The adsorption of ozone on B80 and endohedral Be@B80 fullerenes is ascertained with density functionaltheory. The potential energy curve of ozone adsorption on pristine and endohedral B80 fullerenes isinvestigated. Obtained data indicate that the behavior of O3 adsorption on investigated fullerenes canbe better described by Corrected-Morse potential equation. On the basis of results, the ozone can beadsorbed onto the outer surface of pristine B80 molecule with adsorption energy of −327.34 kJ mol−1.The tangible adsorption energy of O3 onto the B80 and relatively bond length of B O (∼1.45 A) in B80/O3
system imply that this structure is of highly stability. Compared the calculated adsorption energy of O3
adsorbed on Be@B80 system (−498.87 kJ mol−1) with pristine B80, indicate that when the metal atomencapsulated into the B80 fullerene, the adsorption of ozone on Be@B80 is more favorable than pristineB80. Also, the presence of Be metal atom can be improved the oxidation process of B80 fullerene. Duringthe oxidation process of Be@B80 fullerene using ozone, the Be metal atom translated from the center towall of the B80 and strongly bonded to the boron atoms of inner walls. Based on our results, it seems thatozone tends to be chemisorbed onto the B80 and Be@B80 fullerenes with appreciable adsorption energy,
whereas the Be@B80 fullerene is more favorable than pristine B80. Furthermore, due to the disappearedsome energy level near the LUMO and decreased the Eg, the electrical conductance of the B80/O3 andBe@B80/O3 systems are increased. In conclusion, pristine B80 and Be@B80 fullerenes can be converted thepresence of O3 molecule directly to an electrical signal, and therefore, it can be potentially used as ozonesensor.
The discovery of C60 fullerene has motivated significant interestn the field of hollow carbon clusters or fullerenes. As boron andarbon are neighbors in the periodic table, it is expected that boronould also form nano cluster similar to carbon [1,2]. Boron is nonetallic element and is electron-deficient, possessing a vacant p-
rbital. Therefore, it tends to form sp2 hybridization like carbon andtrong covalent bond with other elements. Because of its electron-eficiency, the bonding features in boron clusters are very diverse3]. Unlike carbon, bulk boron cannot be found in nature and allnown boron allotropes where obtained in the laboratory. All ofhem are based on different arrangements of B12 icosahedrons. The
xistence of pristine boron clusters was predicted recently [4]. Also,he boron clusters, nanotubes, and sheets are activity prospected inecent year [2]. Based on computational study, Boustani offered an
“Aufbau principle”, stating that the most stable boron clusters canbe constructed using two basic units: pentagonal and hexagonalpyramids B6 and B7, respectively [5]. At small cluster size, all of thetheoretical and excremental studies indicate that (quasi-)planarisomers are most stable [6–9]. It is very interesting to note thatresearchers from Brown, Shanxi and Tsinghua University in Chinahave shown that a cluster of 40 boron atoms forms a hollow molec-ular cage similar to a carbon buckyball. It’s the first experimentalevidence that a boron cage structure previously only a matter ofspeculation does indeed exist [10]. On the computer, Wang’s col-leagues modeled over 10,000 possible arrangements of 40 boronatoms bonded to each other. The computer simulations estimatenot only the shapes of the structures, but also estimate the elec-tron binding energy for each structure a measure of how tightly amolecule holds its electrons. Swzacki et al. investigated the stabilityof a few boron fullerenes in the size range 12–110 atoms [4]. They
obtained that the symmetric B80 fullerene is more stable than oth-ers. The B80 molecule is energetically favorable and symmetricallyalmost icosahedral and has characteristics similar to B12 [11]. TheB80 molecule contains three basic motifs: hollow pentagon, hollow
Z. Mahdavifar, M. Poulad / Sensors and Actuators B 205 (2014) 26–38 27
Table 1Electronic properties such as highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), gap energy (Eg), electronic chemical potential (�),hardness (�), softness (S), electrophilicity (ω) and structural parameters of pristine ozone molecule and B80 molecule calculated by PBEPBE/6-31G(d) method (atom numberingis according with Fig. 1Sa and b).
HOMO (eV) LUMO (eV) Eg (eV) � (eV) � (eV) S (eV−1) ω (eV)
exagon, and filled hexagon, respectively [2]. In general, the B80ullerene can be constructed from C60 fullerene by replacing car-on atoms with boron atoms and adding 20 extra boron atoms athe center of 20 hexagons [9]. It should be noted that like C60, B80lmost has Ih symmetry; even though Szwacki et al. reported an ico-ahedral shape for B80 fullerene [4]. Later, Gopakumar et al. [12,13]nd Baruah et al. [14] investigated the stability of B80 fullerene andhey claimed that the icosahedral shape is unstable. They found thathe ground state configuration of B80 has Th symmetry. Sadrzadeht al. [15] also studied the symmetry of B80 fullerene and obtainedhat a very small energy differences between three Ih, Th, and C1somers with the icosahedral shape [16]. The geometry, physicalnd chemical properties of B80 fullerene are studied extensivelynd potential application are already forecasted.
