water is about 10 gm/ml. This water-based

dispersion can be used to transfer graphene onto a number of different substrates. To increase

the yield of this process, it is suggested that the filter membrane be removed and sonicated in

clean water for several minutes until the filtrate is re-dispersed, and then filtration is repeated

with a new filter membrane to recover graphene trapped by graphite microparticles saturating

the filter membrane.

2. Characterizations:

The PCA exfoliated graphene solution contains a distribution of single, few-layer, and multi-

layer graphene (thickness >10nm, also known as multigraphene1), whose lateral dimensions

range from 100 of nanometers to a few microns. For large-scale applications, this material needs

to be transferred onto an application specific substrate. Owing to the homogeneous and stable

nature of our graphene dispersion, it was possible to store the dispersion for days and transfer

onto a number of different substrates. To transfer graphene onto SiO2/Si substrates, a

previously cleaned SiO2/Si wafer was placed vertically (with a slant) inside a vial containing the

graphene suspension which was then allowed to evaporate (at 60 C) completely. The substrate

was then washed with water and isopropanol and dried with pressurized nitrogen gas. For

transport measurement, graphene was pressure transferred from graphene films on nanoporous

membranes (see later) onto borosilicate glass substrates which had pre-fabricated electrodes

with a separation distance of 15 µm. These transferred films were not clearly visible under an

optical microscope, but could be tested electrically to ascertain the presence of a continuous film

between the electrodes.

2.1 Raman Spectrometry: Figure 1b (main text) shows a Raman spectrum taken on a single-layer

graphene flake using a 532 nm excitation laser (WITEC alpha300R confocal Raman

spectrometer). A strong peak indicative of the ordered graphitic crystal lattice known as the G-

band was observed around 1583 cm-1, which corresponds to the Raman active doubly

degenerate zone center E2g phonon (in-plane optical mode) of sp2 hybridized carbon, close to the

point. In addition, other peaks are observed at around 1349 cm-1 (corresponding to the D-peak

or the first order edge or defect-induced zone boundary phonons) and 2693 cm-1 (the Dʹ-band or

second-order zone boundary phonons). Together, these peaks represent the presence of

graphitic carbonaceous materials and were observed in all the samples, with slight variations in

peak position and height. Past work has shown that the Raman spectra of single-layer graphene

have at least two unique signatures2,3. First, the Dʹ-band peak intensity is larger than the G peak

(a feature that reverses itself in graphene with number of layers >1 and in graphite). Second, the

Dʹ-band peak can be fitted with a single Lorentzian function, which is not the case of graphitic

flakes with a higher number of layers, where two or more Lorentzian functions are required to

fit the Raman data. Figure 1c shows the single Lorentzian fit to the Dʹ-band peak. The top left

inset in figure 1b shows the spatial Raman map (with spatial resolution approximately 350 nm)

of the graphene flake obtained from the integrated Dʹ-band peak. Overall, the signature peaks

for graphitic materials in the Raman spectrum, the comparative intensities of the G-band and

Dʹ-band peaks, and the single-Lorentzian fit to the Dʹ-band peak led us to conclude that the

observed flake consisted of a single layer. In a number of samples, we found the ratio of the

intensities of the D-band and the G-band peaks (ID/IG) to be ~0.15, a value which can be

compared to that of highly purified single wall carbon nanotubes. We note that since the flake

size is comparable to the Raman spatial resolution, some contribution to the D band definitely

comes from the edges. This strongly indicates that our process is indeed able to produce high

quality individual graphene flakes in a scalable manner.

2.2 AFM and SEM: The presence of single-layer graphene was further confirmed by AFM

investigations. Figure S2a-c shows AFM images of graphene flakes transferred onto SiO2/Si

substrates which is typically required for building electronic devices such as transistors.. To

properly establish the single-layer nature of the flake, the step height at the edge of the flake

was measured after averaging out the surface roughness of the flake and its neighboring

substrate (see inset, figure S2c). The sub-nanometer step height (~0.7 nm) observed compares

with similar values obtained for single-layer graphene flakes by other groups4. Further, AFM

step-height measurements were used to obtain the layer statistics of our dispersion. Figure S2d

shows the statistical distribution of number of layers per flake. We find that the transferred

graphene flakes comprised of slightly less than 10% of monolayers. Figure S2e shows an SEM

image of similar graphene flakes on an SiO2/Si substrate. The flakes seen in the SEM image are

mostly multi-layer graphene flakes, since the thinnest layers are practically invisible to SEM

imaging.

