Photoluminescent CNPs Produced by Confined Combustion of Aromatic Compounds
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Transcript of Photoluminescent CNPs Produced by Confined Combustion of Aromatic Compounds
8/2/2019 Photoluminescent CNPs Produced by Confined Combustion of Aromatic Compounds
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Photoluminescent carbon nanoparticles produced by confined
combustion of aromatic compounds
Abdelaziz Rahy a, Chen Zhou a, Jie Zheng a, S.Y. Park b, Moon J. Kim b, Ikjun Jang c,Sung June Cho c, Duck J. Yang a,*
a Department of Chemistry and the Alan G. MacDiarmid NanoTech Institute, The University of Texas at Dallas, Richardson, TX 75080, USAb Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, TX 75080, USAc Department of Applied Chemical Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea
A R T I C L E I N F O
Article history:
Received 7 August 2011
Accepted 29 October 2011
Available online 10 November 2011
A B S T R A C T
We report new photoluminescent carbon nanoparticles having an average particle size of
50 nm. When dispersed in chloroform and excited with 325 nm wavelength, the solution
showed strong photoluminescence at 475 nm with 12–13% quantum yield. A well dispersed
photoluminescent solution can also be prepared with ethanol, xylene or hexane using the
nanoparticles. The nanoparticles were prepared by a simple confined combustion of an
aromatic compound such as benzene, toluene, xylene or a mixture thereof in air.
Published by Elsevier Ltd.
1. Introduction
Carbon is unique among the elements in the vast number and
variety of compounds it can form. Without carbon, the basis
for life would be impossible. Production of carbon nanoparti-
cles has become an area of increasing interest in material
research because they are biocompatible, chemically inert
and can be surface modified [1]. It was discovered that carbon
nanoparticles display intense light emission, and it is
expected to yield new insights into practical applications such
as in bioimaging [2–4], their ability to suppress fluorescence in
resonance Raman spectroscopy [5], sensor [6] and catalyst
support in DMFC [7,8]. To date, a variety of techniques have
been developed for fabricating carbon nanoparticles including laser ablation [1,9], non-thermal plasma [10], microwave plas-
ma chemical vapor deposition [11], microwave of conducting
polymers [12], thermal carbonization of bis(2-chloroethyl)
amine hydrochloride at 260 °C [13], arc in water method with
forced convective jet [14] and others [15,16]. Liu et al. reported
the preparation and fluorescent carbon nanoparticles derived
from candle soot [17]. Tian et al. adopted a procedure to
synthesize carbon nanoparticles from the combustion soot
of natural gas instead of candles [18]. Stasio et al. investigatedthe microstructure of propane–air diffusion flame soot using
TEM. He observed three classes of nanoparticles: the class pri-
mary particles 20–50 nm, the sub-primary graphitical particles
6–9 nm and elementary particles <5 nm [19]. These methods
have limitations in terms of size selectivity and economical
production capability because of their non-selective harsh
synthesis conditions, low production rate and high capital
investment. In this regard, the synthesis of carbon nanoparti-
cles with tailored composition, structure, morphology and
size by a simple and cheap method is very attractive.
Here, we report a new method to obtain carbon nanoparti-
cles by a controlled combustion of an aromatic compound
(benzene, toluene, or xylene) or a mixture thereof in aconfined space (Pyrexâ glass container) in air. Under our pres-
ent experimental conditions, the majority of the product
resulting from the combustion is deposited in the wall of
the container (yield: 45%). Only a small amount was deposited
in the bottom. Since the combustion process can be contin-
ued until the liquid aromatic compound is completely con-
sumed and no complex apparatus is needed, the present
technique could be readily scalable for mass production.
0008-6223/$ - see front matter Published by Elsevier Ltd.doi:10.1016/j.carbon.2011.10.052
* Corresponding author: Fax: +1 9728832925.E-mail address: [email protected] (D.J. Yang).
