World Journal of Nano Science and Engineering, 2012, 2, 148-153 Published Online September 2012 (
A Study on Synthesis and Characterization of Biobased
Carbon Nanoparticles from Lignin
Prasad Gonugunta1,2, Singaravelu Vivekanandhan1,2, Amar K. Mohanty1,2, Manjusri Misra1,2*
1School of Engineering, University of Guelph, Guelph, Canada
2Bioproducts Discovery and Development Centre, Department of Plant Agriculture,
University of Guelph, Guelph, Canada
Email: *
Received May 12, 2012; revised June 27, 2012; accepted July 2, 2012
Carbon nanoparticles were synthesized using lignin as a renewable feedstock by employing a freeze-drying process
followed by thermal stabilization and carbonization. The effect of adding various amounts of KOH to a lignin solution
on the solubility of the lignin, the freeze-drying process, the thermal stabilization of the freeze-dried lignin, and carbon
nanoparticle formation was investigated through FTIR, DSC, SEM, TEM and surface area analysis. SEM investigations
confirmed that the freeze-drying process caused the formation of lignin with a porous microstructure. TEM analysis
indicates that the thermal stabilization of freeze-dried lignin prevented the formation of agglomerated carbon nanoparti-
cles during the carbonization process. The smallest carbon nanoparticles were found to be 25 nm and were prepared
from the lignin precursor with 15% KOH.
Keywords: Carbon Nanoparticles; Lignin; Freeze-Drying; Carbonization
1. Introduction
Carbon nanostructures such as fullerenes, carbon nano
tubes/fibres/particles, and graphene sheets have been
extensively studied due to their unique properties such as
good electrical/thermal conductivity, excellent corrosion
resistance, and enhanced chemical/bio compatibility [1-
5]. Hence, they have found a wide range of applications
which include polymer composites, electrochemical en-
ergy storage and conversion, catalysis, filtration, hydro-
gen storage, and biotechnology [6-10]. Among the vari-
ous carbon allotropes, particulate nanostructures receive
more attention due to their versatility in fabrication and
their extensive applications in polymer nanocomposites as
nanofillers, waste-water treatment, biomedical imaging,
and optical devices [11-14]. In general, these carbon
nanoparticles have been synthesized using various syn-
thetic processes including thermal carbonization, laser
irradiation, sonication, and exfoliation [3,15-17].
One of the key factors in controlling the morphology
and the yield of the carbon nanoparticles is the precursor
material. Various carbon precursors such as graphite
powders, petroleum pitch, carbon rich polymers, and
other kinds of liquid/gaseous hydrocarbons have been
extensively used for the fabrication of carbon nanoparti-
cles [5,16]. However, there is a need for alternate carbon
sources for the synthesis of carbonaceous materials due
to increasing oil prices, depleting petroleum resources,
their negative environmental impacts, and increasing
demand for carbon-based nanomaterials in various emerg-
ing fields. Hence, renewable carbon resources such as
plant biomasses, biobased oils, and hydrocarbons have
been explored for the fabrication of carbon nanostruc-
tures [18, 19]. Among the various renewable precursors,
lignin, which is widely known as a co-product of pulp
and second generation cellulosic ethanol industries, re-
ceives great attention due to its 1) carbon rich chemical
structure, 2) abundance in nature, 3) chemical compati-
bility, and 4) cost effectiveness. Thus, lignin has been
investigated for the fabrication of carbonaceous materials
such as carbon fibres and activated carbons. However,
the synthesis of carbon nanoparticles from lignin has not
been explored to a great extent [20,21]. Synthesis of
carbon nanoparticles with controlled microstructures is
possible through chemical modification and alteration of
the processing parameters. In this, the challenging issue
is to inhibit the agglomeration of lignin molecules during
the carbonizetion process.
Herein, we report the synthesis of carbon nanoparticles
using lignin as a renewable feedstock by adopting a
freeze-drying process in order to overcome the issues
related to the formation of lumps during carbonization.
Our ultimate aim is to investigate the effect of KOH ad-
*Corresponding author.
opyright © 2012 SciRes. WJNSE
dition on the solubility of lignin, the freeze-drying proc-
ess, and thermal stabilization as well as the formation of
carbon nanoparticles. Freeze-drying of solubilized lignin
can effectively produce ultra porous lignin structures.
The thermal stabilization is involved in the retention of
the obtained microstructure during the carbonizing proc-
ess. This will help to avoid the agglomeration of carbon
particles as well as the formation of lumps during the
carbonization process and result in ultrafine nanoparti-
cles. The complete process was investigated using FTIR,
DSC, SEM, and TEM.
2. Experimental
2.1. Materials
Protobind 2400 lignin (PL) is a by-product of the paper
industries and was obtained from A L M Pvt. Ltd. India.
Potassium hydroxide (KOH) in pellet form was procured
from Sigma Aldrich and both the precursors were used
as-received without further purification.
