Comparative Study of the Adsorption of O 2 on Al (110) Surface in Different Coverage

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International Journal of Material and Mechanical Engineering (IJMME), Volume 5 2016 doi: 10.14355/ijmme.2016.05.007

Comparative Study of the Adsorption of O2 on Al (110) Surface in Different Coverage Z. X. Yan*, Q. Chen, W. Liu, A. Gong and Q. Liao College of Science, Xi’an University of Science and Technology, Xi’an, 710054, China E-mail: zhengxinyan163@163.com

†

Abstract Via First-Principle based on Density Functional Theory, we comparatively analyze the geometric structure, adsorption energy, work function and electronic structure of the adsorption of O2 on Al(110) surface at three different initial adsorption sites (top, bridge and hollow site) in three different coverage (0.25ML, 0.5ML and 0.75ML) respectively. It is found that different coverage results in different adsorption consequences. With the increasing of coverage, the adsorption energy decreases and work function increases, and the bridge site is the most stable adsorption site. The density of states of the adsorption of O2 on Al (110) surface shows that the orbital interactions take the main role between 3p orbit of Al and 2s orbit of O2.

Keywords DFT (Density Functional Theory); Al (110) Surface; Chemical Adsorption; Coverage

Introduction Aluminum is widely used in industry equipment and daily life1, but it is inevitably effected by H2O, CO2, O2 and many other gas molecules once aluminum is exposed in air environment, which may result in the change of surface states and influence the property of aluminum2. Therefore, many researchers devoted themselves to study the adsorption of O2 on Al surface recently. Zhang3 et.al calculated the adsorption of O2 on Al (001) surface at 0.25ML coverage by First-Principles5-8, they came to the conclusion that the parallel site was the easiest adsorption site, and there were two adsorption process when O2 was adsorbed on the O2/Al (001) boundary, which was related to the initial adsorption states of Al (001) surface. Lei2 et.al studied the adsorption of O2 on Al (111) surface at 0.167ML coverage by DFT, they reached a decision that the most stable adsorption site was parallel site and a large number of electrons transferred from Al (111) surface to O2 molecule; and the process was strong chemical adsorption. Guo4 et.al studied the dissociation and reconstruction of O2 on Al (111) surface at 0.22ML, 0.5ML and 1ML coverage by First-principles, they drew the conclusion that the dissociation and reconstruction was not only related to the initial adsorption site of oxygen molecule, but also related to the coverage, the adsorption energy of the same structure may be different due to different coverage.

In this work, the focus will be mainly on the comparison of O2 on Al (110) surface in different coverage. Our discussion will start with a consideration of various computational parameters, energies and details by applying First-principles based on GGA (Generalized Gradient Approximate) 9-12 of DFT; the O-Al interaction, bond length and bond angle changes are then analyzed at three important sites in different coverage, followed by a brief discussion concerning the construction of group orbits; and their electrons transfers in surface-adsorbed analysis.

Finally, the adsorption condition at different coverage is recognized in terms of the present analysis.

Calculation Methods and Models The structure optimization and electronic energy calculation are realized by applying CASTEP (Cambridge Serial Total Energy Package) module13-14of Materials Studio software. In this work, we use Al fcc structure to make up 6 atom layers models; at the same time, vacuum thickness is 2nm, bottom three layers are fixed and top three layers are relaxed.

Degree of coverage is a magnitude used to describe the quantity of chemical adsorption1, and it can be shown by θ and be defined as θ=n/N

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(1)

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In this formula, n is the number of oxygen molecule in the adsorption system, N is the atomic number of monolayer in super cell. In this review, we are mainly concerned with adsorption of oxygen molecule on the 2×2, 1×2 and 1×1 super cell structures, and the coverage are 0.25ML, 0.5ML and 1ML respectively. In the Fig.1, (a)-(c) show the structures of top(T), bridge(B) and hollow(H) site at 0.25ML,0.5ML and 1ML coverage after adsorption respectively. The bond length of O1-O2 is 0.177nm, Al-O1 is 0.128nm and cutoff energy is 270eV. It is found that the settings above can ensure the precision of the calculation after testing the convergence by changing sampling point density of k-point and cutoff energy.