Since fullerenes were prepared in 1985 [17], various methodsave been devised to put atoms and small molecules inside theirollow cages. It should be mentioned that the first endohedral
ullerene was synthesized in 1985 by Smalley et al. [18]. Endohedralullerenes have many potential applications in energy storage, gasdsorption, nanoelectronics, drug delivery, and radioactive traces.any experimental and theoretical studies investigated the stabil-
ty of endohedral carbon fullerenes [19–24]. In the case of boronullerenes, theoretical studies predict the stability of Fe@B80 andi@B80 [25,26] endohedral fullerene. Muya et al. [27] investigated
he encapsulation of small base molecules in the B80buckyball. Theyound that the stability of the complexes will be governed by theize and electron donating character of the encapsulated clusters.lso, Jin et al. demonstrated that the Sc3N@B80 and La@B80are sta-le [28] and they found that the electronic structure of B80 fullerene
s depending on the metal and its position in B80 fullerene. Likearbon fullerenes, boron fullerenes are also used as a substrate forransition metal [29,30] and alkali metal atom [31,32] doping inydrogen storage and as adsorbent for the adsorption of small gasolecule such as NH3 [33] and CO2 [34] applications.Just as know stratospheric ozone (O3) is obstacle to arrive UV
njurious radiation of sun to the earth and so this gas is necessary forife [35–37]. However, it is appearing that ozone has two differentaces because the presence of this gas in troposphere is very danger-us [36,38–40]. This pollutant is generated in the days with brownog, high temperature and low humidity, when said that the pho-ochemical smog is accomplished. Since ozone is a strong oxidant38,41] it could injure to live organism. In recent years, Scientifics
ay attention to utilization of nano particles such as nanotubes andullerenes for the adsorption of ozone. Kim et al. [42] have shownhat ozone can provide a facile route to functionalize basal planef highly oriented pyrolytic graphite (HOPG) leading to uniform
Fig. 1. Potential energy curves of O3 adsorption on (a) B80 and (b) Be@B80 fullerenesusing PBEPBE/6-31G(d) level of theory.
growth of Al2O3 by atomic layer deposition (ALD). The oxidationof SWNTs by ozone molecule has been considered experimentally[43–48] and theoretically [49–56]. Also, ozone interaction withmulti walled nanotubes has been investigated by several groups[57,58]. Furthermore, Bil et al. [59] have been performed ab-initiomolecular orbital and molecular dynamics calculations to verifythe influence of an endohedral noble gas atom on the reactiv-
ity of the surface of the C70O3. In the case of ozone adsorption,Lee et al. [60] investigated ozone adsorption on graphene usingthe ab initio density functional theory method. They found thatthe ozone molecules adsorb on the graphene basal plane with a
28 Z. Mahdavifar, M. Poulad / Sensors and Actuators B 205 (2014) 26–38
Fig. 2. Typically fitted potential energy curve for adsorption of O3 on B80 fullerene with (a) Morse, (b) Lenard-Jones and (c) Corrected-Morse potential equations.
Table 2Calculated parameters of Mores and Corrected Morse potentials using PBEPBE/6-31G(d) level of theory for different configurations of O3 adsorption on different sites of B80
fullerene.
Configuration Site Equation D (kJ mol−1) � a (kJ mol−1)
B80/O3
�-mode Pentagonal ring Morse 12.32 6.90 –Pentagonal ring True Morse 15.82 6.36 6.16
Be@B80/O3
H-mode Pentagonal ring Morse 16.62 6.52 –
bim
frmihdhstoc
afofo
Pentagonal ring True Morse
inding energy of 0.25 eV, and the physisorbed molecule can chem-cally react with graphene to form an epoxide group and an oxygen
olecule.Currently, gas sensor with optimized features such as low cost,
ast response, high gas selectivity, high sensitivity and small size areequired. A standard method for ozone detection is a UV adsorptionethod. Although this method is reliable and has a high sensitiv-
ty, it has disadvantage in the complexity of the apparatus withigh cost and large detector size. On the other hand, the ozoneetection based on metal oxide thin film such as ZnO, WO3 [61]ave advantage such as compact size and high sensitivity. Theseensor devices also have limitation due to their high operationalemperature. In order to overcome these disadvantages, studiesf ozone detection with B80 and Be@B80 gas sensor have beenonducted.
