Chemical properties of Bimetallic (Au+Pt) using Density Functional Theory

 

Yamina Benkrima¹*, Abdelkader Souigat¹, Mohammed Elbar Soudani²,

Mohammed Seyf Eddine Bougoffa³, Zineb Korichi¹, Omar Bentouila⁴

¹Ecole normale supérieure de Ouargla, 30000 Ouargla, Algeria.

²Laboratoire 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.

³laboratory of Materials Technology, Department of Materials Science, University of Science and Technology Houari Boumediene, Bp 32 El Alia, Bab Ezzouar, 16111, Algeria.

⁴Department of Matter Sciences, Lenreza Laboratory, Optoelectronics Team, Faculty of Mathematics and Matter Sciences, Kasdi Merbah Ouargla University, Ouargla 30000, Algeria.

*Corresponding Author E-mail: benkrimayamina1@gmail.com

 

 

INTRODUCTION:

Over the past years, the physics and chemistry of nanocluster science has become very important to researchers. The researchers' work was directed to research to find the unique properties of these clusters, whose unique structure between the molecule and the size (mass) was the main reason for the theoretical researcher to delve into the understanding of the transition from atoms to clusters, molecule and finally to solid state.

 

Where in recent years, a lot of attention has been paid to the structural and chemical properties of mixed bimetallic clusters, this type of cluster is very important in its uses thanks to the possibility of using it according to special requests.

 

Nano-sized bimetallic groups have received great attention, due to their wide applications in many fields, including optics, magnetism and catalysis^1,2, and because they have physical and chemical properties that change in size as a result of the surface change in size, The nanoclusters made of noble metals, especially the PtAu_n nanocluster, are attractive catalysts^3,4. The physical and chemical properties of bimetallic clusters depend not only on the size and shape, but also on the atomic structure of the two metallic elements⁵. Therefore, the researchers were interested in conducting the current studies to find the new structural and electronic variables possessed by the groups due to their new size^6,7.

 

Both particles of noble metals such as gold and platinum have wide uses, whether in organic chemistry, where they play an important role in protein delivery⁸, or their important role in cancer treatment⁹, It also has a great ability to resist fungi¹⁰, Because of their potential as optical sensors contributing to phototherapy, they generally play a broad role in sensor synthesis and biomedicine^11-13, Platinum particles are also included in the catalytic oxidation of blue carbon, as well as in the general electrochemical behaviors of amino compounds, and generally in many applications in various fields^14-16.

 

DETAIL OF CALCULATIONS:

The electronic structure calculations of PtAu_n (n = 1-9) clusters were performed using the density functional theory (DFT)¹⁷, as implemented in the SIESTA program ¹⁸. This code uses norm-conserving Troullier-Martins nonlocal Pseudopotentials¹⁹, it also uses flexible basis sets of atomic orbital’s that are of the positional Gaussian type. The exchange correlation energy was evaluated using the generalized gradient approximation (GGA) parameterized by Perdew Burke and Ernserh of (PBE)²⁰, and local density approximation LDA²¹.

 

Self-consistent field (SCF) calculations were performed with the estimated convergence criterion of 1 × 10⁻⁶ a.u.

For the total energy, we use the double (DZ) basis with polarization function for Pt and Au atoms. With energy shift parameter of 50meV, the change density was calculated in the regular real space network with cut-off energy of 170 Ry. The simulation sets were placed within a large cubic cell with a parameter of 40 Å, Including the necessary spaces between adjacent groups and imposed periodic boundary conditions.

 

RESULTS AND DISCUSSION:

Structural properties of clusters Au_n+1  and PtAu_n (n = 1-9)

Structural characteristics:

The calculated structural properties of pure gold and platinum-doped gold clusters depend on the groups structure, in addition to the positions of the atoms and the average bond length between them, the density function theory (DFT) was chosen. Using generalized gradient approximation (GGA) and local density approximation (LDA) to reach the most stable structures with lower energy. In this work we have come up with the electronic structures of the most stable groups using the application of annealing simulation (SA), which has gone through the following stages:

 

·       The first stage: a random group of atoms is placed in the block simulation box.

·       Stage 2: We raise the temperature of the system until it is about 1,000 K in a total of 1,000 iterations.

·       Stage 3: System temperature is stable at T = 1000 K for about 500 iterations.

·       Stage 4: We gradually lower the temperature of the system until t = 0 K in 1,000 iterations.

 

Figures 1 and 2 represent the most stable pure gold and platinum-doped groups, respectively.

 

Figure 1. The most stable Au_n (n=2-10) clusters.

 

Figure 2. The most stable PtAu_n (n=1-9) clusters.

 

Adiabatic electronic affinity (AEA) and Vertical electronic affinity (VEA):

using approximation (GGA):

In the following figure, we obtained the values of adiabatic electronic affinity (AEA) and vertical electronic affinity (VEA) for Au_n+1 and PtAu_n (n = 1-9) clusters in terms of cluster size in (GGA) approximation.

 

Figure 3. Adiabatic electron affinity (AEA) and vertical electron affinity (VEA) of Au_n+1 and PtAu_n (n=1-9) clusters in approximation (GGA).

