Chemical

Communication

Biosci. Biotech. Res. Comm. 9(4): 712-717 (2016)

Toluene adsorption on C

, Si-doped C

, and C

fullerenes

Mohammad T. Baei

* and Adel Shojaei

Department of Chemistry, Azadshahr Branch, Islamic Azad University, Azadshahr, Golestan, Iran

Department of Physics, Behbahan Branch, Islamic Azad University, Behbahan, Iran

ABSTRACT

T he absorption feasibility of toluene molecule in the C

, Si-doped C

, and C

fullerenes has been studied based on

calculated electronic properties of those fullerenes using Density functional Theory (DFT). It is found that energy of

toluene adsorption upon the pure and Si-doped C

fullerenes were in range of -1.80 and -15.72 kJ/mol with slight

changes in their electronic structure. The results showed that the C

and Si-doped C

fullerenes cannot be used as a

chemical adsorbent or sensor for toluene molecule in nature. Also, silicon doping cannot signi cantly modify both

the adsorption energy and electronic properties of C

fullerene to toluene. On the other hand, toluene molecule

exhibits a high sensitivity upon C

fullerene, so that the energy gap of the fullerene is changed about 91.21% after

the adsorption process. We concluded that the C

fullerene can be served as a reliable material for toluene detection.

KEY WORDS: C

AND C

FULLERENE, TOLUENE, SENSOR, DFT STUDY

712

ARTICLE INFORMATION:

*Corresponding Author: Baei52@yahoo.com

Received 22

Oct, 2016

Accepted after revision 10

Dec, 2016

BBRC Print ISSN: 0974-6455

Online ISSN: 2321-4007

Thomson Reuters ISI ESC and Crossref Indexed Journal

NAAS Journal Score 2015: 3.48 Cosmos IF : 4.006

reserved.

Online Contents Available at: http//www.bbrc.in/

INTRODUCTION

Toluene, a kind of important volatile organic compound

(VOC), is used in many kinds of industries, such as paint-

ing, printing, coating, automotive, and petrochemical

industries. Emission of the toluene from these industries

causes air pollution and the environment, odor prob-

lem,  ammability problem and affects human health.

Due to these toxicological effects, air contaminated with

toluene needs to be treated before it can be released

to atmosphere. Therefore, adsorption and detection of

toluene molecule has high importance in environmental

systems, (Chang et al 2000, Vandenbroucke et al 2011).

In recent years, a wide variety of investigations have

been done upon the adsorption of toluene. For example,

the adsorption of toluene on ZSM5 and mordenite zeo-

lites modi ed with Cs was investigated both theoretically

and experimentally. Toluene removal by oxidation reac-

tion in spray wet scrubber has been also studied. It has

been shown that the highest toluene removal ef ciency

Mohammad T. Baei and Adel Shojaei

was of 91.7%. Their results showed that the sequence to

prepare the catalyst affected the adsorption and plasma

catalytic of adsorbed toluene, (Chungsiriporn et al 2006,

Serra et al 2012 and Qin et al 2016).

After the synthesis of fullerene C60 by Kroto et al

(1985) fullerenes have attracted great interest because of

their physical and chemical properties and applications

in nanomaterials and biomedical science, (Akasaka and

Nagase 2002 and Muthukumar and Larsson 2008). Also,

they play a fundamental role in medical sciences, chem-

istry, biology, materials, electronics, and related  elds

(Senapati et al 2004, Yoon et al 2009, Chamberlain et

al 2011).

Among the smaller fullerenes, C20 and C24 fuller-

enes are a favorable candidate for examining in mol-

ecule electronic devices, nanotechnology, and biomedi-

cal engineering. Liang Xu et al (2012) have studied the

interaction between empty C24 fullerene and the small-

est amino acid (glycine). Their results showed that the

glycine molecule is energetically favorable to interact

on the C24 fullerene through the amino nitrogen active

site. Also, orientation effects on the electronic transport

properties of C24 fullerene were studied by Wen-Kai

Zhao et al (2013) between the electrodes (Au–C24–Au).

Their  ndings showed the application of the C24 fuller-

ene in the  eld of single molecular devices or nanometer

electronics. Prinzbach et al (2000) have synthesized C

fullerene by using C

. They replaced the hydrogen

atoms with bromine atoms, and then debrominated to

produce C

fullerene in gas phase.

An et al (2011) have tried to stabilize the highly per-

fect I

symmetry C

fullerene cage by placing interstitial

atoms at the center of the fullerene using  rst-principles

density functional theory (DFT). They was also investi-

gated the transport properties of C

fullerene and the

endohedral Li@C

metallofullerene coupled to three-

dimensional electrode system using DFT methods . Using

different DFT methods, the transformation processes

from the physisorption state to the chemisorption state

of a H

molecule in C

fullerene and B-doped fullerene

B system was investigated.

