First principles study of the adsorption of a NO molecule on N-doped anatase nanoparticles

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Applied Surface Science 258 (2012) 8312–8318

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First principles study of the adsorption of a NO molecule on N-doped anatase nanoparticles Juan Liu a , Qin Liu a , Pengfei Fang a , Chunxu Pan a , Wei Xiao a,b,∗ a b

Department of Physics, Wuhan University, Wuhan 430072, PR China Division of WCU Multiscale Mechanical Design, School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-744, Republic of Korea

a r t i c l e

i n f o

Article history: Received 9 April 2012 Received in revised form 9 May 2012 Accepted 9 May 2012 Available online 17 May 2012 Keywords: N-doped Anatase Nanoparticle Adsorption First principles calculation NO

a b s t r a c t The adsorption of a NO molecule on 72 atom N-doped TiO2 nanoparticles has been studied by first principles calculations. Two types of adsorption are considered in the calculations. In one type of the adsorption, the NO molecule forms one bond with the particle, while in the other type of adsorption, the NO molecule forms two bonds with the particle. The second type of adsorption is more energetic favorable. The adsorption energies, bond lengths, density of the states (DOSs), and the difference of the charge density are calculated to investigate the adsorption. In the adsorption process, the unpaired electron of the NO molecule transfers to the empty state of the particle, making the Fermi levels lower. As a result, the electrons of the N-doped system occupy lower energy states, making the system energy lower than that of the undoped particle. Since the adsorption of a NO molecule on N-doped nanoparticles is stronger than that on undoped particles, N-doped particles can adsorb more NO molecules on their surfaces than the undoped particles do. Meanwhile, there are more adsorption sites on the N-doped particles, on which the adsorption energies are much higher than that of the undoped particle, some of them are even higher than the highest adsorption energy of the undoped particle. It suggests that N-doped particles are more active and they can adsorb more small toxic gas molecules in the air. So, the doping method can be used to remove NO molecules for the air pollution control through the surface adsorption strategy. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The reduction of nitrogen oxides produced by fuel combustion is a challenge in environment protection. Numerous research groups from academic and industrial laboratories invest a lot of effort to investigate it [1]. Catalysts, such as metals (Au, Rh, Pd, and Pt) [2–8] or metal oxides (ZnO, CeO2 , and TiO2 ) [9–13] are used for the air pollution control. Titanium dioxide attracts many scientific interests because of its unique physical and chemical properties. It has been widely used in photocatalysis [14,15], heterogeneous catalysis [16], and photovoltaics [17]. Since the band gap for TiO2 is 3.0–3.2 eV, TiO2 can only absorb 4–5% of the solar spectrum. Doping of TiO2 can introduce doping energy levels inside the band gap which can let TiO2 absorb photons in the visible region [18–21]. Nitrogen doped TiO2 (N-doped TiO2 ) has received many attentions in those doping approaches [22,23]. Since TiO2 nanoparticles have large surface area and they are relatively more open, the particles can accommodate a nitrogen atom easier, and the doping formation energy for particles is lower

than that of the bulk material [24]. The hole introduced by the N doping atom in TiO2 may oxidize toxic gas molecules [24], but the adsorption of toxic gas molecules on the N-doped TiO2 nanoparticles has not been intensively studied. In this work, the adsorption of a NO molecule on N-doped TiO2 nanoparticles is investigated by density-functional theory (DFT) calculations and which is compared with the adsorption on undoped TiO2 nanoparticle. The adsorption energy, bond length, density of state (DOS), and charge density difference are analyzed. When the NO molecules are adsorbed on the surfaces of particles, the unpaired electron of the NO molecule transfers to the empty state, making the Fermi levels lower, and which can lower the system energy. Since the adsorption of a NO molecule on Ndoped nanoparticles is stronger than that on undoped particles, N-doped particles can adsorb more NO molecules than the undoped particles can do. As a result, we can use the doping particles to adsorb more toxic gas molecules and clean the air pollution. 2. Calculation methods 2.1. Calculation detail

∗ Corresponding author at: Department of Physics, Wuhan University, Wuhan 430072, PR China. E-mail addresses: [email protected], [email protected] (W. Xiao). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.05.053

Density functional theory (DFT) [25,26] calculations are carried out to study the adsorption of a NO molecule on a stoichiometric

J. Liu et al. / Applied Surface Science 258 (2012) 8312–8318

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Table 1 TiO2 anatase lattice constants. Compared with the experimental data. a and c are the lattice constants and d is the Ti O bond length.

