INTRODUCTION:
Over the past two decades, nanomass physics and chemistry have become of great interest to researchers, As researchers were interested in finding the unique properties of these groups, their unique structure between the molecule and 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. In recent years, emphasis has been placed on the structural, electronic and optical properties of bimetallic mixed groups. 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 a lot of attention, due to their promising applications in several fields, including optics, magnetism, and catalysis1,2, This is because it has physical and chemical properties that change in size as a result of changing the surface in terms of volume, Nanoclusters made of noble metals, especially PtAun nanoclusters, are attractive catalysts3,4. 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 elements as well5. Therefore, current studies are done on finding the new structural and electronic variables that groups have due to their new size6,7.
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 biomedicine11-13, 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 fields14-16.
In this work, we focus on the study of clusters and their effect on catalysis, and by comparing the results reached, and then deducing which clusters showed the best catalysis property.
DETAIL OF CALCULATIONS:
The electronic structure calculations of PtAun (n = 1-9) clusters were performed using the density functional theory (DFT)17, as implemented in the SIESTA program18. 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 × 10−4 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 Aun+1 and PtAun (n=1-9)
Structural characteristics:
The calculated structural properties of the metallic free gold and platinum-saturated gold blocks depend on the structure of the clusters, As well as the locations of the atoms and the average bond length between them, the DFT theory was chosen using generalized gradient approximation (GGA) and local density approximation (LDA) in order to reach the most stable structures with less energy, in this work we have come up with the electronic structures of the most stable clusters using the application of annealing simulation (SA), Which goes 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 Aun (n=2-10) clusters.
Figure 2. The most stable PtAun (n=1-9) clusters.
The electronic properties:
The second difference in binding energy:
In the field of cluster physics, the second difference in binding energy is very important; it shows the stability of the electronic structure, as it can be calculated from the following relationship:
Whereas:
E (n+1), E (n-1) and E (n): binding energies for each atom corresponding to the sizes (n+1), (n-1), and n respectively. And the results of the values of the second difference in binding energy ∆2E(n)(eV) are shown in figure 3.
Figure 3. The second difference in binding energy of Aun+1 and PtAun (n = 1-9) clusters.
From the general form of the curves for the second difference in binding energy in terms of cluster size, We note the oscillation pattern for the pure gold Aun+1 cluster according to the approximations of (GGA) and (LDA) for the values (n = 8,6,4,2) at n = 6, we find it is the most stable structure compared to its neighboring clusters, Whereas, for PtAun clusters according to approximations (GGA) and (LDA), they have the largest values for the energy of the second difference in binding energy for (n = 8, 6,3,1) compared to the neighboring clusters, They are the ones who enjoy the most stability. The results of the second difference in binding energy are close to the results obtained in the work22,23.
Homo Lumo gap:
The energy gap is defined as the minimum energy required to move an electron from the upper occupied orbital (HOMO) to the lower unoccupied orbital (LUMO), Where scientific research has proven that energy gaps play a major role in representing the ability of the cluster to achieve chemical stability, and knowing the extent of its interaction with external influences24.
In large-sized systems, interactions between partial orbital’s increase, and thus the upper-level energy (HOMO) increases and the lower-level energy (LUMO) decreases, and thus the gap between them decreases. The results were obtained and represented in figure 4, which represent the upper and lower bound values of the gap for clusters Aun+1 and PtAun in terms of cluster size.
Figure 4. Homo lumo gap energy of Aun+1 and PtAun (n = 1-9) clusters.
It is evident from the curve of figure 4, that the values of the upper and lower limits of the energy gaps record fluctuations, sometimes increasing and sometimes decreasing and this is along the size n, we also recorded high values in the upper and lower bounds of the energy gap in the cluster Au3 by approximation (GGA) and (LDA) and cluster PtAu2 by approximation (GGA) estimated at 1.2 eV, 1.12 eV, 1.1 eV, respectively, which indicates that it is more stable compared to the rest of the clusters. It also shows us the lowest values of the upper and lower bounds of the energy gap at Aun clusters by approximating (LDA) starting from (n = 3-9) are the lowest in values, this means that they are the most chemically active.