To our knowledge, a few works have been done on the inter-ction of ozone molecule on the fullerenes especially boron
ullerenes. Therefore, we have performed density functional the-ry (DFT) calculations to investigate the ozone oxidation of B80ullerene and ascertain the influence of metal atom on the ozonexidation of B80 fullerene.
17.89 6.35 2.25
2. Computational method
The geometry optimizations, stabilities, electronic properties,and natural bond orbital (NBO) analysis of B80, B80/O3 andBe@B80/O3 systems are computed with spin-polarized general-ized gradient approximation (GGA). The Perdew–Burke–Ernzerhof(PBEPBE) functional [62] and a conventional 6-31G(d) basis setfor all atoms is chosen. The density functional theory is car-ried out using Gaussian03 [63] package. We first determinedthe site that ozone molecule could be adsorbed onto the B80and Be@B80 surfaces. There are four possible adsorption sites ofB80 fullerene such as (a) above center of pentagonal ring (P-site), (b) above center of hexagonal ring (H-site), (c) the edgebetween hexagonal and pentagonal rings (T1-site), and (d) the edgebetween two hexagonal rings (T2-site) are considered (see Fig.S1 in Supplementary materials). Furthermore, three different ori-entations of ozone molecule on B80 surface are investigated: (a)
central oxygen atom of ozone is up (�-mode), (b) central oxy-gen atom of ozone is down (V-mode) and (c) ozone moleculeis horizontally (H-mode) lying above the hexagonal or pentago-nal rings. The various modes of ozone could be adsorbed on B80
Z. Mahdavifar, M. Poulad / Sensors and Actuators B 205 (2014) 26–38 29
n on pristine B80 fullerene using PBEPBE/6-31G(d) level of theory.
ft
ndeho
�
�
wn
ot
ω
i
3
3
gtgs∼[fp(
Table 3Calculated nearest intermolecular distance (Å), Wiberg bond indices and partialcharges of atoms of ozone molecule adsorbed onto the B80 and Be@B80fullereneusingPBEPBE/6-31G(d) method (atom numbering is accordance with Figs. 3 and 7).
Fig. 3. Fully optimized geometry of ozone molecule adsorptio
ullerene are plotted in Fig. S2 in Supplementary materials sec-ion.
Using successive derivatives of energy with respect to either theumber of electrons or the external potential, both local and globalescriptors have been defined. For an N-electron system with a totalnergy (E) and external potential v(r), electronegativity (�) [64] andardness(�) [65] are defined as the following first order and secondrder derivatives, respectively:
= −(
∂E
∂N
)(r),T
= −� (1)
= 12
(∂2
E
∂N2
)(r),T
= 12
(∂�
∂N
)(r),T
(2)
here � is the electronic chemical potential that is defined as theegative of the electronegativity.
Parr et al. [66] have proposed electrophilicity index (ω) in termsf the chemical potential and chemical hardness as a measure ofhe electrophilic power of a molecule as:
= �2
2�(3)
Using the Janak’s approximations, this relation for electrophilic-ty has the simple forms of Eq. (3).
. Results and discussion
.1. Adsorption of O3 on B80 fullerene
Fig. S3 in Supplementary materials show the fully optimizedeometries of B80 fullerene in the framework of density functionalheory using PBEPBE/6-31G(d) level of theory. The optimizedeometry of B80 was found to be asymmetric geometry with C1ymmetry. The calculated B B bonds of B80 fullerene is obtained1.725 A, which are in well agreement with other researches
4,12]. The natural bond orbital (NBO) calculations are also per-ormed to obtain electronic properties of B80 and O3 such asartial atomic charge and bond order using entitled methodsee Table 1). The calculated HOMO (highest occupied molecular
B(6) – −0.216Be – 0.922
orbital)–LUMO (lowest unoccupied molecular orbital) energy gapof B80 fullerene is obtained about 1.01 (eV) that is in accordancewith Szwacki et al. results [4,12]. In the case of ozone, the bondlengths and HOMO–LUMO energy gap are obtained 1.33 A and
1.67 eV, respectively.
In order to investigate the interaction of ozone with pristine B80fullerene, the potential energy curve of O3 molecule adsorbed onto
30 Z. Mahdavifar, M. Poulad / Sensors and Actuators B 205 (2014) 26–38
Fig. 4. Calculated partial charge distribution of (a) B80, (b) B80/O3, (c) Be@B80
Table 4Adsorption energy (Eads), interaction energy (Eint) and deformation energy (Edeform)of O3 adsorbed on the B80 and Be@B80 fullerenes using PBEPBE/6-31G(d) method.
Type Eads (kJ mol−1) Eint(kJ mol−1) Edeform (kJ mol−1)
able 5ighest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO)
ω) of B80/O3, Be@B80, Be@B80/O3systems calculated by PBEPBE/6-31G(d) method.