 

It is clear from the curve represented in Figure 3 that the adiabatic electron affinity property increases with the size of the studied cluster and this is for both Au_n+1 and PtAu_n clusters in general, It is also clear that the clusters for (n= 4,6,9) are the least stable clusters compared to the other clusters and this is for Au_n+1 clusters, While we find that the clusters for the value of (n = 3,6,8) are also less stable compared to the rest of the clusters of type PtAu_n , In general, it is clear that PtAu_n clusters are characterized by high stability compared to the Au_n+1 clusters, except what was recorded in Au₉ cluster which showed higher stability than PtAu₈ cluster.

 

It also shows from the vertical electron affinity change curve for Au_n+1 clusters that clusters Au₇ and Au₉ appear to be less stable compared to the rest of Au_n+1 clusters, while PtAu₄ and PtAu₇ clusters are also less stable compared to other clusters. Thus, in general, we find that PtAu_n clusters are the most stable compared to Au_n+1 clusters, except what was recorded in each of PtAu₄ and PtAu₈ clusters which are characterized by less stability compared to Au₅ and Au₈ clusters, we find that they are roughly in agreement with the results of²².

 

Using approximation (LDA):

We also obtained the curve shown in figure 4, that gives the values of adiabatic electronic affinity (AEA) and vertical electronic affinity (VEA) for Au_n+1 and PtAu_n clusters in approximation (LDA).

 

Figure 4. Adiabatic electron affinity (AEA) and vertical electron affinity (VEA) of Au_n+1 and PtAu_n (n=1-9) clusters in approximation (LDA).

 

It is clear from the curve represented in figure 4, that the adiabatic electron affinity property increases with the size of the studied cluster, and this is for each of the Au_n+1 and PtAu_n clusters in general. As it turns out, the clusters are Au_n+1 which are for (n =5,8) are the two least stable clusters compared to other pure clusters, in the case of doping clusters and for values (n = 5,9) they are also less stable compared to the rest of the clusters.

 

It can be concluded that all PtAu_n clusters show high stability compared to Au_n+1 clusters, except in the case of Au₁₀ which showed greater stability compared to its counterpart PtAu₉, By comparing our results for gold clusters, we find that they are roughly in agreement with the results of²².

 

Enthalpy:

The enthalpy values are calculated for clusters of both pure gold and gold impregnated with a platinum atom when we use the approximation (GGA), which is given by the relationship:

 

H(eV) = E + PV                                                            (1)  

 

Where:

E is the energy of the cluster, PV is the value of the pressure exerted on the cluster times the volume of space it occupies.

 

Where the following two figures (5-a) and (5-b) represent the enthalpy of Aun+1 and PtAun clusters in terms of cluster size in approximations (GGA) and (LDA) respectively.

 

Figure 5. Enthalpy of Au_n+1 and PtAu_n clusters in approximation: (a) (GGA), (b) (LDA).

 

Through the curve shown in figure (5-a) and (5-b), it is clear that the enthalpy values of Au_n+1 clusters take the lowest values, as the larger the cluster size, the enthalpy remains approximately fluctuating at the same value, Whereas in PtAu_n clusters, we find that PtAu cluster has the largest value in enthalpy, then it decreases with increasing cluster size, eventually reaching the lowest value at PtAu₉ cluster. The enthalpy values strongly indicate that Au_n+1 clusters are more stable than PtAu_n clusters.

 

Chemical hardness:

The hardness is symbolized by the symbol η, and it is calculated for clusters in order to understand their chemical stability, where the large values of chemical hardness indicate that the cluster is less reactive^23,24 and this is in the basic state of the mineral.

 

The chemical hardness relationship is given by:

                                                             (2)

 

The obtained results are represented in figure 6, where it represents the chemical hardness values for Au_n+1 and PtAu_n clusters in terms of cluster size in both approximations (GGA) and (LDA).

 

Figure 6. Chemical hardness of Au_n+1 and PtAu_n (n=1-9) clusters in approximate (GGA) and (LDA).

 

We notice that in general, the chemical hardness values generally decrease with increasing cluster size, and this is for Au_n+1 or PtAu_n clusters in both approximations used, This means that clusters of large size are less chemically reactive and more stable than clusters of small size, in addition, other peaks were recorded at Au₅ in (GGA) and (LDA) approximations, as well as the peak at the PtAu₄ cluster in (LDA) approximations, where these clusters showed that they are the least chemically reactive than other clusters.

 

CONCLUSION:

In this paper, we performed DFT simulating annealing calculation of geometric, Chemical and electronic structure of Au_n+1 and PtAu_n clusters with (n = 1-9). Our results show that new structures are obtained for each cluster size. Vertical electronic affinity (VEA) and adiabatic electronic affinity (AEA) are found where it was concluded that for PtAu_n clusters are the most stable compared to Au_n+1 cluster, except for the cluster Au₁₀ Where showed greater stability compared to its cluster counterpart. The enthalpy values strongly indicate that the Au_n+1 clusters are more stable than the PtAu_n clusters, Also the chemical hardness values generally decrease with increasing cluster size.

 

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Received on 09.04.2022                    Modified on 03.07.2022

Accepted on 16.08.2022                   ©AJRC All right reserved