C A R B O N 5 0 ( 2 0 1 2 ) 1 2 9 8 – 1 3 0 2
A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c a r b o n
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2. Experimental
Carbon soot (aggregates of carbon nanoparticles (CNPs)) wascollected after a confined combustion of benzene, toluene,
xylene or mixture in air in a Pyrexâ jar as shown in Fig. 1.
As-synthesized sample ($1.70 mg) was dispersed in 20 ml
chloroform using sonication and then further diluted with
chloroform to have $6.8 mg/l concentration. This dispersed
solution was used for the measurement of photolumines-
cence at the excitation wavelength range of 250–420 nm.
Additional solvents such as ethanol, xylene or hexane were
also used to prepare a well dispersed solution with the sam-
ple. The photoluminescence spectrum was measured by a
PTI QuantaMasterTM 30 Fluorescence Spectrophotometer (Bir-
mingham, NJ). The quantum yield was calculated by using
POPOP dye as a standard reference. BET surface area wasdetermined on ASAP 2020 (Micromeritics) by N2 adsorption
at 77 K. TEM images were recorded using a JEOL Company
Instrument (Model: 2100) operating at 200 kV. Sample prepa-
ration for the imaging was done by ultrasonically dispersing
the particles in ethanol, and using the solution, the particles
were deposited on a copper grid coated with carbon film.
The morphology of each sample was determined by using a
Zeiss-LEO model 1530 variable-pressure field-effect scanning
electron microscope, and the dried sample for SEM was pre-
pared by coating each sample on carbon conductive grid. An
as-synthesized sample was sent to the Galbraith Laboratory
for carbon, oxygen and hydrogen analysis.
3. Results and discussion
Large quantities of carbon nanoparticles (CNPs) were col-
lected from the wall of the glass container. Fig. 2a, b and d
shows the TEM of the as-produced carbon nanoparticles col-
lected from the container wall. One can see that the sample
consisted of aggregated nanoparticles having flat and round
morphology with relatively regular size as also supported by
SEM result shown in Fig. 2c. Based on statistical analysis of
several samples, the average size of the nanoparticles was
found to be approximately 50 nm. Fig. 2d’s TEM image shows
a flat and round structure. Yan et al. reported the synthesis of
carbon nanoparticles 3–6 nm, carbon onions 30–80 nm and
carbon nanoropes using commercial mesophase pitch as
carbon precursor and a block copolymer P123 through solu-
tion phase synthesis below 200 °C [15]. To the best of ourknowledge, such small CNPs (carbon nanoparticles) of ca.
50 nm size having flat structure have not been reported as
of today. Based on elemental analysis of our CNP sample [Gal-
braith Laboratory], it is consisted of mainly carbon with small
amount of hydrogen and oxygen presence (C:H:O = 94.9:
2.4:2.8 by weight%). The resulting CNPs might be synthesized
by the assembly of the aromatic molecules in liquid. At the
same condition, the confined combustion of non-aromatic
hydrocarbon compound such as hexane was found to
produce only a small amount of amorphous carbon. Thus,
our hypothesis is that the p–p stacking interaction among aro-
matic compounds in liquid resulted in the formation of the
flat nanoparticles via a self-assembly and polymerization. Ithas been reported that carbon nanoparticles has to be sur-
face-passivated in order to become highly photoactive having
strong photoluminescence in the visible and infrared spectral
region [2]. Our as-prepared CNPs were insoluble in water so
they are relatively hydrophobic in nature. However, when
the sample was sonicated in ethanol, it was dispersible and
filterable through 0.45 lm filter. The filtrate shows blue color
fluorescence under UV exposure.Fig. 3a shows the photolumi-
nescence with peak intensity at 475 nm with the excitation
wavelength at 325 nm. Our CNPs do not need to be passivated
to be photoactive. Luo et al. reported that in order to have
fluorescence from carbon material, it needs to have oxygen
atoms in carbon material [21]. Elemental analysis of ourCNP sample showed that it has oxygen, and FTIR also shows
that it has C–O group corresponding to 1100 cmÀ1 and C@O
group corresponding to 1720 cmÀ1 absorption peaks. A broad
photoluminescent peak can be seen at 490 nm. Our CNPs, in
addition to fluorescing in the visible light, are essentially
non-photobleaching and retaining their fluorescence indefi-
nitely. Even quantum dots, which exhibit longer lifetimes,
will ultimately photobleach. However, our CNPs have very
stable photoemission over many hours under UV light with
quantum yield of 12–13%. This property would allow one to
modulate this emission for sensor application. The CNPs re-
main stable for many hours even under high illumination
conditions, i.e., they do not exhibit the blinking compared
Fig. 1 – Synthesis of carbon nanoparticles (CNPs).