HOMO (eV) LUMO (eV) Eg (eV)
B80/O3 −4.93 −4.50 0.43
Be@B80 −5.18 −4.15 1.03
Be@B80/O3 −5.03 −4.70 0.33
and (d) Be@B80/O3 fullerenes using PBEPBE/6-31G(d) level of theory.
the B80 surface is calculated (Fig. 1a). As we know the depth ofwell is a measure of the strength of binding to the surface-in factit is a direct representation of the adsorption energy, whilst thelocation of the global minimum on the horizontal axis correspondsto the equilibrium bond distance (re) for the adsorbed molecule
on this surface. As can be seen in Fig. 2a, the value of adsorptionenergy is negative, and since it corresponds to the energy changeupon adsorption which can be represented as Eads. However, it isalso often found that the depth of this figure well associated with
, energy gap (Eg), electronic chemical potential (�), hardness (�), and electrophilicity
Z. Mahdavifar, M. Poulad / Sensors and Actuators B 205 (2014) 26–38 31
Fig. 5. Total density of state (DOS) spectra of (a) (a) B80 (b) B80/O3 fullerenes using PBEPBE/6-31G(d)level of theory.
3 rs and Actuators B 205 (2014) 26–38
tcB
tMts
U
U
U
w2bCmtstapMrtaftw
a
E
wtlnptrbwf
E
E
E
war
fcr(aBm
ti
2 Z. Mahdavifar, M. Poulad / Senso
he enthalpy of adsorption, Hads. In this particular case, there islearly no barrier to be overcome in the adsorption of O3 on B80 ande@B80 fullerene.
Next, the obtained potential energy curves are also fitted withhree model potentials such as Lenard-Jones, Morse and Corrected-
orse potentials (Eqs. (4)–(6), respectively), in order to evaluatehe interaction potential of O3 molecule adsorbed onto the B80urface.
i = 4ε
[(�
ri
)12−
(�
ri
)6]
(4)
i = 2D[x2 − 2x
], x = exp
(−�
2
(ri
re− 1
))(5)
i = 2D[x2 − 2x
]+ a, x = exp
(−�
2
(ri
re− 1
))(6)
here ri is the intermolecular distance of O3 from B80 surface,D, re, and � denote the dissociation energy, the equilibriumond distance, and the adjustable parameter and also a is theorrected-Morse potential, respectively. The parameters of theseodel potentials are listed in Table 2. These results could be applied
o perform the pairwise potential energy between two non-bondedystems which expressed as a function of intermolecular separa-ion. Also, it may be used in the molecular simulation study of ozonedsorption onto the nanocage. Fig. 2 shows the typically fittedotential energy curves with Lenard-Jones, Morse and Corrected-orse potentials. These curves show the typical features of the
eal intermolecular interactions and reflect the salient features ofhe real interactions in general way. These potential energy curveslso provide a reasonable description for the properties B80/O3ullerene. As can be seen in Fig. 2, the fitted potential curves showhat the attractive and repulsive portions have more correlationith Corrected-Morse potential.
In continue, to investigate the stability of formed complexes, thedsorption energy of B80/O3 system (Eads) is considered as below:
ads = E(B80/O3) −(
EB80 + EO3
)(7)
here E(B80/O3) is the total energy of B80/O3 system, EB80 is theotal energy of pristine B80, and EO3 is the total energy of the iso-ated ozone molecule. Note that, according to this equation, theegative adsorption energy imply to formed stable complex andositive adsorption energy belongs to the local minimum in whichhe adsorption of ozone onto the fullerene is prevented by a bar-ier. It should be noted that the adsorption energy encompassesoth interaction (Eint) and deformation (Edef) energy contributions,hich are both occurred during the adsorption process. Hence, the
ollowing equations are applied to calculate these contributions:
ads = Edef + Eint (8)
int = EB80/O3−
(EB80 in complex + EO3 in complex
)(9)
def = Edef O3+ Edef B80
(10)
here EB80 in complex; EO3 in complex is the total energy of B80; O3nd Edef B80
; Edef O3is the deformation energy of B80; O3 in its
elaxed geometry.We have explored an O3 molecule adsorbed onto the B80 sur-
ace. Different adsorption sites of B80 molecule such as (a) aboveenter of pentagonal ring (P-site), (b) above center of hexagonaling (H-site), (c) the edge between hexagonal and pentagonal ringsT1-site), and (d) the edge between two hexagonal rings (T2-site)re considered. Our calculations prove that the most stable site of80 for O3 adsorption is (P-site) (see Table S1 in Supplementary
aterials).To further examine the effect of O3 configuration on the adsorp-
ion process, the different configurations of O3 molecule are alsonvestigated. For this object, ozone molecule in three situations are
Fig. 6. Typically contour plots of (a) HOMO and (b) LUMO of B80/O3 system calcu-lated by PBEPBE/6-31G(d)level of theory.