LDA GGA Expt. [36]

˚ a (A)

c/a

˚ c (A)

˚ d (A)

3.77 3.83 3.78

2.51 2.51 2.51

9.46 9.61 9.50

1.97 2.00 1.98

TiO2 anatase nanoparticle and the N-doped TiO2 anatase nanoparticles. The system energy calculations are performed by using the Vienna ab initio simulation package (VASP) code [27,28]. The projector augmented wave (PAW) method [29,30] is used to describe the potential. Both of the local density approximation (LDA) of Ceperly and Alder parametrized by Perdew and Zunger (CA) [31] and generalized gradient approximation (GGA) parameterized by Perdew, Burke and Enzerof (PBE) [32,33] are used in this work. A plane-wave kinetic energy cutoff of 400 eV is used. In the electronic relaxation calculations, the residual minimization method with direct inversion in the iterative subspace (RMM-DIIS) is used while the energy convergence criterion is set to 10−4 eV. The conjugate gradient method is used to minimize the Hellmann–Feynman forces in the ionic relaxations with the energy convergence criterion of 10−3 eV. Spin-polarization is used in all calculations due to the odd electron number in the N-doped particles. The lattice constants and surface energies for TiO2 anatase are calculated and listed in Tables 1 and 2. The data from both LDA and GGA agrees with the experimental data or other literature data well. It suggests that both of the pseudo-potentials are reasonably accurate to simulate the anatase particles. The nanoparticle energy calculations are performed at the  point. The 72 atom nanoparticle and the 72 atom N-doped TiO2 ˚ nanoparticle are placed in a 20 A˚ × 15 A˚ ×30 Abox which is much larger than the size of the particles. The shortest distance to the neighbor particles is 11.4 A˚ in three directions due to the periodic boundary condition. The larger vacuum layer is sufficient to reduce the interaction between the neighbor particles. The adsorption energy is defined as the following formula: Ead = Eparticle+NO − Eparticle − ENO

(1)

Eparticle+NO is the total energy of the system after adsorption, Eparticle is the energy of the particle without any adsorbed gas molecule, and ENO is the energy of a nitric oxide (NO) molecule. All of the energies are calculated in the same box. If the adsorption energy is negative, the adsorption process is exothermic. 2.2. Structure of nanoparticles For the N-doped anatase particles, the nitrogen atom substitutes an oxygen atom in the TiO2 nanoparticle and introduces a hole in the particle. The empty state may be on the top of the valence band or inside the band gap as an impurity band [24]. The hole in the particle may oxidize the adsorbed small molecules. A 72 atom TiO2 nanoparticle is selected from the previous work [24,34] and a NO molecule is adsorbed on the particle. The structure of the particle is shown in Fig. 1. The front and side view of the 72 atom particle are shown in Fig. 1(a) and (b). N1 and N2 are the two substitutional Table 2 Surface energies for the anatase (0 0 1) and (1 0 1) surfaces. Unit in J/m2 . Surface

(0 0 1)LDA

(0 0 1)GGA

(1 0 1)LDA

(1 0 1)GGA

Present work Lazzeri et al.’s [37]