In previous study, we have studied the chemical func-

tionalization of C

fullerene with NO

molecule. In sum-

mary, there are few studies on the C20 and C24 fuller-

enes and further study on the structures is of important

duties. On the other hand, the doped C

and C

fuller-

enes show dramatic changes in electronic properties

with respect to their pristine. Therefore; the aim of this

study was to investigate the ability of toluene adsorp-

tion onto C

, Si-doped C

, and C

fullerenes, to deter-

mine whether the fullerenes are applicable for  ltering

or sensing toluene molecule (An et al., 2010, Tian et al

2011 and Baei 2013).

COMPUTATIONAL METHODS

In this study, the adsorption of toluene on the C

, Si-

doped C

, and C

fullerenes are considered. C

fullerene

with a D

d symmetric form consisting of two 6-mem-

bered rings joined by twelve 5-membered rings and C

fullerene with an I

symmetric form consisting of 12

pentagons and 30 bonds are selected for this purpose.

DFT calculations at the level of B3LYP (Becke 1993),

with the standard 6-31G* basis set were carried out on

the C

, Si-doped C

fullerenes and the PBE (Perdew and

Ernzerhof,1996) level in GAMESS package, (Schmidt et

al 1996) with the standard 6-31G* basis set were per-

formed on the C

fullerene. For the C

fullerene, the PBE

functional show better results with respect to B3LYP

method of (Perdew and Ernzerhof, 1996). These meth-

ods were used to calculate the adsorption energy (E

)

of toluene on the surface of the fullerenes using the fol-

lowing equations:

= E

toluene/C24

– [E

C24

+ E

toluene

] +



BSSE

Eq. (1)

= E

toluene/Si-doped C24

– [E

Si-doped C24

+ E

toluene

] +



BSSE

Eq. (2)

= E

toluene/C20–

C20

+ E

toluene

] +



BSSE

Eq. (3)

Where E

toluene/C24

and E

toluene/C20

are the total energy of

complex of adsorbed toluene on the C

and C

fuller-

enes. E

C24

, EC

, and E

toluene

are the total energy of the

pure C

and C

fullerenes and toluene molecule. E

toluene/

Si-doped C24

is the total energy of complex of adsorbed tolu-

ene on the Si-doped C

fullerene. E

Si-doped C24

is the total

energy of Si-doped C

fullerene, and 

BSSE

is the BSSE

correction. The negative values of E

reveal that the

adsorption is exothermic. For the structures, the geom-

etry optimization, natural bond orbital (NBO)

density

of states (DOS), energies, and frontier molecular orbital

(FMO) were calculated according to Glendening et al

(1998). The energy gap (E

) of the optimized structures

was obtained by the energy difference between the high-

est occupied molecular orbital (HOMO) and the lowest

un-occupied molecular orbital (LUMO). Also, Fermi level

energy (E

) and work function (



) of the considered

structures are calculated.

RESULTS AND DISCUSSION

The optimized structure of C

, Si-doped C

, and C

fullerenes are shown in Fig. 1. The C-C bond lengths of

pure C

and C

fullerenes are in the range of 1.36-1.53

and 1.44-1.51 Å. The electronic property analysis based

on DOS shows a HOMO-LUMO gap (E

) of 1.82, 1.63,

and 0.74 eV for the C

, Si-doped C

, and C

fullerenes,

respectively. E

of the C

and C

fullerenes are very

BIOSCIENCE BIOTECHNOLOGY RESEARCH COMMUNICATIONS TOLUENE ADSORPTION ON C

, SI-DOPED C

, AND C

FULLERENES 713

Mohammad T. Baei and Adel Shojaei

close to the results of Liang Xu et al (2012) and Kumar

et al (2011) respectively. Change of E

of the Si-doped

fullerene is about 10.44% with respect to the pristine

model, suggesting that the electronic properties of C

fullerene is not very sensitive on the Si adsorption.

TOLUENE ADSORPTION ON THE C

, SI-DOPED

, AND C

FULLERENES

First, we computed the optimized structures of the indi-

vidual toluene molecule and C24, Si-doped C24, and C20

fullerenes. Then, for investigation of toluene adsorp-

tion on the out surface of the fullerenes, several pos-

sible initial adsorption geometries including the parallel

and perpendicular orientations of toluene molecule close

to the surface of the fullerenes are considered. After

full structural optimization without any constraints,

the three most stable structures are obtained upon the

relaxation processes for toluene adsorption on the C

fullerene which are shown in Figs. 2 and their electronic

properties are shown in Table 1.