(T)

(B) (a)

(H)

(T)

(B)

(H)

(T)

(b)

(B) (c)

(H)

FIG. 1. ADSORPTION STRUCTURES OF O2 MOLECULE ON AL (110) SURFACE AT TOP(T), BRIDGE(B) AND HOLLOW (H) SITE OF THREE DIFFERENT COVERAGE RESPECTIVELY:(a) 0.25ML; (b) 0.5ML; (c) 1ML.

Results and discussion Adsorption Energies and Structure Change Parameters of O2 on Al (110) Surface After geometry optimization, lattice parameter of Al bulk is 0.45nm, which is correspondent with experience value 0.45nm 3, we take Al (110)-(2×2) supercell model with 6 layer atoms as an example to calculate the surface energy and use σ to express it, σ=(Eslab-nEbulk)/2A

(2)1

n is the number of aluminum atoms in the structure, A is the superficial area of a cell, because of two faces in a cell, we consider it as 2A, Ebulk is average energy of each aluminum atom in Al bulk and Eslab is the total energy of relaxation system. After calculation, surface energy is 0.93 J/m2, it is smaller than experience value 1.11J/m2, and it is thought that there is no consideration with the influence of temperature in the calculation15.

The adsorption energy is defined as Ead, the formula is shown in (3) below1:

E

ad

(3)

1 ) = − (E − − Al.O 2 E Al NE O 2 N

EAl.O2 is the total energy of the adsorption system, EAl is the total energy of aluminum supercell, EO2 is the total energy of oxygen molecule, and N is the number of oxygen molecule.

TABLE 1. ADSORPTION ENERGIES, WORK FUNCTION AND THE STRUCTURE CHANGES OF O2 ON AL (110) SURFACE

Bond length/nm

△Bond Angle/°

Coverage

Adsorption site

Φ/eV

Adsorption energy/eV

Al-O1

O1-O2

AlO1Al

O2O1Al

0.25ML

T

4.605

2.165

0.1703

0.1349

0

0

B

4.55

3.054

0.1838

0.1482

-4.648

+2.324

H

4.449

1.774

0.1798

0.1487

0

0

T

4.92

2.079

0.1707

0.1338

0

0

B

4.979

3.000

0.1843

0.1467

+2.648

-1.325

H

4.53

2.031

0.1791

0.1530

0

0

T

5.079

1.783

0.1728

0.1319

0

0

B

5.535

2.621

0.1820

0.1441

+4.94

-2.469

H

5.725

2.206

0.1712

0.1466

0

0

0.5ML

1ML

According to the results from Table 1, it is found that the adsorption energies decrease with the increasing coverage, and the adsorption energy at bridge site is the largest, which means the bridge site is the most favorable

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site. Both of bond lengths of Al-O1 and O1-O2 at three sites stretch after adsorption and the stretch decreases with the increasing coverage. In this table, positive means the bond angle increases and negative means the angle decreases after adsorption, we can see the bond angle only changes at bridge site after adsorption; at low coverage, bond angle AlO1Al decreases and AlO1O2 increases. However, at high coverage, bond angle AlO1Al increases and AlO1O2 decreases; in addition to this, with the increasing of coverage, the change of bond angle increases at high coverage; therefore, the property of bond angle exhibits the irregularity. It is also found that the work function of Al (110) surface is 3.853eV and it also increases with the increasing coverage.