1.40 1.38

0.95 0.98

1.18 0.84

0.49 0.49

Fig. 1. Undoped TiO2 anatase nanoparticles. (a) Front view of a 72 atom undoped particle. (b) Side view of the particle, N1 and N2 are the two substitutional positions which generate the impurity states while other substitutional positions generate empty state on the top of the valence band. (c) A NO molecule adsorbed on the undoped particle. The NO molecule forms one bond with the dangling O atom on the particle. (d) A NO molecule adsorbed on the undoped particle. The NO forms two bonds with the particle, one with the dangling oxygen atom and the other with the Tia .

positions which generate the impurity states inside the band gap while other substitutional positions generate empty state on the top of the valence band. Two substitutional positions are chosen in the particle corresponding to the two types of doping effects. One doping configuration is that a N atom substitute the O atom at N1 position, and the other is a N atom substitute an O atom in the middle of the particle (see Fig. 2). Although the NO molecule can be adsorbed on all surface oxygen atom of the particle, after adsorption, the NO molecule forms strong chemical bonds with the particle at the dangling oxygen atom adsorption site. The adsorption energies at other sites are not as strong as that at the dangling oxygen site. The absolute values of the adsorption energy on different surface oxygen atoms are shown in Fig. 3. The star-point is the adsorption energy for the dangling oxygen atom adsorption. Consequently, the adsorption on the dangling oxygen atom is mainly studied in this paper. The adsorption of a NO molecule on a particle is shown in Fig. 1(c) and (d). The NO molecule may form one or two bonds with the particle. These two types of adsorptions are studied in this work. 3. Results and discussion 3.1. A NO molecule on a stoichiometric undoped anatase particle 3.1.1. Geometric optimization There exist two adsorption configurations for a NO molecule on an undoped anatase particle. In type A (see Fig. 1(c)), the N atom in the NO molecule forms a N O bond with the dangling O atom in the anatase particle; while in type B (see Fig. 1(d)), except the newly formed N O bond, the O atom in the NO molecule forms a O Ti bond with a Ti atom in the anatase particle. The adsorption energy for the adsorption type A is −1.52 eV by LDA (see Table 3). The bond length of the newly formed Ob Nc bond

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J. Liu et al. / Applied Surface Science 258 (2012) 8312–8318 Table 3 Adsorption energies for a NO molecule on anatase nanoparticles. Unit in eV. Type

Ead (LDA)

Ead (GGA)

Undoped (A) Undoped (B) M1 (A) M2 (A) M1 (B) M2 (B)

−1.52 −1.76 −2.92 −3.16 −3.29 −3.57

−0.65 −0.78 −1.98 −2.51 −2.23 −2.72

Table 4 ˚ Bond lengths for a NO molecule adsorbed on anatase nanoparticles. Unit in A. Type

Fig. 2. A NO molecule on a N-doped anatase nanoparticle. (a) The doping N atom generates an impurity band inside the band gap and the NO molecule forms one bond with the particle, type M1A. (b) Same doping position as (a), the NO molecule forms two bonds with the particle, type M1B. (c) The doping N atom generates a hole on the top of the valence band. The NO molecule forms one bond with the particle, type M2A. (d) Same doping position as (c), the NO molecule forms two bonds with the particle, type M2B.

˚ The bond length of the Tia Ob bond is 1.86 A˚ and that of is 1.39 A. ˚ These two bonds are both longer than the Nc Od bond is 1.18 A. those before the adsorption, for example, they are 1.66 A˚ and 1.16 A˚ before the adsorption, respectively. Because the electrons transfer from the dangling bond of the particle and the NO molecule to the

Tia

Ob

Ob

Nc

Nc

Od

LDA

GGA

LDA

GGA

LDA

GGA

Undoped Non-adsorbed Undoped (A) Undoped (B)

1.66 1.86 2.01

1.68 1.88 2.05

– 1.39 1.29

– 1.44 1.31

1.16 1.18 1.25

1.17 1.20 1.26

N-doped Non-adsorbed M1 (A) M1 (B)

1.72 1.87 2.03

1.73 1.89 2.08

– 1.39 1.28

– 1.43 1.30

1.16 1.19 1.26

1.17 1.19 1.27

Non-adsorbed M2 (A) M2 (B)

