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

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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

http://dx.doi.org/10.1016/j.carbon.2011.10.052

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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).

C A R B O N 5 0 ( 2 0 1 2 ) 1 2 9 8 – 1 3 0 2 1299



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).

1300 C A R B O N 5 0 ( 2 0 1 2 ) 1 2 9 8 – 1 3 0 2



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 )

0

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Fig. 6 – N2 adsorption ( s ) and desorption ( d ) isotherms.

100

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1400

I n t e n s i t y

500 1000 1500 2000 2500

(a.u.)

Evaluation Copy

Fig. 4 – Raman spectrum of as-synthesized CNPs.

Fig. 5 – TGA analyses.

C A R B O N 5 0 ( 2 0 1 2 ) 1 2 9 8 – 1 3 0 2 1301



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photoluminescent cnps produced by conﬁned combustion of aromatic compounds

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