2.2. Methods
Figure 1 shows a schematic representation of the various
steps involved in the synthesis of carbon nanoparticles
from lignin. Step 1: 2 g of lignin samples were dissolved
in 500 ml of deionised water with various KOH concen-
trations such as 0 wt%, 5 wt%, 10 wt%, and 15 wt% un-
der sonication. The obtained solutions were labeled as
PL-0, PL-5, PL-10, and PL-15 respectively. Step 2:
Brown coloured lignin solutions were transferred into
steel beakers and solidified using liquid nitrogen. The
solidified lignin samples were freeze-dried in order to
achieve porous lignin. Step 3: Thermal stabilization of
the freeze-dried lignins was performed by heating them
up to 250˚C at a 1˚C/min ramp rate. This helps in reten-
tion of the porous microstructure during the carbonize-
tion process. Step 4: After the thermal stabilization, proc-
ess, thermo stabilized lignin samples were carbonized in
a tubular furnace at 700˚C for 2 hours under nitrogen
atmosphere by employing a 5˚C/min heating rate. The
obtained carbon nanoparticles were used for further char-
2.3. Characterization Techniques
A SAVANT-MODULYO (Model No: B1576) freeze
dryer was used to dry lignin solutions, which allows the
formation of porous lignin. The effect of thermal stabili-
zation on the structural coordination of freeze-dried lig-
nin was investigated using Fourier transform infrared
spectroscopy (FTIR), Thermo Scientific Nicolet TM
6700 FT-IR Spectrometer, USA employing attenuated
total reflection infrared (ATR-IR) mode between 400
cm1 and 4000 cm1 with a resolution of 4 cm1. Differ-
ential Scanning Calorimetric (DSC) analysis of the
as-obtained as well as the thermo-stabilized freeze-dried
lignin was performed using a TA Q-200 DSC in order to
identify the glass transition temperatures (Tg). 3 to 5 mg
of lignin samples were sealed in an aluminum pan sup-
plied by TA instruments. Initially, the sample was heated
to and maintained at 80˚C for 30 minutes in order to re-
move existing moisture and then cooled to 0˚C. The DSC
thermogram was recorded up to 250˚C employing a 20˚C
/min ramp rate. All analysis was carried out in a nitrogen
atmosphere.The micro-structure of the lignin samples
were analyzed using a scanning electron micro-scope
(SEM), FEI Inspect S50 Netherlands. Gold coating was
Figure 1. Schematic of the various steps involved in the synthesis of carbon nanoparticles fr om lignin.
Copyright © 2012 SciRes. WJNSE
performed for all the samples in order to enhance the
SEM images. Synthesized carbon nanoparticles were
characterized using a transmission electron microscope
(TEM), JEOL 2010F FEG TEM/STEM, employing a 200
kV operating voltage. Brunauer-Emmet-Teller (BET)
surface area analysis of the freeze-dried lignin samples
and the synthesized carbon nanoparticles were measured
with a NOVA station-C, Quantachrome through nitrogen
gas sorption at 77.3 K. First, the samples were flow de-
gased at 55˚C for 8 - 16 hours to remove the volatiles.
BET surface areas were taken from a multipoint plot over
a P/Po range of 0.05 - 0.35.
3. Results and Discussion
The freeze-drying process caused the formation of po-
rous lignin samples. However, these may fuse together
and form lumps during the carbonization process, which
results in bulk carbon material rather than well-defined
carbon nanoparticles. In order to retain this porous struc-
ture, lignin samples underwent a thermal stabilization
process employing a low heating rate of 1˚C/min at
250˚C for 2 hours. The effect of thermal stabilization on
their structural coordination, thermal behavior, and mi-
crostructure were investigated by FTIR, DSC, and SEM
analysis respectively. Figure 2 shows the FTIR spectra
of the as-obtained and thermo stabilized lignin with dif-
ferent KOH formulations. From Figure 2, the FTIR
spectra indicates a characteristic peak of the lignin at
1590 cm1 and 1500 cm1, which represents the aromatic
skeletal vibration [22]. Thermal stabilization of the
freeze-dried lignin caused shifting of the peak at 1590
cm1, which also increases with increasing KOH concen-
tration for both the freeze-dried as well as thermo stabi-
lized lignins. During the thermal stabilization, lignin un-
dergoes condensation reactions in the presence of alkali
metals, which also caused the formation of various or-
ganic compounds such as metal formates as well as ace-
tates [22]. These metal carboxylates have a very strong
absorbance in the region of 1695 - 1540 cm1, which
caused the increased peak intensity at 1590 cm–1 [23].
Thermal stabilization also caused the formation of new
peaks at 1385 cm1 and 1315 cm1, which is attributed to
C-O-C stretching, which evidences the formation of ex-
cess ether groups through condensation. This results in
higher cross-linking and caused significant improvement
in the glass transition temperature.