2.3 Temperature dependence of resistance: Multigraphene flakes were also transferred onto

borosilicate glass substrates pre-fabricated with Au electrical leads for investigation of low-

temperature transport in these materials. The Au leads were separated by a distance of 15 µm

and typically had more than one flake attached across the leads. Figure S2f shows the

temperature dependence of resistance in a typical two-terminal device. We find that as the

temperature decreases, the resistance of the system increases gradually. This is in sharp contrast

with bulk graphite which behaves like a metal with the resistance value decreasing with

temperature. It has been shown recently that in graphene-based films5 and multigraphene flakes

with 10-20 layers of graphene1,6, both the carrier density as well as the mobility develop

temperature dependences that, in a quantum-confined geometry, cause the resistance to

increase with decreasing temperature. The results of the temperature dependence study agree

very well with previously reported data.

2.4 XPS analysis of the exfoliated graphene:

We have performed XPS experiment on our graphene films and have compared it with the

parent graphite powder. The figures below show the XPS spectrum near the C1s peak. The data

Figure S2 Characterization of flakes transferred onto various substrates: (a)-(c) AFM images

of graphene flakes transferred onto an SiO2/Si substrate, with typical images at different scan

sizes as indicated. Also shown in (c) is an AFM image of a single-layer graphene flake of step

height ~0.7 nm. (d) Distribution statistics of graphene layers/flake on the SiO2/Si substrate

estimated from AFM height measurements (e) An SEM image of graphene flakes transferred

onto a SiO2/Si substrate. The thinnest flakes are invisible to the SEM (f) Temperature

dependence (5K<T<300K) of resistance of overlapping multigraphene flakes transferred onto

a borosilicate glass with prefabricated Au electrodes.

analysis is based on a very detailed experimental investigation of defective graphitic carbon by

Estrade-Szwarckopf7. The salient features of the XPS data are:

1. The parent graphite powder has four recognizable peaks, assigned to the graphitic carbon

(~284.7 eV), a defect peak (~285 eV), a single-bonded carbon C-O peak (~286.5), and a broad -*

transition peak (290-291 eV) (figure S3a). The presence of these peaks is in excellent agreement

with past work by Estrade-Szwarckopf.

2. For the XPS data of the washed graphene films the defect peak (figure S3b) is larger in area,

indicating larger percentage contribution if defects to the spectrum. This is easily explained by

the fact that flakes in the flakes have much smaller size and hence much larger amount of edges

is present compared to the parent powder, and is in direct agreement with our Raman data.

Compared to the parent graphite spectrum, we now find and additional C=O peak, and a C-O

peak whose relative contribution is larger than that of the parent graphite powder. The

simultaneous increase in the relative contribution of C-O and C=O bonds is possibly due to

residual PCA functionalization on the graphene flakes even after washing.

Figure S3: XPS spectra of the C1s peaks of the (a) parent graphite powder sample, and (b)

exfoliated graphene sample.

(a) (b)

Comparing the XPS spectra of the parent graphite powder and the graphene flakes, we can

conclude that no significant additional covalent bonding has appeared in the exfoliated

graphene.

2.5 HRTEM and SAED data

We have performed high resolution transmission electron microscopy (HRTEM) to observe the

graphene flakes which were transferred from the aqueous dispersion onto lacey carbon coated

copper TEM grids. The characterization was carried on a JEOL model JEM-2010 microscope.

Figure S4 shows the HRTEM images of two typical graphene flakes and their corresponding

selected area electron diffraction (SAED) patterns shown as insets.

3. Applications

3.1 Fabrication of graphene films on nanoporous membranes (graphene membranes)

Figure S4 HRTEM images of (a) a bilayer graphene flake and (b) a 5-layer graphene flake at

their fold-edges. The insets show their corresponding SAED patterns taken from a plane

close to the edges.

The washed dispersions were vacuum filtered onto nanoporous Teflon membranes of pore size

100 nm. About 10-15 ml of suspension, when transferred onto the membranes gave a grey

contrast to the white membrane when dried, and the graphene-coated membrane was

electrically conducting. Figure S5 shows how these membranes are fabricated and characterized

using optical imaging and Raman spectroscopy. These membranes can be directly used to

fabricate chemical sensors and electrolytic double-layer capacitors.

