Ab initio Calculations of Structural and Electronic Properties of
Pt3 and Cu3 clusters adsorbed on ZnO(000 ̅1)
Yamina Benkrima1*, Abdelkader Souigat1, Yassine Chaouche2, Zineb Korichi1,
Mohammed Elbar Soudani3, Assia Belfar1
1Ecole Normale Supérieure de Ouargla, 30000 Ouargla, Algeria.
2Larbi Tebessi University, Tebessa, Laboratoire de Physique Appliquée et Théorique,
Route de Constantine 12002 Tebessa, Algeria.
3Laboratoire de Développement des
Energies Nouvelles et Renouvelables dans les Zones Arides et Sahariennes,
Faculté des Mathématiques et des Sciences de la Matière, Université Kasdi
Merbah Ouargla, Ouargla 30000, AlgérieTebessi University, Tebessa, Laboratoire
de Physique Appliquée et Théorique,
Route de Constantine 12002 Tebessa, Algeria.
*Corresponding Author E-mail: benkrimayamina1@gmail.com
ABSTRACT:
To increase its application range, from both methods; experimental and theoretical, the researchers focus their work to adapt the electronic structures of ZnO by doping or deposing diverse atoms into ZnO bulk compound.
The most stable phase of ZnO is Wurtzite, in precedent years, the subject of ZnO surfaces intensive the studies owing the utilization as active catalysts and catalyst supports and, because of their opposite polarity ZnO-Zn:(0001) and ZnO-O: (000 1 ̅) surfaces6-9. However, an interesting field of investigation in previous years is noble metal/oxide systems10-12. These model systems result the cluster/oxide interface, which in turn has a strong influence on the catalytic activity, it has a big role in the geometric and electronic structure due the interaction between the metal clusters and the oxides13-15.
Through the earliest research, it seems that ZnO supported Cu catalysts are essential subject of wide investigations in current years, which are practical applications and surface properties become a modest bit different from those of bulk copper16.
Theoretical part, in particular, the semi-empirical and Density Function Theory (DFT) methods are useful to investigate the different structures and electronic properties as the deposition of atom or molecule surfaces of ZnO17-19.
The literature is rich in scientific work; surface structures and electronic properties on ZnO surface as other noble metal clusters deposition from theoretical and experimental techniques like Agn and Aun20.
In this paper to simplicity the noble metal adsorbed on ZnO-O or ZnO(0001 ̅) surface, we study the structures, stabilities and electronic proprieties of Pt3 and Cu3 clusters adsorbed on ZnO-O surfaces in the Wurtzite stable structure. This work is concentrate on the size effect of supported metal clusters using ab initio calculations with SIESTA code in the framework of Density Function Theory (DFT) method. The rest paper is in order as follow; section 2 is the detail of calculations, section 3 is the results and discussions obtained by SIESTA code, and ended by conclusion.
DETAIL OF CALCULATIONS:
These calculations are based on the first principles pseudopotential method in the framework of DFT21, the description of the interactions of the electronic exchange-correlation were treated by the (PBE) functional of the Generalized Gradient Approximation (GGA)22 as executed in the SIESTA code23. The inputs variables used are about 250 eV chosen for the energy cutoff of the plane wave, the model of the Brillouin Zones is sampled by extremely dense Monkhorst Pack K points grids of (6 × 6 × 1) for bulk ZnO, in the opposite we have taken the K points grids of (3 × 3 × 1) for ZnO-O surface and M/ZnO-O schemes. Our calculations of the self-consist field (scf) in the convergence criteria of total energy were set about 5 × 10-4 eV, and 0.05 Å is taken for the greatest ionic movement tolerance within the cluster and atoms. Since the atomic forces were lesser than 0.005 eV/Å, we have used the conjugated gradient technique for the structure relaxation. Our calculation of constants parameters of bulk ZnO are a=b=3.284 Å and c=5.330 Å, which agree very well and close with the values of experience method; 3.25 Å and 5.207 Å24. We included four Zn-O double layers of ZnO to the repeated the slab supercell. The similar lattice spacing’s like those of the bulk in the x and y directions of slab possesses, except in the z direction while the atoms are relaxed.
In geometric optimization, the fully relaxed atoms are the top two ZnO bilayers the same as the ad atom layer of the slab are allowed. Even as the fixed one are the two bottom ZnO bilayers.