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with most fluorophors. Fig. 3b shows that the photoemission
of carbon nanoparticles shifts in different media. One way to
explain this emission shift is the solvatochromism, which is
the change in nanoparticle photo emission due to a change
in solvent polarity. We found that in more polar solvent, such
as in ethanol, carbon nanoparticles exhibit a blue shift. Fig. 4
shows the Raman spectrum of as-produced carbon nanopar-
ticles. The presence of the typical G-band at 1594 cmÀ1 and
D-band at 1347 cmÀ1 usually correspond to the E2g nodes of
graphite and disordered graphite, respectively. The intensity
ratio [ID /IG], which is often used to correlate the structural
purity of graphite, also indicates that the carbon nanoparti-
cles are composed of mainly nanocrystalline material [3,20].
Thermal gravimetric analysis (TGA) shows a significant ther-
mal stability consistent with the formation of carbon materi-
als (Fig. 5). Graphene is stable in air until 600 °C and at higher
temperature, it oxidizes to CO2. Our CNPs begin to oxidize at
550 °C, approximately 50 °C lower than graphene. Owing to its
nanometric size, our CNPs have a specific surface area of
44 m2 /g. The pore structures were analyzed further by the
Fig. 2 – TEM images: a, b & d and SEM image: c.
Fig. 3a – Photoluminescence spectrum of CNPs. Photograph
of CNPs in ethanol solution under UV light (excitation peak
at 325 nm).
Fig. 3b – Photoluminescence spectra of CNPs in different
solvents (excitation peak at 325 nm).
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BJH (Barret–Joyner–Halenda) method and t-plot method from
N2 adsorption–desorption isotherm. The results of the BET
and analysis of our CNPs are summarized in Table 1. The
shape of the adsorption isotherm on the CNPs as shown in
Fig. 6 suggested that there were macro pores in addition to
micro pores.
4. Summary
We reported new photoluminescent carbon nanoparticles
(CNPs) having an average size of 50 nm. The CNPs were found
to emit strong fluorescence in the visible with peak intensity
at 475 nm when excited with a wide wavelength range with
peak excitation at 325 nm with 12–13% quantum yield. It is
postulated that CNPs become photoluminescence due to the
oxygen presence. Its presence was confirmed by FTIR and
element analyses. We will also measure other physical prop-
erties of the CNPs to find their potential applications in
bio-imaging, electrodes of fuel cell and super-capacitor, and
catalyst support.
Acknowledgment
The authors would like to thank the Department of Chemistry
and the AG MacDiarmid NanoTech Institute at the University
of Texas at Dallas for providing support.
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Table 1 – Characteristics of carbon nanoparticles samples byN2 BET.
BET surface area (m2 /g) 44BJH desorption cumulative volume of poresbetween 1.7 nm and 300 nm diameter (cm3 /g)
0.046
BJH desorption average pore diameter (4 V/A) nm 8.66
P/P 0
0.0 0.2 0.4 0.6 0.8 1.0
Q u a
n t i t y A d s o r b e d ( c m 3 / g S T P )
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(a.u.)
Evaluation Copy
Fig. 4 – Raman spectrum of as-synthesized CNPs.
Fig. 5 – TGA analyses.
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