Analysis of Electronic Structure

(1)

(2)

FIG.2 DENSITY OF STATES AT TOP, BRIDGE AND HOLLOW SITE IN 0.25ML COVERAGE (1) O2 ON AL (110); (2) O2 MOLECULE

Fig.2 (1) is used to express the density of states of O2 on Al (110) surface at top, bridge and hollow site in 0.25ML coverage; and Fig.2 (2) shows the density of states of O2 molecule and it at top, bridge and hollow site in 0.25ML coverage. Clean Al (110) surface is mainly the role of 3p orbit of Al atom, and O2 molecule near Fermi level is mainly effected by 2p orbit, but in the range of -10eV~-30eV, O2 molecule is mainly effected by the electrons of 2s orbit. Beside this, in the figure (b)-(d) of Fig.2 (1), two new crests are produced in the range of -15eV~ -25eV after adsorption, it is mainly the role of 2s orbit of O2 molecule, and in the range of -10eV~7eV, it is mainly the role of 3p orbit of Al atom, wave peak move to lower range, which tells us there is an interaction between 3p orbit of Al atom and 2s orbit of O2 molecule, so that it is the strong chemical adsorption produced; and after adsorption, the system is more stable than before adsorption. Compared with the figure (a)-(d) in the Fig.2 (2), the peak enhances in the range of -10eV~5eV because of the influence of 3p orbit of Al atom.

Frontier Orbits Analysis of O2 on Al (110) Surface at Three Different Sites in Different Coverage To reveal the basic picture that how oxygen is bonded to different sites, we calculate the frontier orbits HOMO (Highest Occupied Molecule Orbital) and LUMO (Lowest Unoccupied Molecule Orbital) of O2 on Al(110) surface at top, bridge, and hollow site in 0.25ML, 0.5ML and 1ML coverage, respectively.

(a) (b)

(c)

(d)

(e)

(f)

FIG.3 HOMO AND LUMO AT 0.25ML

(a)

(b)

(c)

(d)

(e)

FIG.4 HOMO AND LUMO AT 0.5ML

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International Journal of Material and Mechanical Engineering (IJMME), Volume 5 2016

(a)

(b)

(c)

(d)

(e)

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(f)

FIG.5 HOMO AND LUMO AT 1ML

Fig.3-Fig.5 are the HOMO and LUMO figures at three adsorption sites in 0.25ML, 0.5ML and 1ML coverage, respectively; in particular, (a) and (b) are HOMO and LUMO at top site, respectively; (c) and (d) are HOMO and LUMO at bridge site, respectively; and (e)and (f) are HOMO and LUMO at hollow site, respectively.

TABLE 2. HOMO AND LUMO OF O2 ON AL (110) AT THREE ADSORPTION SITES IN 0.25ML, 0.5ML AND 1ML COVERAGE

Coverage

Sit Top

-4.841969

-4.566019

0.275950

0.25ML

Bridge

-4.897508

-4.566182

0.331326

0.5ML

1ML

HOMO(eV)

LUMO(eV)

Energy gap(eV)

Hollow

-4.583624

-4.522916

0.060708

Top

-5.167336

-4.758403

0.408933

Bridge

-5.149785

-4.694293

0.455492

Hollow

-4.867412

-4.511541

0.355871

Top

-5.278576

-5.147254

0.131322

Bridge

-5.977256

-5.511642

0.465614

Hollow

-4.790295

-4.318640

0.471655

Combined with Table 2 and Fig.3-Fig.5, it is found that the energy of HOMO for bridge site is the lowest one, which indicates that the electron transferring from Al πz to π*of O is the biggest one. The bond lengths of O2 in table also prove this viewpoint.

Conclusion The geometric structure, adsorption energy, work function and electronic structure of the adsorption of O2 on Al (110) surface at three different initial adsorption sites were investigated. It is found that the adsorption energy decreases and work function increases with the increasing of coverage. Bond length of two oxygen atoms can be found at three sites with different coverage stretch, and the stretch decreased with the increasing of coverage. For bridge site, because of the interaction of neighbor orbits in crystal, the change of AlO1Al exhibits the irregularity.

We also find that the interaction between 3p orbit of Al and 2s orbit of O2 plays a vital role in electrons’ transferring from Al surface to O2 molecule, which produces strong chemical adsorption.

ACKNOWLEDGMENTS

The work was partly supported by National natural science foundation of China (No.11404257 and No. 11405127) and Department of Shanxi province natural science foundation of China (2015JM1027).

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