In addition, to check the unphysical charge transfer between the two slabs, top and bottom, we saturated the dangling bonds on the bottom layer with pseudohydrogens.
RESULTS AND DISCUSSION:
1. Comparison of Pt3 and Cu3 clusters:
1.1. The structure of Pt3 and Cu3 clusters:
Density function theory (DFT) was selected, as well as the use of generalized gradient approximation (GGA) to get the most stable structures to have less energy. The obtained of the stable structure is based on the application of simulated annealing, in which we primary heated the both clusters to a temperature of 1000 K in 1000 iterations, we let them settle at this temperature for about 500 iterations, to cool them down again in 1000 iterations, we obtained the structures stable as follow:
Figure 1. The most stable structure of Pt3 and Cu3 clusters.
1.2. Binding energy:
Our studied system consists three atoms of each metal, Cu and Pt, so to study the stability of the clusters of copper and platinum metals, we compute the binding energy between the atoms of the two clusters by knowing the total energy of the cluster and the energy of one atom when it is in the free state according to the following relationship:
Ebin(Kn) = Etot(Kn) – nEat(Kn) /n (1)
Whereas:
n: the number of atoms in the cluster.
Etot: The total energy of the cluster.
Eat: The energy of one atom in the Free State.
Kn: cluster code.
The results obtained of the values by this method of binding energy are: -2.069253 (eV) and -3.413209 (eV) for Cu3 and Pt3 respectively. It seems from these values that the binding energy of the platinum cluster Pt3 is greater than the binding energy of the copper cluster Cu3. It is noted, in the knowledge, that the higher binding energy indicates a more stable system. Therefore, the platinum cluster is more stable than the copper cluster.
1.3. Average bond length:
Via the Xcrysden program, the average bond length between the two cluster atoms; copper and platinum (n = 3), was calculated, by the addition of the bond lengths between the atoms and divided them by the number of bonds present in the cluster, and accordingly we arrived at, 4.1758 (Å) and 4.3614 (Å) for Cu3 and Pt3 respectively.
We note that the average bond length of the Pt3 cluster is greater than the average bond length of the Cu3 cluster, as there is a rapport between the stability of the clusters and the average bond length. We find that the stability of the clusters increases as the average bond length of the cluster increases, and from this point of view we find that a Pt3 cluster is more stable than a cluster Cu3.
1.4. Vertical ionization potential (PIv):
The vertical ionization potential is a variable that shows us the stability of the elements, and it is the difference between the energy of the element, which is ionized, and the energy of the element. According to the following relationship:
PIv = E (Kn+) – E (Kn) (2)
The results obtained are 6.532811 and 7.427115 for Cu3 and Pt3 respectively.
It is noticed that the vertical ionization potential of platinum is greater than the vertical ionization potential of copper, as we know that the greater value of vertical ionization potential indicates the more stability one, and accordingly, the Pt3 cluster is more stable than the Cu3 cluster.
1.5. Density of states for Pt3 and Cu3 clusters:
The change in energies of both clusters is accompanied by differences in their electronic and optical properties, and the following figure 2 shows the density of the electronic effective states of the copper and platinum clusters and set the Fermi level EF at 0 point by shifting, all the values of the density of electronic states relative to its value.
Figure 2. Density of states for Cu3 and Pt3 clusters.
The previous figure shows that the highest peak of the density of states at the Fermi level is about 11.5 (states/eV) for copper, the value 7.4 (states/eV) was also recorded for platinum. We also note that the density of the electronic states of the copper cluster is greater than the density of the electronic states of the platinum cluster. As it is known that there is a relation between the chemical activity of the clusters and the density of the electronic states located near the Fermi level, which the higher electron density near the Fermi level increases the possibility of electrons moving from the valence band towards the conduction band. Chemically active, while those that record a low density of states are considered chemically stable25-26.
So we find that the Pt3 cluster is more stable than the Cu3 cluster, as it confirmed by the results obtained previously.
2. Study of the surface of ZnO oxide:
2.1. Unit cell structure:
The obtained values of lattice constants of ZnO have been verified many times over several decades27-28. The stable structure of unit cell is Wurtzite (B4) of ZnO, this form is the hexagonal compact which it is characterized by the set is C46vP63mc group with the lattice parameters as follow: a=b=3.249 Å and c=5.20 Å29.