1.74 1.82 2.02

1.78 1.85 2.09

– 1.51 1.29

– 1.48 1.30

1.16 1.16 1.25

1.17 1.18 1.27

newly formed N O bond between the NO molecule and the particle, both of the dangling bond of the particle and the N O bond of the molecule are weakened after the adsorption. GGA calculation shows the similar results about the bond length change at the interface (see Table 4). The NO molecule forms two bonds with the particle in configuration B. There exists another Tia Od bond. The adsorption energy is −1.76 eV (LDA) which is lower than that of type A. Although the adsorption energy from GGA is higher than the data from LDA, both of the methods show that the type B is more energetic favorable than the type A (see Table 3). The newly formed Ob Nc bond ˚ which is shorter than that in the configuration A. length is 1.29 A, The Tia Ob bond length is 2.01 A˚ and the Nc Od bond length is

4

a (undoped)

c (N−doped:M2)

b (N−doped:M1)

3.5 3

2

ad

|E | (eV)

2.5

1.5 1 0.5 0 0

2

4

6

8

10

12 0

10

20

30

40 0

10

20

30

40

50

N Fig. 3. The absolute values of the adsorption energy of a NO molecule on different surface oxygen adsorption sites of the anatase particle (from LDA calculations). Star-point is the adsorption energy at the dangling oxygen site. Panel (a): adsorption energies of a NO molecule on undoped nanoparticle. Because of the symmetry, there are only 13 different adsorption sites. Panel (b): adsorption energies of a NO molecule on the N-doped nanoparticle M1. Panel (c): adsorption energies of a NO molecule on N-doped nanoparticle M2.

J. Liu et al. / Applied Surface Science 258 (2012) 8312–8318

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Non−adsorbed

a

adsorbed

a

adsorbed

Non−adsorbed

O

adsorbed

b

b

N

b

DOS

DOS

c

Ob

adsorbed

Nc

c

−14

−12

−10

−8

−6

−4

−2

0

−14

O

d

adsorbed

Tia

−12

−10

−8

−6

Energy (eV)

Energy (eV)

(a)typeA

(b)typeB

−4

−2

0

Fig. 4. DOS for the adsorption of a NO molecule on an undoped 72 atom particle. The vertical lines are Fermi energies. (a) DOS for the adsorption type A, the NO molecule forms one bond with the particle. Panel (a): the total DOS for the adsorption type A which is compared with the total DOS for the particle without adsorption. Panel (b): the LDOS of the dangling oxygen of the particle and the N atom in the NO molecule after adsorption. (b) The DOS for the adsorption type B, in which the NO molecule forms two bonds with the particle. Panel (a): the total DOS. Panel (b): LDOS of the dangling oxygen atom and the N atom in the NO molecule after adsorption. Panel (c): LDOS of the Tia atom and the O atom in the NO molecule atom after adsorption. (For interpretation of the references to color in the text, the reader is referred to the web version of the article.)

˚ Both of them are also longer than that before the NO molec1.25 A. ular adsorption. The bond length change from GGA agrees with that from LDA too (see Table 4). 3.1.2. Electronic structure of the adsorption In order to further investigate the NO adsorption on anatase nanoparticles, we calculate the local density of states (LDOSs), charge density difference (CDD) for the adsorption. Since the results from GGA are similar to that from LDA calculations, the DOS and CDD from LDA are discussed in this part. The LDOSs for a NO molecule adsorbed on an undoped anatase particle at the dangling oxygen atom position are shown in Fig. 4. For the adsorption type A, panel (a) shows the total DOS of the particle before the adsorption and the total DOS for the adsorption complex after adsorption. Panel (b) shows the LDOS of the dangling oxygen atom and the N atom in NO molecule after the adsorption. The large DOS overlap for these two atoms shows that the dangling oxygen atom of the particle and the nitrogen atom in the NO molecule form a chemical bond after the adsorption. Further study shows that this chemical bond comes from the O 2p and N 2p orbital. For the adsorption type B, we can see the newly formed Ob Nc bond and the Tia Od bond from the large DOS overlaps. The electron configuration of a NO molecule can be written as K 1 0 ∗ )2 ( ∗ ∗ 2 2 2 K ( 2s )2 (2s 2px ) ( 2py ) ( 2pz ) (2px ) (2py ) [35]. Its electronic structure is characterized by one unpaired electron in the 2* antibonding orbital, which determines a formal bond order of 2.5, with an inherent instability relative to the NO+ ion [6]. The total DOS for the adsorption type A is shown in panel (a) of Fig. 4(a). The blue dash line represents the total DOS before adsorption and the red solid line is the DOS for the adsorption system. The vertical lines are the Fermi energy before and after the adsorption. Before the adsorption, the Fermi energy is on the top of the valence band; after the adsorption, the Fermi energy rises to the conduction