located on the B80 surface such as: (a) central O atom up (�-mode),(b) central O atom down (V-mode) and (c) O3 horizontally (H-mode)located above the hexagonal or pentagonal rings of B80 fullerene(see Fig. S2). Results indicate that the most negative adsorptionenergy is related to �-mode of O3 molecule on top of the center ofpentagonal ring (see Table S1). In conclusion, the most stable site
of B80 for O3 adsorption is pentagonal site and the best orientationof O3 molecule is �-mode. Therefore, we only investigated the sta-bility, electronic and structural properties of above configurationin the relaxed geometry. For this purpose, the �-mode of ozone
Z. Mahdavifar, M. Poulad / Sensors and Actuators B 205 (2014) 26–38 33
ne an
miFBbastpbombttT
afasa
Fig. 7. Fully optimized geometry of (a) endohedral Be@B80 fullere
olecule is located above pentagon ring of B80 surface and thent is fully optimized. As can be seen in the relaxed geometry (seeig. 3), the O(1) and O(3) atoms of ozone bonded to the B(1) and(2) atoms of B80 surface with bond length about 1.45 A. It shoulde noted that the new pentagon ring including B(1), B(2), O(1), O(2)nd O(3) atoms is formed when the ozone adsorbed onto the B80urface (see Fig. 3). During the ozone adsorption on B80 surface,he bond lengths of B B and O O are increased compared withristine components. When O3 adsorbed onto the B80 surface theond length of O O is increased from 1.28 to 1.45 A as well as thef B(1) B(2) bond is increased from 1.73 to 2.08 A. It should beentioned that in this situation the new B(1) O(2) and B(2) O(3)
onds with bond lengths about 1.45 A are formed. It is noteworthyhat the O3 molecule significantly changed the structural proper-ies of B80 fullerene (atom numbering is accordance with Fig. 3).he calculated structural parameters are collected in Table 3.
To investigate the stability of B80/O3, the adsorption energy islso considered. The calculated adsorption energy of O3 on B80 sur-
ace was found to be −327.34 kJ mol−1 (see Table 4). The strongdsorption of O3 on B80 and relatively bond length of B O in B80/O3ystem imply that this structure is of highly stability. Due to the highdsorption energy and bond length of B O, we can conclude that
d (b) Be@B80/O3 fullerene using PBEPBE/6-31G(d) level of theory.
the adsorption of O3 on B80 is chemisorption and the reaction wasvery facile. Furthermore, our calculations accordingly to PBEPBE/6-31G(d) have shown that the B80 fullerene react with ozone andit can be oxidized by ozone molecule. It is noteworthy that thechemisorption of O3 on B80 fullerene is in well agreement withresearch done on the oxidation of single walled carbon nanotubeusing ozone molecule [4,49,67].
The monitoring mechanism of O3 molecule on B80 surface isrelated to the change in the geometry of B80/O3 system due to thedeformation energy and electronic band structure due to the partialelectron charge transfer between ozone molecule and B80 fullerene.Therefore, the deformation energy, Natural Bond Orbital (NBO)analysis of pure B80 fullerene and B80/O3 system are considered. Thecurvature in the geometry of B80/O3 system is investigated by com-paring the relaxed geometry of B80/O3 with pristine fullerene. Thedeformation and interaction energies of B80/O3 system are calcu-lated using Eqs. (9) and (10) (Table 4). According to these results, it isclear that the curvature in the geometry of B80 is occurred when O3
molecule adsorbed onto the B80 surface. The value of deformationenergy for B80/O3 system is ∼244 kJ mol−1 while the contribution ofinteraction energies for O3 adsorption on B80 is ∼−571.6 kJ mol−1,which indicate that O3 molecule can be strongly chemisorbed onto
34 Z. Mahdavifar, M. Poulad / Sensors and Actuators B 205 (2014) 26–38
llerene
t(cct
t(idNt
BOOrt00wia
cTscTO
Fig. 8. Typically fitted potential energy curve for adsorption of O3 on Be@B80 fu
he B80 surface. The considerable amount of deformation energy244 kJ mol−1) indicates that the geometry of B80 is significantlyhanged when ozone adsorbed onto the fullerene surface. In con-lusion, B80 can be acted as sensor for O3 molecule because it fulfillshe mechanism of the sensing condition.