Our calculations in this work applied on the SIESTA program, which was used to calculate the primary cell constants for ZnO, and the results are tabulated in the following table:
Table 1. The lattice constants of ZnO oxide and their comparison with available results.
|
Eg (eV) |
c (Å) |
a (Å) |
Method |
|
0.65 |
5.33 |
3.284 |
Our results |
|
0.71 |
5.305 |
3.288 |
(PBE) GGA30 |
|
0.74 |
5.299 |
3.286 |
GGA (PBE)31 |
|
1.02 |
5.27 |
3.288 |
GGA (PBE)32 |
|
3.44 |
5.207 |
3.25 |
Experimental33 |
By means of the SIESTA program based on the density function theory (DFT) within the pseudopotential method enabled us to calculate the unit cell constants. The values of a and c constants are found, 3.284 Å and 5.33 Å respectively, and given that the structure is hexagonal, in all cases we will find that a = b, and it is noticeable that these results are in very well agreement with the theoretical and experimental results mentioned in the previous table of references30-32. It also seems that the value of the energy gap obtained by this theory was somewhat far from the experimental results, and this is the problem that the density function theory suffers from.
2.2. Surface energy:
To calculate the surface energy in this study, the cell size estimated at: (1x1) and its depth was taken from four layers, containing 36 atoms. Relaxation (a change in the x and y coordinates of atoms while preserving their z coordinates), using the following relationship:
S= (Eslab – nEbulk)/2A
S surface energy
Eslab: class energy.
Ebulk: energy of primary cell.
A: surface space
n: class number.
The following table presents the calculated surface energy results for both surfaces according to the number of layers that make up them.
At number class, n=4, the Surface energy of ZnO-Zn : (0001) and ZnO-O : (0001 ̅) is 3.11 (J/m2) and 2.47 (J/m2).
From the previous presentation of the surface energy results for four layers of both surfaces, we notice that the ZnO-O surface has lower surface energy values, the following value was recorded: (J/m2) 2.47, while the value (J/m2) 3.11 was recorded for the surface ZnO-Zn for the same number of layers as the previous ones, which are very close results for the work of K. Sun et al.16.
On the other hand, it was concluded that the surface energy of ZnO-Zn for four layers is greater than the surface energy of ZnO-O for those layers, which indicates that the surface ZnO-Zn is the most chemically active compared to the surface of ZnO-O. The results are largely consistent with what is applied, as experiments showed that this surface is characterized by pits and islands, which gave the surface the possibility of adhesion on its level, in contrast to the other surface that appeared in the form of smooth terraces and this is evidenced by the work of O. Dulub et al.17.
2.3. The placement of Cu3 and Pt3 clusters on the ZnO-O surface:
2.3.1. Surface Structure Pt3/ZnO-O and Cu3/ZnO-O:
In order to study the placement of the copper and platinum clusters consisting of three atoms on the ZnO-O surface, which contains four layers, we have come to highlight the structure resulting from the process of placing each of the two clusters on this surface, which is the most stable structure, and can be clarified in the following forms:
(a) (b)
Figure 3. (a) top view, (b) side view of Cu3/ZnO-O structure of dimension (1x1x1) respectively.
(a) (b)
Figure 4. (a) top view, (b) side view of the Pt3/ZnO-O structure of dimension (1x1x1) respectively.
2.3.2. Binding energy:
We included the atoms which are not in direct contact with the surface to calculate the interaction, binding, energy for each atom of the cluster to the surface. The calculation of this parameter mean for two motives: the first is to consider the interact of the non-bonded atoms and the bonded ones as interacting indirectly with the surface. The second is to compare the interaction energy for clusters of diverse size. We can predict the EB as binding energy for each atom by the equation as follow:
Such as, we define n as the number of atoms, Pt or Cu, E(M/ZnO) the total energy of Pt3 or Cu3 adsorbed on the surface and E(M) is the energy of the clusters for Pt3 or Cu3 respectively.
The more stable of M/ZnO-O system is the value which has a more negative value. Our values of binding energies obtained are -2.38 (eV) and -2.65 (eV) for the systems Pt3/ZnO-O and Cu3/ZnO-O respectively.