band. One electron at the 2* state transfers to the conduction band, delocalized in the particle. The charge density difference is defined as:  = (particle + NO) − (particle) − (NO),

(2)

here, (particle + NO) is the electron charge of the adsorption system, (particle) and (NO) are the electron charge density of the particle without the NO molecule and the charge density of the NO molecule without the particle. The charge transfer between the NO molecule and the anatase nanoparticle is shown in Fig. 5. The charge density calculation shows that the electron density at the center of the newly formed N O bond increases for type A adsorption. The electron charge transfers from the negative charge area. For the type B adsorption, electron density at both of the two newly formed bonds increases. It agrees the results from the DOS calculations and it suggests that the chemical bonds form during the adsorption process. 3.2. A NO molecule adsorbed on 72 atom N-doped anatase nanoparticles For the N-doped anatase particles, a nitrogen atom substitutes an oxygen atom in a TiO2 nanoparticle and introduces a hole in the particle. The empty state may be on the top of the valence band or inside the band gap as an impurity band [24]. The doping positions are shown in Fig. 1(b). N1 and N2 are the two substitutional positions which generate the impurity states while other substitutional positions generate empty state on the top of the valence band. A possible configuration which may generate an impurity band is shown in Fig. 2(a) and (b) (M1); while a possible configuration which can generate a hole on the top of the valence band is shown in Fig. 2(c) and (d) (M2). These two particles are chosen to

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J. Liu et al. / Applied Surface Science 258 (2012) 8312–8318

for the three particles (see the stars in the figure). Especially, in the undoped particle, the adsorption energy at the dangling oxygen site is much higher than that of all the other sites. Consequently, the dangling adsorption site is the energy favorable adsorption site. Second, the adsorption energy for the N-doped particles is much higher than that for the undoped particle. Third, in the N-doped particles, many adsorption energies at different adsorption sites are higher than the highest adsorption energy for the undoped particle. It suggests that on the N-doped particles, there are more active adsorption sites rather than only the dangling oxygen site.

Fig. 5. Charge density difference (CDD) for a NO molecule adsorbed on a 72 atom undoped anatase particle. (a) CDD for the adsorption type A. The orange color represents positive value of the electron transfer. The electron density increases at the center of the newly formed N O bond. (b) CDD for the adsorption type B. The orange color is the positive area. (c) CDD for the adsorption type A. The blue color represents the negative value area. (d) CDD for adsorption type B. The blue color is the negative value area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

study the doping effect on the NO adsorption on the particle in this section. The NO molecule can be adsorbed on all surface oxygen atoms of the particle. The absolute values of the adsorption energy on different oxygen atoms for the NO adsorption on the undoped particle and two N-doped particles are listed in Fig. 3. Panel (b) and panel (c) are the adsorption energies for the N-doped nanoparticles M1 and M2. There are three comments for this energy figure. First, the adsorption energy at the dangling oxygen site is the highest one