Vertical ionization potential (VIP) and adiabatic ionization potential (AIP):
The obtained results are represented in figure 5, which represents the values of the vertical ionization potential (VIP) and the adiabatic ionization potential (AIP) for of Aun+1 and PtAun clusters in terms of cluster size if we use the approximation (GGA).
Figure 5. Vertical ionization potential (VIP) and adiabatic ionization potential (AIP) for Aun+1 and PtAun (n=1-9) clusters in (GGA) approximation.
It is clear from the curve represented in figure 5, that both the vertical ionization potential (VIP) and the adiabatic ionization potential (AIP) are decreasing with the increase in the size of the cluster; it is known that the smaller the values of the vertical ionization potential, the more the cluster will have similar properties to metals, Therefore, it turns out that the clusters of each of Aun+1 or PtAun for values equal to or greater than (n = 7) are the ones that clearly bear the metallic properties, While the smaller values of the vertical ionization potential and the adiabatic ionization potential showed that the clustersAu10, PtAu9, Au8 and PtAu8 respectively, ionize more easily than others, It is also noted that the Au2 cluster is the most stable cluster compared to the PtAu cluster, and this also applies to the Au10 cluster when compared to the PtAu9 cluster.
When using approximation (LDA):
The results obtained are represented in figure 6, which represents the values of the vertical ionization potential (VIP) and the adiabatic ionization potential (AIP) for Aun+1 and PtAun clusters in LDA.
Figure 7. Vertical ionization potential (VIP) and adiabatic ionization potential (AIP) for Aun+1 and PtAun (n = 1-9) clusters in LDA approximation.
It is clear from figure 7, that the values of both the vertical ionization potential (VIP) and the adiabatic ionization potential (AIP) are decreasing with the increase in the size of the cluster in general, Where it was shown to us through the small values of each of the vertical ionization potential and the adiabatic ionization potential that some clusters have properties similar to the properties of minerals, we show that the Aun+1 or PtAun clusters for values equal to or greater than (n = 8) have the outstanding metallic properties. It is clear from this that the minimum values of (VIP) and (AIP) show that the following clusters Au9, PtAu8, Au10, and PtAu9 are the ones that ionize more easily than other clusters, It is also noted that the Au2 cluster is the most stable cluster compared to the PtAu cluster, while the PtAu9 cluster shows this characteristic compared to its Au10 counterpart.
Fragmentation energy:
Among the computed properties also, we calculated the fragmentation energy, which is also a good criterion for predicting the relative stability witnessed by the studied clusters, depending on the automatic fragmentation energy, in this work it was found for Aun+1 and PtAun (n = 1-9) clusters.
The calculated fragmentation energy is subject to the following relationship:
where: E(Au), E(Aun+1 ) and E(PtAun+1) are the total energies of the free gold atom, the gold cluster and platinum doped gold cluster, respectively.
Based on the above formula, the evolution of the calculated fragmentation energy value compared to the cluster size is shown in figure 8.
Figure 8. The fragmentation energy of Aun+1 and PtAun (n = 1-9) clusters in (GGA) and (LDA) approximations.
It is noticed from the curve that there are fluctuating behaviors of this energy, we find it decreasing sometimes in some clusters, and increasing in other times for other clusters, as it appears that clusters with values of n = 4,6,7 are the most stable clusters, This is in both approximations compared to the clusters adjacent to them, we also see through the curve that the two clusters Au7 and PtAu6 are the most stable among all the studied clusters and this is in both approximations.
In general, all the calculated properties indicate the importance of these clusters in the field of catalysis; these results are very close to what was achieved in the work25,26.
CONCLUSION:
In present work, the structural and electronic
properties of Aun+1
and PtAun (n = 1-9) clusters with are investigated by density
functional theory calculations
(DFT) with generalized gradient approximation (GGA) and local density approximation
(LDA). Clusters with total number of atoms up to five were found
to have planar structures, The second differences energies of cluster show that
the lowest energy Au3 is more stable than neighboring clusters in (GGA)
and (LDA), and the cluster 
withe generalized gradient approximation (GGA),
the ionization potential and adiabatic generally decreases with the increase of
clusters size, the HOMO–LUMO gap and fragmentation energy shows
clear odd-even oscillation.
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Received on 07.04.2022 Modified on 28.05.2022
Accepted on 04.07.2022 ©AJRC All right reserved
Asian J. Research Chem. 2022; 15(4):251-255.