In Con gurations A and B in Fig. 2, the hydrogen

atoms of toluene is interacted perpendicular to 5 and

6-membered rings of the C atoms of the fullerene with

the minimum distance of about 3.45 to 3.22 Å, respec-

tively. Calculated E

values of the con gurations are

about -1.80 and -3.22 kJ/mol, respectively and a maxi-

mum NBO charge of 0.01|e| is transferred from the

fullerene to the toluene. The structural parameters of

the con gurations upon the adsorption process remain

unchanged. The results show that the Con gurations

have a weak interaction between toluene molecule and

the C

fullerene. Also, in the con gurations, the in u-

ence of toluene adsorption on the electronic properties

of the fullerene was investigated and almost remains

unchanged. HOMO and LUMO energies, energy gap (E

Fermi level energy (E

), and work function (



) of the

con gurations are shown in Table 1.

The E

in a molecule is approximately middle of E

and



for a semiconductor is de ned as the energy dif-

ference between the E

and the LUMO [24] which is

important in  eld emission applications. The important

sensing mechanisms in nanostructure devices is change

of E

the nanostructure and subsequently change of its

conductivity upon the adsorption process as per Zhou

et al (2010). Therefore, it is very important to calculate

the DOS of the C

fullerene in the con gurations before

and after toluene adsorption. DOS for the con gurations

is shown in Fig. 2. In comparison with the pure model,

FIGURE 1. Optimized Structures of C

, Si-doped C

and C

fullerenes and their density of states (DOS).

FIGURE 2. Different models of toluene adsorp-

tion on the C

fullerene and their density of

states (DOS). Distances are in Å.

714 TOLUENE ADSORPTION ON C

, SI-DOPED C

, AND C

FULLERENES BIOSCIENCE BIOTECHNOLOGY RESEARCH COMMUNICATIONS

Mohammad T. Baei and Adel Shojaei

FIGURE 3. Toluene adsorption and its density of states (DOS) on

the Si-doped C24 fullerene.

their E

values remain almost unchanged (changed by

about 0.55 %). Also, for the con gurations, Fermi level

energy (E

), and work function (



) values remain almost

unchanged (changed by about 1.10 %). The results indi-

cate that the toluene adsorption via these con gurations

has no sensible effects on the electronic properties of the

fullerene.

In Con gurations C in Fig. 2, the hydrogen atoms of

toluene is interacted parallel to 5 and 6-membered rings

of the C atoms of C

fullerene with the minimum dis-

tances of about 3.94 Å. The E

value of the con gura-

tion is about -3.22 kJ/mol. In comparison with the pure

fullerene, its E

, E

, and



values remain almost

unchanged (see Table 1).

The above results show that C

fullerene cannot be a

potential ef cient adsorbent or sensor for adsorption or

determine of toluene from environments systems. There-

fore, to solve this problem, introduced various func-

tional groups and or the doping methods, which enables

chemical covalent bonding between the fullerene and

foreign atoms or molecules. Pure silicon can be doped

with other elements to adjust its electrical response by

controlling the number and charge of current carriers.

Such control is necessary for transistors (Cui and Lieber

(2001), solar cells, semiconductor detectors, and other

semiconductor devices which are used in electronics and

other high-tech applications.

Therefore, doping of C

fullerene by Si atoms may

be able to yield changes in the interactions between the

fullerene and foreign atoms or molecules. Fig. 3 shows

adsorption con guration of toluene molecule and its

density of states (DOS) on the C

fullerene doped with Si

atom. The E

value of the con guration is about -15.72

kJ/mol, which is stronger than that in the pure C

states.

Nevertheless, the adsorption con guration is in the

range of physisorption and cannot be used as potential

ef cient adsorbent for adsorption of toluene. For fur-

ther study, the changes of E

, E

, and



of the con-

 guration is shown in Table 1. The values do not show

notable changes. Therefore, the C

fullerene doped with

Si atom cannot be a potential ef cient sensor for deter-

mine of toluene molecule. Also, Silicon doping cannot

Table 1: Adsorption energy (Ead) of toluene on C

, Si-doped C

, and C

fullerenes, HOMO energies (EHOMO), LUMO energies

(ELUMO), HOMO–LUMO energy gap (Eg), Fermi level energy (EFL), and work function (



) for the studied systems.