Non−adsorbed

a

Nc

adsorbed

b

adsorbed

a

adsorbed

Non−adsorbed

b

adsorbed

c

adsorbed

N

c

Ob

DOS

DOS

3.2.1. Doping N atom generates an impurity band For the substitution positions N1 and N2, in Fig. 1(b), the N doping atom introduces an impurity band inside the band gap [24]. The adsorbed NO molecule on the dangling oxygen atom of the particle can form one bond or two bonds with the anatase particle. Two N-doped particles M1A and M1B are shown in Fig. 2(a) and (b). The adsorption energies (by LDA) are −2.92 eV and −3.29 eV for the two types of adsorption on M1 doping particle (see Table 3). These energies are 92% and 87% lower than the corresponding undoped particle adsorption energies. Although the energy values for the GGA calculations are different, it also shows that the adsorption energies for the N-doped particle are much lower than that of undoped system. So, for M1 particle, the NO adsorption is much stronger than on the undoped particles. The DOSs for the NO adsorption on the M1 particle are shown in Fig. 6. After adsorption, the unpaired electron in the 2* orbital of the NO molecule transfers to the empty state of the particle. As a result, the Fermi level moves from the top of the valence band to the middle of the band gap (see the total DOS in Fig. 6(a)). We also can see the newly formed Nc Ob bond from the large overlap in the LDOS. Unlike the undoped adsorption system, there is no electron in the conduction band since the impurity state is empty which can contain an extra electron.

O

b

Tia Od

−14

−12

−10

−8

−6

−4

−2

0

−14

−12

−10

−8

−6

Energy (eV)

Energy (eV)

(a) M1A

(b) M1B

−4

−2

0

Fig. 6. DOS for the adsorption of a NO molecule on a N-doped particle. The vertical lines are Fermi energies (LDA). (a) DOS for the adsorption type M1A. Panel (a): the total DOS. The blue dash solid line is the total DOS of undoped nanoparticle before adsorption, the red solid line is the total DOS of the adsorption system. Panel (b): the LDOS of the dangling oxygen of the particle and the N atom in the NO molecule after adsorption. (b) DOS for adsorption type M1B. Panel (a): the total DOSs. Panel (b): LDOS of the dangling oxygen atom and the N atom in the NO molecule after adsorption. Panel (c): LDOS of the Tia atom and the O atom in the NO molecule atom after adsorption. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

J. Liu et al. / Applied Surface Science 258 (2012) 8312–8318

Non−adsorbed

a

8317

adsorbed

a

adsorbed

Non−adsorbed

N

adsorbed

b

c

O

Nc

adsorbed

b

DOS

DOS

b

Ob

Ti

adsorbed

c

a

O

d

−14

−12

−10

−8

−6

−4

−2

0

−14

−12

−10

−8

Energy (eV)

Energy (eV)

(a) M2A

(b) M2B

−6

−4

−2

0

Fig. 7. DOS for the adsorption of a NO molecule on a N-doped particle (M2). The vertical lines are Fermi energies. (a) DOS for the adsorption type M2A. Panel (a): The total DOS. The blue dash solid line is the total DOS of the M2 particle before adsorption, the red solid line is the total DOS of the adsorption system. Panel (b): the LDOS of the dangling oxygen of the particle and the N atom in the NO molecule after adsorption. (b) DOS for adsorption type M2B. Panel (a): the total DOS. Panel (b): LDOS of the dangling oxygen atom and the N atom in the NO molecule after adsorption. Panel (c): LDOS of the Tia atom and the O atom in the NO molecule atom after adsorption. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

In the type B adsorption configuration, the unpaired electron of the NO also transfers to the empty state and the Fermi level rises to the middle of the band gap which is lower than the bottom of the conduction band. The overlap of the DOS in Fig. 6(b) shows the NO molecule forms two bonds with the particle. Because the adsorption energy of type B is even lower than that of type A, the two chemical bonds configuration is more stable than the configuration of forming one bond with the particle.