3.1.1 Fabrication of conductometric chemical sensors: Graphene membranes of suitable shape

and size were contacted on two sides by physically masking the central area. Au electrodes are

directly evaporated on the two ends of this membrane, and then attached to external leads

using a conductive silver paint. After the silver paint dries and hardens, the electrode area is

coated with the flexible polymer polydimethylsiloxane (PDMS) to provide mechanical and

electrical stability to the contacts, and also to prevent any ‚thinning/dissolving‛ of the silver

paint, or spurious sensor response due to exposure of the silver paint to the various analytes.

Figure S6a shows a graphene membrane sensor built into the cap of a typical laboratory vial.

Such a cap can be directly screwed onto a matching vial (figure S6b) containing a small quantity

of liquid analyte, and the sensor is held above the surface of the liquid in the presence of its

saturated vapour. In case of CO2 detection, dry ice was used as a source, and the emanating CO2

was first thermalized to room temperature and then exposed to the sensor surface, and so in

this case, the detection was that of nearly 100% CO2 gas.

3.1.2 Fabrication of electrolytic double-layer capacitors: For supercapacitor measurements two

pieces of graphene coated Teflon membrane were placed back to back on top of each other

(which gives a layered geometry of graphene film - Teflon membrane - Teflon membrane-

graphene film, see figure S7). This layered structure was dipped in 6 M KOH solution

sandwiched between two stainless steel plates for performing the capacitor related

measurements that are presented in figure 6 (a&b) of the main manuscript. These

measurements were performed using a Potentostat/Galvanostat (Model # PARISTAT 6623

manufactured by Princeton Applied Research Inc.).

The specific capacitance (capacitance/mass) for the device was calculated from the constant

current discharge curves figure 4 (b) using the formula

( )mdtdV

ICsp /

= (1)

Where, I is discharge current, dtdV / is slope of discharge curve, and m is mass of each

electrode of double layer capacitor (in our case, the mass was less than 10 g, but we used 10 g

as a maximum upper limit for our calculations8).

Similarly, the energy density was calculated using equation

mVCWsp′2/)×(= 2 (2)

Figure S5: Fabrication of flexible graphene membranes using the graphene dispersion. (a)

Aqueous dispersion of graphene is transferred onto nanoporous membranes via vacuum

filtration. (b) The deposited graphene flakes gives the membrane a grey color. (c)-(d) The

membrane can be cut to desired geometries for fabricating rigid or flexible sensor

membranes or ultracapacitor electrodes. (e) A digital optical image of a typical membrane

surface. (f) Typical Raman spectrum of the membrane shows the signature graphene peaks.

and the power density was calculated using equation

mIVPsp′/=

(3)

where C is capacitance, V is the operating voltage of the

capacitor, I is discharge current, and m′is mass of both

electrodes.

4. Advantages

4.1 Chemical sensors:

1. The chemical sensors are lithography-free, macroscopic,

flexible, scalable gas/vapor sensors.

2. In contrast to most of the previously reported graphene-

based sensors, which were fabricated on substrates, the

overlapping graphene flakes that form the active surface of

our sensors have at least part of both sides of the graphene

film available for analyte adhesion, allowing them to be more

sensitive.

3. By selecting the membrane material (e.g. cellulose

acetate, Teflon, anodized alumina, etc.) it is possible to

have either flexible or rigid sensors, with various chemical

compatibilities, and various porosities.

4. The membrane structure provides a useful method for

transferring the graphene films onto other substrates by

mechanical contact and subsequent dissolving, following

the methods reported by others.5

(a) (b)

Figure S6 Conductometric

Chemical sensors from

graphene membrane. (a)

Electrode contacted

sensor. (b) Sensor

mounted into a laboratory

vial (sensing chamber).

Figure S7 Schematic of the

ultrathin device arrangement

used for measuring the

ultracapacitor performance.

5. By covering the exposed surface with flexible polymers such as PDMS, the lower surface of

the graphene sensor (covered by the nanoporous membrane) can potentially be used for size-

selective analyte detection determined by the pore size – a technique useful in biochemical

analyte detection.

6. We find that our sensor is highly responsive to alcohol vapours, and by comparing the

response to ethanol, water vapour and CO2 gas, we have demonstrated the potential for the

fabrication of lightweight, portable and low-cost breath alcohol analyzers.