In order to investigate the electronic properties of B80/O3 sys-em, NBO calculations are also studied. The Wiberg bond indexWBI) which demonstrate the strength of the covalent character,s also considered. The WBI [68,69] comes from the attaint of theensity matrix in the orthogonal natural atomic orbital based on theBO analysis. The WBI closely relates to the bond order character,
he larger WBI implies to stronger covalent character.NBO analysis indicates that the bond orders of B(1) O(2) and
(2) O(3) are about 0.69. In other words, the total bond order of3 molecule on B80 surface is 1.38. These results suggest that the3 molecule strongly bonded to the B80 surface. In addition, this
esult is in well accordance with result of adsorption energy. Onhe other hand, the bond order of O O is decreased from 1.33 to.92 as well as the bond order of B(1) B(2) of B80 is decreased from.71 to 0.36 (see Tables 1 and 3). Obtained data are in agreementith results of bond lengths. As we know, when the bond length
s increased the bond order will be decreased (atom numbering isccordance with Fig. 3).
Next, the electron population analysis reveals that considerableharge transfer is occurred during the O3 adsorbed on B80 fullerene.he partial charges of atoms obtained from the NBO calculations are
ummarized in Table 3. It is clear from Table 3 that the considerableharge transfers take place to the O3 molecule from B80 fullerene.he calculated partial charge of atoms show the charges of the O(1),(2) and O(3) atoms (atom numbering is depicted in Fig. 3) of O3
with (a) Lenard-Jones, (b) Morse and (c) Corrected-Morse potential equations.
molecule are changed. For example the partial charges of O(1) andO(2 or 3) is changed from 0.24 esu to −0.005 esu and −0.12 esu to−0.37 esu, respectively. Furthermore, the partial charges of B(1),B(2) atoms of B80 are increased from −0.03 to ∼0.33 esu. Theseresults indicate that the strong electrostatic interaction betweengas molecule and B80 is occurred. It seems that the considerablecharge transfers from B80 fullerene to O3 molecule are take place.Fig. 4a and b shows the partial charge distribution on B80 and B80/O3fullerenes.
In order to verify the effects of the adsorption of O3 on elec-tronic properties of B80 fullerene, total electronic densities of states(DOSs) are plotted using GaussSum [70] package. In DOS plot, Eg
(gap energy) generally refers to the energy difference betweenthe valence (HOMO) and the conduction levels (LUMO) in insu-lators and semiconductors. Therefore, the Eg is one of the mostimportant factors in determining the electrical conductivity of thematerials. The total DOS plot of pristine B80 and B80/O3 systemsare shown in Fig. 5. It is obvious from this figure that significantchange is occurred in the overall feature of DOS plot of B80 fullerenewhen O3 adsorbed onto the B80 surface. As can be seen in the DOSplot of B80/O3 system, some local energy levels near the LUMOare disappeared. On the other hand, compared the DOS spectra ofB80/O3 system with pristine fullerene demonstrate that a dramati-cally mutation in Fermi level during adsorption process is occurred,which imply to change in HOMO–LUMO gap energy (Eg). Therefore,the Eg of B80 is decreased from 1.01 eV to 0.43 eV when O3 adsorbed
onto the B80 fullerene (see Tables 1 and 5). Due to the disappearedsome energy level near the LUMO and decreased the Eg, the elec-trical conductance of the B80/O3 system is increased. According tothis result, it can be concluded that B80 fullerene can be converted
Z. Mahdavifar, M. Poulad / Sensors and Actuators B 205 (2014) 26–38 35
b) Be@
ttms
atoppmtB
Fig. 9. Total density of state (DOS) spectra of (a) Be@B80 (
he presence of O3 molecule directly to an electrical signal, andherefore, it can be potentially used as ozone sensor. Further-
ore, Fig. 6 shows the HOMO and LUMO contour plots of B80/O3ystem.
The global reactivity indices in the conceptual of the DFT for O3dsorption on B80 fullerene are summarized in Table 5. Accordingo the global reactivity indices, it is found that since O3 adsorbednto the fullerene, the hardness values of B80/O3 is decreased com-ared to pristine fullerene. Note that based on maximum hardness
rinciple (MHP) of Person [71], the system with more hardness hasore stability. Therefore, these hardness value variations suggest
hat through ozone interaction with B80 fullerene the reactivity of80 significantly will be increased.
B80/O3 fullerenes using PBEPBE/6-31G(d) level of theory.
3.2. Adsorption of O3 on Be@B80 endohedral fullerene
In this section, the influenced of a Be atom encapsulated in theB80 cage on the adsorption of ozone molecule is investigated. Forthis purpose, the Be atom located in the center of B80 cage andalso the new configuration is fully optimized without any restric-tion using PBEPBE 6-31G(d) level of theory. As can be seen in therelaxed geometry of Be@B80 presented in Fig. 7a, the Be atom isremained in the center of B80 cage the same as initial configu-
ration. It should be noted that by encapsulation of Be atom ontothe B80 cage, the structural properties of Be@B80 has remainednearly constant comparing with pristine B80 fullerene (seeTables 1 and 5).