3.2.2. Doping N atom generates a hole on the top of the valence band For the other 21 possible substitutional configurations except N1 and N2 substitutional sites, the empty state is on the top of the valence band and the hole mainly distributes on the nitrogen atom and the two dangling oxygen atoms [24]. We choose one of these substitutional configurations to investigate the doping effect of the NO adsorption on M2 particle (Fig. 2). The adsorption energies for the M2 particle are −3.16 eV and −3.57 eV (LDA) respectively for the type A and B adsorption (see Table 3). They are 108% and 103% lower than the adsorption energies for the undoped particle. GGA calculations show the similar results that the doping N atom makes the adsorption energies much lower than that for the undoped particle. Bond lengths are listed in Table 4. Both GGA and LDA calculations show that the dangling bond and the bond length of the NO molecule are elongated after the adsorption because of the electrons transfer to the newly formed bonds. The DOSs of the NO adsorption on the M2 particle are shown in Fig. 7. We can see the charge transfer process from the DOS analysis. After adsorption, the Fermi level rises, but it is still lower than the bottom of the conduction band. The unpaired electron of the NO molecule fills in the empty state of the particle and forms a small pick on the top of valence band. Since the energy of the unpaired

electron of the molecule is higher than the empty state [24], the system energy is lowered after the adsorption. A new Nc Ob bond forms for type A adsorption and there is another new Od Tia in the type B adsorption configuration from the large overlap of the DOS.

4. Conclusions The adsorption of a NO molecule on 72 atom N-doped anatase nanoparticles has been studied by first principles calculations. Because the adsorption energy at the dangling oxygen atom site is the highest for the undoped and N-doped particles, two types of adsorption at the dangling O site have been intensively studied. In one type of the adsorption, the NO molecule forms one bond with the particle, while in the other type of adsorption, the NO molecule forms two bonds with the particle. The two bonds configuration is energetic favorable. The length of the N O bond of the adsorbed NO molecule and the length of the dangling bond on the particle surface are longer than that before the adsorption. Compared with the adsorption on the undoped anatase particles, the adsorption energies of a NO molecule on the N-doped nanoparticles are lower. As a result, a NO molecule can be easily adsorbed on N-doped anatase particles. In one word, the N-doped anatase particles are more efficient to remove the NO molecules to reduce the air pollution by surface adsorption. After the surface adsorption on the N-doped particle, the unpaired electron of the NO molecule transfers to the empty state of the N-doped particle. It makes the Fermi level lower. Consequently, the system energy is lower than that of the adsorption on the undoped particle. It agrees our previous study that the HOMO energy of a NO molecule is higher than the maximal Fermi energy of the N-doped particles and the electron in the NO molecule can transfer to the N-doped anatase particles [24]. For Ndoped particles, the NO molecule can be adsorbed on many oxygen atom sites on the particle surfaces because the adsorption energies

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J. Liu et al. / Applied Surface Science 258 (2012) 8312–8318

at those sites are much higher than that of the undoped particle. It suggests that the N-doped anatase particles are more active for surface adsorption to remove the small toxic gas molecules, such as NO.

[12] [13] [14] [15] [16]

Acknowledgements

[17] [18]

This work is supported by the Chinese National Science Foundation project 10704058, the Chinese National Basic Research Program (973 Program) project 2009CB939705. One author acknowledges the World Class University (WCU) program Grant No. R31-2010-000-10083-0 of the Korea Research Foundation. References [1] V. Pârvulescu, P. Grange, B. Delmon, Catalysis Today 46 (1998) 233–316. [2] D. Torres, S. González, K.M. Neyman, F. Illas, Chemical Physics Letters 422 (2006) 412–416. [3] C. Vinod, J.N. Hans, B. Nieuwenhuys, Applied Catalysis A: General 291 (2005) 93–97. [4] A. Hussain, D.C. Ferré, J. Gracia, B. Nieuwenhuys, J. Niemantsverdriet, Surface Science 603 (2009) 2734–2741. [5] D. Loffreda, D. Simon, P. Sautet, Chemical Physics Letters 291 (1998) 15–23. [6] M.P. Jigato, K. Somasundram, V. Termath, N.C. Handy, D.A. King, Surface Science 380 (1997) 83–90. [7] R.T. Vang, J.-G. Wang, J. Knudsen, J. Schnadt, I. Stensgaard, F. Besenbacher, The Journal of Physical Chemistry B 109 (2005) 14262–14265. [8] R.B. Getman, W.F. Schneider, The Journal of Physical Chemistry C 111 (2007) 389–397. [9] M. Breedon, M. Spencer, I. Yarovsky, Surface Science 603 (2009) 3389–3399. [10] Z. Yang, T.K. Woo, K. Hermansson, Surface Science 600 (2006) 4953–4960. [11] Z. Ai, L. Zhu, S. Lee, L. Zhang, Journal of Hazardous Materials 192 (2011) 361–367.