Structure Ead(kJ/mol) EHOMO(eV) ELUMO(eV) Eg(eV) aΔEg(%) bQT|e| EFL(eV) (eV) cΔ%

Toluene - -6.40 0.14 6.54 - - -3.13 3.27 -

- -5.64 -3.82 1.82 - - -4.73 0.91 -

Fig. 2A -1.80 -5.68 -3.86 1.82 0.00 -0.01 -4.77 0.91 0.00

Fig. 2B -3.22 -5.68 -3.85 1.83 0.55 -0.01 -4.76 0.92 1.10

Fig. 2C -3.18 -5.57 -3.76 1.81 0.55 0.00 -4.66 0.90 1.10

Si-doped C

- -5.62 -3.99 1.63 - - -4.80 0.82 -

Fig. 3 -15.72 -4.38 -3.02 1.36 -16.53 -0.01 -3.70 0.68 17.04

- -4.45 -3.71 0.74 - - -4.08 0.37 -

Fig. 4A -6.26 -4.49 -3.76 0.73 0.72 0.00 -4.12 0.37 0.00

Fig. 4B -60.16 -4.60 -3.18 1.41 91.21 0.17 -3.89 0.70 91.21

The change of Eg of C

, Si-doped C

, and C20 fullerenes after toluene adsorption

QT is de ned as the total natural bond orbital charges on the toluene molecule (positive values show charge transfer from toluene molecule to the

fullerenes)

The change of work function of C

, Si-doped C

, and C

fullerenes upon toluene adsorption

BIOSCIENCE BIOTECHNOLOGY RESEARCH COMMUNICATIONS TOLUENE ADSORPTION ON C

, SI-DOPED C

, AND C

FULLERENES 715

Mohammad T. Baei and Adel Shojaei

signi cantly improve both the adsorption energy and

electronic properties of C

fullerene to toluene.

In the next step, the in uence of the toluene adsorp-

tion on the electronic properties of the C

fullerene was

studied. After full structural optimization, the two most

stable structures are obtained upon the relaxation pro-

cesses which are shown in Fig. 4. In Con gurations A

and B in Fig. 4, the toluene is interacted perpendicular

and parallel to surface of the fullerene. The E

value of

toluene in Fig. 4A is -6.26 kJ/mol (a weak interaction)

and in comparison with the pure C

fullerene, its E

, E

and



values remain almost unchanged (see Table 1).

The results showed that in this con guration, C

fuller-

ene cannot be a potential ef cient adsorbent or sensor

for adsorption or determine of toluene molecule. How-

ever, the E

value of toluene in Fig. 4B is -60.16 kJ/

mol and also, the results show that the toluene adsorp-

tion through this con guration has sensible effects on

the electronic properties of the fullerene. In this state,

the E

and



values are changed about 91.21% after the

toluene adsorption. The important sensing mechanisms

in nanostructure devices is change of E

the nanostruc-

ture and subsequently change of its conductivity upon

the adsorption process, (Zhou et al 2010). Therefore, the

presence of the toluene molecule can be detected by

computing the conductivity change of the C

fullerene

before and after the toluene adsorption. This behavior

can be explained according to the following equation,

(Li 2006).

Eq. (4)

Where  is the electric conductivity of the structure

and k is the Boltzmann’s constant. According to equa-

tion (4), smaller E

at a special temperature leads to the

larger electric conductivity. Therefore, the considerable

change in E

of the C

fullerene shows the high sensitiv-

ity of electronic properties of C

fullerene towards the

toluene molecule. Also, it is well known that one of the

most important factors in sensor devices is their recov-

ery time () that can be described as:

Eq. (5)

Where k

is the Boltzmann’s constant, T is the tempera-

ture, and 

is the attempt frequency. According to equa-

tion (5), more negative E

will prevent the recovery of

the device. In other words, very strong interactions are

not favorable in sensor devices due to long recovery

times (). However, the E

value of the con guration

is -60.16 kJ/mol (Table 1) that is not too large to hinder

the recovery of the fullerene. The results show the high

sensitivity of the fullerene towards toluene molecule and

can be used as toluene sensor.

CONCLUSION

We have investigated the toluene adsorption on C

, Si-

doped C

, and C

fullerenes using DFT calculations. The

results show that toluene molecule presents a weak phys-

ical adsorption with the pure and Si-doped C

fullerene

and the fullerenes are not a suitable adsorbent for tolu-

ene molecule. In addition, the results suggest that the

pure and Si-doped C

fullerene has low sensitivity to the

presence of toluene and the in uence of toluene adsorp-

tion on the electronic properties of the fullerenes remain

almost unchanged. On the other hand, toluene molecule

exhibits a high sensitivity upon C

fullerene, so that the

energy gap of the fullerene is changed about 91.21%

after the adsorption process. The results show the high

sensitivity of C

fullerene towards toluene molecule and

therefore can potentially be used for toluene sensors.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the  nancial sup-

port received from Islamic Azad University, Azadshahr

Branch.

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BIOSCIENCE BIOTECHNOLOGY RESEARCH COMMUNICATIONS TOLUENE ADSORPTION ON C

, SI-DOPED C

, AND C

FULLERENES 717