36 Z. Mahdavifar, M. Poulad / Sensors and
Fc
frttai
f
ig. 10. Typically contour plots of (a) HOMO and (b) LUMO of Be@B80/O3 systemalculated by PBEPBE/6-31G(d)level of theory.
Like the B80 fullerene, our calculations show that the best siteor O3 adsorption on Be@B80 fullerene is above center of pentagonaling and also it could be found that the best configuration of O3 onop of pentagonal rings of Be@B80 fullerene is H-mode. In continuehe Be@B80/O3 in the above configuration is fully optimized without
ny restriction using PBEPBE/6-31G(d) level of theory (see Table S2n Supplementary materials).
The potential energy curve of O3 adsorption on Be@B80ullerene, the same as O3 adsorbed on B80, is also obtained (see
Actuators B 205 (2014) 26–38
Fig. 2b). In this case, the potential energy curve is fitted withLenard-Jones, Morse and Corrected-Morse potential equation (Eqs.(4)–(6), respectively). Obtained data indicate that the behavior ofO3 adsorption on Be@B80 can be better described by Corrected-Morse potential equation (Fig. 8). This result is also in accordancewith O3 adsorption on pristine B80 fullerene (Table 2).
To better understand the Be@B80/O3 interactions, the adsorp-tion energy (Eads) of ozone onto the endohedral fullerene is alsocalculated as below:
Eads = E(Be@B80/O3) −(
EBe@B80 + EO3
)(11)
where E(Be@B80/O3) is the total energy of Be@B80 complex with anadsorbed O3 molecule, EBe@B80 is the total energy of Be@B80 endo-hedral fullerene, and EO3 is the total energy of the isolated ozonemolecule. The final optimized geometries of Be@B80/O3 system isdepicted in Fig. 7b. It is obvious from Fig. 7b that while ozonemolecule adsorbed onto the outer surface of endohedral fullerene,the metal atom is moved from the center of B80 cage to wall of theB80 fullerene and directly bonded to the inner surface of fullerene(see Fig. 7b). The structural parameters of relaxed geometries ofBe@B80/O3 consist of the nearest intermolecular distance betweenozone molecule and fullerene as well as the equilibrium distancebetween the metal atom and B80 wall are collected in Table 5. Itcould be found from this table and relaxed geometry of Be@B80/O3that the O atoms directly bonded to B atoms with bond lengthsabout 1.45 A. On the other hand, the nearest intermolecular dis-tance of Be atom and B80 wall is obtained ∼1.95 A. Furthermore,the bond length of O O of ozone molecule adsorbed on endohedralfullerene is increased from 1.28 to 1.49 A as well as the bond lengthof B(1) B(2) is increased from 1.73 to 2.78 A (Table 5).
The calculated adsorption energy of O3 molecule on Be@B80 sur-face is obtained about −498.87 kJ mol−1 (see Table 4). This valueindicates that when the metal atom encapsulated into the B80fullerene, the adsorption of ozone onto the Be@B80 is more favor-able than pristine B80. The difference adsorption energy of ozone onBe@B80 and pristine B80 fullerene is obtained about 171.5 kJ mol−1.These results show that the presence of Be metal atom can improvethe oxidation process of B80 fullerene. The strong adsorption of O3onto the Be@B80 and relatively bond length of B O in Be@B80/O3system imply that this structure is of highly stability. Accordingto the high adsorption energy and bond length of B O, it canconclude that the adsorption of O3 on Be@B80 is chemisorptions(similar to the O3 adsorption on B80). To understand the influ-enced of O3 adsorption on the structural and electronic propertiesof Be@B80, the curvature in the geometry of Be@B80 as well as NBOcalculations are also investigated. Since O3 molecule chemisorbedonto the Be@B80 surface, the deformation and interaction ener-gies of Be@B80/O3 system are calculated using Eqs. 9 and 10. Theobtained data are summarized in Table 4. The calculated defor-mation energy value for Be@B80/O3 structure is ∼147 kJ mol−1
while the contribution of interaction energies for O3 adsorptionon Be@B80 is ∼−646.29 kJ mol−1, which indicate that O3 moleculecan be strongly chemisorbed onto the Be@B80 surface. In compar-ison, the curvature in the geometry of Be@B80/O3 is smaller thanB80/O3 system (147 versus 244 kJ mol−1 for Be@B80/O3 and B80/O3,respectively) while the interaction energy of Be@B80/O3 system ismore than that of B80/O3 (see Table 4).