4.2 Ultracapacitors:

1. These ultracapacitors are built on flexible substrates where the porous substrate itself forms a

‚separator‛. Hence it is possible to distort the shape of the capacitor to desired shapes such as

cylinders.

2. No additional ‚binders‛ or specially fabricated current collectors were needed.

3. Despite its simple arrangement, the specific capacitance obtained was one of the highest

reported for graphitic nanostructures. In addition, they showed impressive performance at high

cycling speeds in terms of power and energy density. We envision that with suitable

modifications, their performance can be improved significantly.

5. Comparison of our ultracapacitor data with other available data

TABLE S1: The table lists Sp. Capacitance (C), Energy Density (ED), Power Density (PD) and electrolyte used for some of the recent

experimental results on electrochemical double layer capacitors fabricated from various carbon nanomaterials.

* These data are taken from the "BASIC RESEARCH NEEDS FOR ELECTRICAL ENERGY STORAGE" Report of the Basic Energy

Sciences Workshop for Electrical Energy Storage, April 2007. Only those manufacturers are mentioned where the capacitors are made from

Electrode Method Treatment C

F/g

ED

Wh/Kg

P D

kW/Kg Electrolyte References

1 MWNT

CVD

ferrocene-

xylene

Cast Ni foam onto a

graphite milk(32

wt% graphite,

55wt%ethanol, 8wt

%binder)

14.1 7 M KOH

Nanotechnology 18,

195607 (2007)

2 MWNT CVD silica freestanding 51 6 M KOH Adv. Mat. 19, 3300-

3303 (2007)

3 MWNT CVD-

acetylene

Coated with

conductive cement

after transferring to

an Al substrate

10-15 2.2 125 Elec. Sol. Lett. 10 (4),

A106-A110 (2007)

4 MWNT CVD-

acetylene

Acid treatment for

purification, KOH

activation at 800˚C,

pellets

90 6 M KOH or

1 M H2SO4

Chem. Phys. Lett. 361,

35-41 (2002)

5 MWNT CVD-

acetylene

Acid treatment,

pellet form (85wt%

CNT, 5wt%

acetylene black, and

10% PVDF binder)

137 6 M KOH Appl. Phys. Lett. 77

(15) (2000)

6 SWNT H2 arc

discharge As produced 20 6 M KOH

Appl. Phys. Lett. 92,

143108 (2008)

7 SWNT H2 arc

discharge

Electrochemically

oxidized 113 6 M KOH

Appl. Phys. Lett. 92,

143108 (2008)

8 SWNT

Arc

discharge

using Ni/Y

As grown 30 ~1 ~1 6 M KOH J. Elec. Soc. 151 (6), 831-

837 (2004)

9 SWNT HiPco/

Pyrrole 350

3.3

kJ/Kg 4.8 6 M KOH

Chem. Mater. 17, 1997-

2002 (2005)

10 SWNT Arc

discharge PVDC binder 180 6-7 20 7.5 N KOH

Appl. Phys. 82, 585-591

(2006)

11 Graphene

sheet 122.6 6 M KOH

Carbon 48, 1731-1737

(2010)

12 Graphene/

SnO2 43.4 1M H2SO4

Nanotechnology 20,

455602 (2009)

13 Graphene

Based 99-135 5.5 M KOH

Nano Lett 8 (10):3498–

3502 (2008)

14 Graphene

Based 35-117 H2SO4

J. Mat. Chem. 18, 1517-

1523 (2008).

15 Graphene Liq. Phase

Exfoliation

As produced on a

membrane ~120 9.2 105 6 M KOH Present work

16 ECOND 2.3 F 0.5 Aqueous *

17 ELIT 0.027 0.8 Aqueous *

18 ESMA 9 2.7 Aqueous *

carbon materials with aqueous electrolytes. The specific energy and power densities are for the whole device and not for the electrode materials as

reported for other experimental findings listed from 1-15.

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8. To obtain the weight accurately, the weights of several large pieces of graphene membranes

with known dimensions were measured. From these measurements, the average weight per

unit surface area (sp. wt.) of the graphene-membrane was obtained. Then, smaller pieces of

known dimensions were cut from these to fit our electrochemical setup. The weights of the

smaller pieces were calculated to be = (sp. wt.) surface area of the membrane. In our

laboratory scales, a pair of membrane electrodes weighed between 10-20 micrograms. We

have chosen the upper limit of 10 micrograms per electrode. This means that the reported

values of specific capacitance and energy and power densities are the lower limit of their real

values.