Figure 3 exhibits the DSC thermograms of freeze-
dried lignin before and after thermo stabilization. The
DSC thermogram of freeze-dried lignin derived without
KOH addition shows a Tg of 89˚C, which increases with
increasing KOH concentration. Ucar et al. reported that
the presence of alkali metal in lignin effectively caused the
cross-linking through condensation, which also results
PL+15% KOH
PL+10% KOH
PL+15% KOH
PL+10% KOH
Figure 2. FTIR spectra of freeze-dried lignin (a) as derived
and (b) thermal stabilized.
in the increased Tg in the DSC thermogram.
Thermo stabilization further enhances the condensa-
tion/cross-linking significantly thereby increasing the
glass transition temperature and retaining the glassy state
of lignin, which is confirmed through the DSC thermo-
grams which do not show a significant Tg point. During
thermal stabilization, lower heating rates increase the Tg
of the lignin samples faster than the actual temperature,
thereby avoiding the possibility of fusing and stabilizing
the foamy structure. SEM analysis confirms this phe-
nomenon, which is shown in Figure 4. The freeze-drying
process caused the formation of porous structures, which
is highly influenced by the presence of KOH. The
freeze-dried lignin solution made without KOH addition
formed a caused the formation of solid mass. Further, the
thermal stabilization caused the successful retention of
Copyright © 2012 SciRes. WJNSE
PL+15% KOH
PL+10% KOH
PL+15% KOH
PL+10% KOH
111 ˚C
Figure 3. DSC thermogram of freeze-dried lignin (a) as
porous structure. This resuis consistent with the re-
were carbonized at
derived and (b) thermo stabilized.
ported literature by Kadla et al. [20].
Thermol stabilized lignin samples
0˚C in nitrogen atmosphere for 2 hours. It was ob-
served that the carbonized lignin without KOH results in
the formation of solid mass where as the lignin samples
modified with KOH yielded ultrafine particles. The chal-
lenging issue in fabricating carbon nanoparticles is the
yield, which indicates the efficiency of the conversion
process. The thermal stabilization yield fraction (YTS) is
the ratio of mass of lignin present after thermal stabili-
zation (mTS) to before (mTS) thermo stabilization process.
Similarly, carbonization yield fraction (YC) is the ratio of
mass of carbonized material (mC) to mass of material
present before carbonization process (mTS). Overall yield
is the product of the yields of thermal stabilization (YTS)
and the carbonization (YC).
(I) A s derived (I I) The rmo-s tabilized
50µm 50µm
50µm 50µm
50µm 50µm
(a) (a)
(b) (b)
(d) (d)
Figure 4. SEM micrographs of freeze-dried lignin (I) as
derived and (II) thermo stabilized ((a) PL; (b) PL + 5 wt%
KOH; (c) PL + 10 wt% KOH; (d) PL + 15 wt% KOH).
mm m
 
  
 
Table 1 summarizes the yields during the various
ages involved in the synthesis of carbon nanoparticles.
The specific surface area of the synthesized carbon
noparticles were measured by employing BET surface
area analysis The measured surface area of the carbon
nanoparticles synthesized from lignin with different
KOH concentrations of 0%, 5%, 10%, and 15 % are 0, 43,
47, and 23 m2/g respectively. From this analysis, it is
confirmed that the addition of KOH to lignin up to 10%
Copyright © 2012 SciRes. WJNSE
increases the surface area and higher concentrations of
KOH decreases the surface area. This may be due to the
tendency of KOH to form agglomerates at higher KOH
concentrations (15%).
The results indicate that the addition of KOH reduces
4. Conclusion
les were successfully synthesized us-
Table 1. Yields for thermaed and carbonized
Sample al Yield of
car Overall yield
e overall yield, which may be due to the oxidation be-
havior of KOH in lignin. Synthesized carbon powders
were further characterized by TEM analysis to confirm
the formation of nanoparticles. TEM micrographs of the
carbon particles prepared from lignin source modified
with different KOH concentration are shown in Figure 5.
Carbon nanopartic
ing lignin (Protobind 2400), a industrial co-product, as a
renewable feedstock. The effect of KOH addition on the
solubility of lignin, the freeze-drying process, thermal
lly stabiliz
carbon nanoparticles.
Yield of therm
name stabilization
(YTS%) bonization
(YC%) (YT%)
PL 92.88 1.3 52.847 1.11 49.08
PL + 5
w88.46 2.5 55.072 1.9 48.71
w 75.97 3.1 56.7 2.1 43.07
w66.82 3.5 57.8 2.5 38.61
t% KOH
t% KOH
PL+ 15
t% KOH
500 nm 100 nm
200 nm 200 nm
(a) (b)
(c) (d)
Figure 5. TEM image of carbon nanoparticles synthe
+ 15 wt% KOH.
he Ontario ministry of agri-
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