To investigate the electronic properties of Be@B80/O3 system,NBO calculations are also performed. NBO analysis indicates thatthe bond orders of B(1) O(2) and B(2) O(3) are about 0.74 (atomnumbering is accordance with Fig. 7b). On the other hand, the bondorders of O O in O3 is decreased from 1.33 to 0.89 (see Table 3). The
electron population analysis reveals that the considerable chargetransfers take place to the O3 molecule from Be@B80 fullerene.The partial charge of atoms obtained from the NBO calculationsare summarized in Table 3. The calculated partial charge of atoms
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how the charge of the O(1) atom is changed from 0.24 esu (inristine ozone molecule) to −0.02 esu (in the Be@B80/O3) and O(2r3) atom is changed from −0.12 esu (in pristine ozone molecule)o −0.36 esu (in the Be@B80/O3). In addition, the partial chargef Be atom is increased from 0.07 (in Be@B80) to 0.922 esu (ine@B80/O3). Obtained data indicate that the strong electrostatic
nteraction between gas molecule and Be@B80 is occurred. It seemshat the considerable charge transfers from Be metal atom to B80ullerene and from B80 to O3 molecule are occurred. These resultsre in accordance with strong adsorption energies of the Ba@B80/O3ystem. The partial charge distributions on Be@B80 and Be@B80/O3ystems are depicted in Fig. 4.
The total DOS plot of Be@B80/O3 systems is shown in Fig. 9.t is obvious from this figure that significant change is occurredn the overall feature of DOS plot of Be@B80 fullerene when O3
olecule adsorbed onto the endohedral fullerene. Compared theOS spectra of Be@B80/O3 system with Be@B80 demonstrate a dra-atically mutation in Fermi level during adsorption process is
ccurred, which imply to change in HOMO–LUMO gap energy (Eg).he HOMO–LUMO gap energies of considered configurations areummarized in Table 3. Typically contour plots of HOMO and LUMOf Be@B80/O3 are depicted in Fig. 10. As can be seen in Table 5,he energy gap (Eg) is decreased from 1.03 eV (for Be@B80) to.33 eV (for Be@B80/O3) when ozone molecule adsorbed on Be@B80ullerene. These results show the more ability of Be@B80 endohedralullerene than pristine B80 for O3 detecting and it can be betterransform the presence of O3 molecule directly to an electrical sig-al than the pristine B80. In conclusion, for ozone molecule, B80nd Be@B80 conductivity was found to dramatically increase uponzone adsorption. B80 present an extreme sensitivity to chargeransfer and chemical doping effect by the interaction with ozone
olecule. Electrical properties of B80 and Be@B80 are modifiedhen ozone adsorbed and interact with them. Furthermore, our
esults provide a microscopic understanding of the ozonization athe B80 and Be@B80 fullerene.
. Conclusion
In summary, we investigated the adsorption of ozone moleculen pristine B80 and endohedral Be@B80 fullerenes using densityunctional theory. The geometrical structures, electronic propertiesnd natural bond orbital (NBO) analysis are performed. Our resultsndicate that the ozone molecule tends to be chemisorbed ontohe B80 and Be@B80 systems with appreciable adsorption energy,hereas the Be@B80 system is more favorable than pristine B80
or adsorption of ozone molecule. Results of the adsorption energyhow the presence of Be metal atom can improve the oxidationrocess of B80 fullerene. It is noteworthy that the Be metal atomoved from the center to wall of the B80 and it strongly bonded to
he boron atoms of B80 when the ozone molecule adsorbed on thee@B80 system. The NBO analysis as well as DOS spectra for B80/O3nd Be@B80/O3 systems show the adsorption of ozone can be affecthe electronic and structural properties of pristine and endohedral80 fullerenes. Furthermore, it seems that the considerable chargeransfers from B80 and Be@B80 fullerenes to O3 molecule occurreduring the O3 adsorbed onto the B80 and Be@B80 fullerenes.
cknowledgment
This work was supported by the Shahid Chamran University ofhvaz—Iran for their supports in this scientific research
ppendix A. Supplementary data
Supplementary data associated with this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.snb.2014.08.059.
[
[
Actuators B 205 (2014) 26–38 37
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Biographies
Zabiollah Mahdavifar was born in Kazerun, Iran (1978) He received his Ph.D. inComputational& Physical Chemistry under the supervisor of Prof. Amir Abbas Rafatiin 2008 from Bu-Ali Sina University in Iran. His Ph.D. thesis was molecular simula-tion and computational study of gas adsorption on nanotubes. Now, his research isfocused on molecular simulation and computational chemistry with special interestin prediction of novel nanomaterial for rechargeable battery, prediction of nanoma-terial for solar cells and gas sensing, adsorption and separation.
Marziyeh Poulad was born in Shoush, Iran (1981). She completed her B.Sc. inPhysical Chemistry in 2008 from Bu-Ali Sina University and received her M.Sc. inComputational& Physical Chemistry under the supervision of Assist. Prof. ZabiollahMahdavifar from Chamran University in 2013.