It is clear from these values, a greatest value of the binding energy of Pt3/ZnO system than those of Cu3/ZnO system for the polar surface ZnO-O. Also in comparison, it is established that the binding of Cu3/ZnO is stronger through -0.27 eV with the binding energy greater than that in Pt3/ZnO. The system of M/ZnO-O with adsorbed Cu3 cluster is more stable than the system with Pt3 cluster. It is known as the more chemically activity of Pt3 cluster adsorbed on ZnO-O in comparison with Cu3 cluster adsorbed on the similar surface.
(a)
(b)
(c)
Figure 5. Structure of the energy bands: (a) for the surface, ZnO, (b) for the Cu3/ZnO structure, (c) for the Pt3/ZnO structure.
2.3.3. The structure of energy bands:
The semiconductors are characterized by the energy gap and the Fermi level, which it is defined as the highest energy level that electrons can occupy at a temperature of 0K, and the following figures represent the structure of the energy bands of the surfaces Pt3/ZnO-O and Cu3/ZnO-O and the structure of the energy bands of the ZnO-O surface before localization34-37.
We note from the figure 4 that the evidence of the energy bands of the surface ZnO before localization shows a value of the energy gap of 1.6 eV, while the value of the energy gap after placement of the Cu3/ZnO-O surface was estimated at 1.5 eV, while the value of the energy gap of the surface Pt3/ZnO-O was estimated at : eV1.3. These results were in large agreement with the results reported in the work38. Through the obtained results, we find that the lowest recorded energy gap is the energy gap at the structure Pt3/ZnO-O, which makes it qualified to be among the current conductors of the electric current.
2.3.4. Density of states:
Our previous study of the density of states for copper and platinum clusters in terms of the electronic properties showed that copper is more chemically active compared to platinum, which shows the catalytic property. The localization is a development in the electronic properties of the new surface.
Figure 6. Density of states for ZnO, Cu3/ZnO and Pt3/ZnO surfaces.
Figure 6 shows the density of states for the copper and platinum clusters after ZnO is deposited on the surface and the density of the surface states when it is in the Free State. We take the study of the density of states for the three surfaces near the Fermi level, for each of the valence band and the conduction band, and compare them to each other. Then compare it with the free ZnO surface before lying, as the density of states near this level gives us information about the presence of electrons near it and thus knowing the state of chemical activity in it.
In the eV range [-8 - 0]: the density of states is very high up to 94 states/eV for both surfaces after positioning compared to the density of states for the free surface which is about 9 states/eV. That is, the high density of these surfaces is within this region (valence band) It has a high chemical activity which shows the catalytic property of (metal oxide / cluster) surfaces. In the eV range [0-20]: the density of states for the two surfaces after placing the two clusters within this field observer a significant increase compared to the free surface ZnO, the higher electronic density in this region (the conduction band) means the higher conductivity of the surface. It is characterized by greater electrical conductivity, as the density of states reaches the value of 22 states/eV for the localization of platinum cluster and the value of 18 states/eV for placing of the copper cluster, meaning that the platinum cluster is placed on the surface ZnO that exhibits greater electrical conductivity. Also, these results showed great convergence with the results of work in38.
CONCLUSIONS:
In this paper, we investigate the structures, stabilities and electronic properties of Pt3 and Cu3 clusters adsorbed on ZnO-O surfaces are systematically investigated. We find that both Pt3 and Cu3 clusters prefer to be adsorbed the Top sites of the ZnO-O surface. In DOS analysis we find that the Pt3 and Cu3 clusters adsorbed on ZnO-O surfaces exhibit metallic characteristics, While the surface Pt3/ZnO exhibits greater electrical conductivity and more catalytic activity, These conclusions were obtained using first principles density functional theory calculations. Finally, the work function of Pt3 and Cu3 clusters adsorbed on ZnO-O surfaces are calculated and discussed for the first time in the paper. We wish the present work provided useful information about noble metal clusters adsorbed on ZnO surfaces.
CONFLICT OF INTEREST:
The authors have no conflicts of interest regarding this investigation.
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Received on 27.02.2022 Modified on 23.05.2022
Accepted on 04.07.2022 ©AJRC All right reserved
Asian J. Research Chem. 2022; 15(4):272-278.