[19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]

M. Calatayud, B. Mguig, C. Minot, Surface Science Reports 55 (2004) 169–236. H. Kizaki, K. Kusakabe, Surface Science 606 (2012) 337–343. U. Diebold, Surface Science Reports 48 (2003) 53–229. A. Fujishima, X. Zhang, D.A. Tryk, Surface Science Reports 63 (2008) 515–582. F. Han, V.S.R. Kambala, M. Srinivasan, D. Rajarathnam, R. Naidu, Applied Catalysis A: General 359 (2009) 25–40. B. Brian O’Regan, M. Grätzel, Nature 353 (1991) 737. A. Nambu, J. Graciani, J.A. Rodriguez, Q. Wu, E. Fujita, J.F. Sanz, Journal of Chemical Physics 125 (2006) 094706. A.K. Rumaiz, J.C. Woicik, E. Cockayne, H.Y. Lin, G.H. Jaffari, S.I. Shah, Applied Physics Letters 95 (2009) 262111. Q. Chen, C. Tang, G. Zheng, Physica B: Condensed Matter 404 (2009) 1074–1078. L. Jia, C. Wu, S. Han, N. Yao, Y. Li, Z. Li, B. Chi, J. Pu, L. Jian, Journal of Alloys and Compounds 509 (2011) 6067–6071. J. Ananpattarachai, P. Kajitvichyanukul, S. Seraphin, Journal of Hazardous Materials 168 (2009) 253–261. S. Hu, F. Li, Z. Fan, Journal of Hazardous Materials 196 (2011) 248–254. H. Li, Y. Lei, R. Tu, Y. Zheng, C. Pan, W. Xiao, Physica Status Solidi b 248 (2011) 1665–1670. P. Hohenberg, W. Kohn, Physical Review 136 (1964) B864–B871. W. Kohn, L. Sham, Physical Review 140 (1965) A1133–A1138. G. Kresse, J. Hafner, Physical Review B 47 (1993) 558–561. G. Kresse, J. Hafner, Physical Review B 49 (1994) 14251–14269. P.E. Blöchl, Physical Review B 50 (1994) 17953–17979. G. Kresse, D. Joubert, Physical Review B 59 (1999) 1758–1775. J.P. Perdew, A. Zunger, Physical Review B 23 (May) (1981) 5048–5079. J.P. Perdew, K. Burke, M. Ernzerhof, Physical Review Letters 77 (October) (1996) 3865–3868. J.P. Perdew, K. Burke, M. Ernzerhof, Physical Review Letters 78 (February) (1997) 1396. Y. Lei, H. Liu, W. Xiao, Modelling and Simulation in Materials Science and Engineering 18 (2010) 025004. K. Ding, J. Li, Y. Zhang, W. Wang, Chemical Physics Letters 389 (2004) 255–260. J.K. Burdett, T. Hughbanks, G.J. Miller, J.W. Richardson, J.V. Smith, Journal of the American Chemical Society 109 (12) (1987) 3639–3646. M. Lazzeri, A. Vittadini, A. Selloni, Physical Review B 63 (March) (2001) 155409.