Shchukin A.I., Vargalyuk V.F., Stets O.S., Posudievska O.R.
Oles Honchar Dnipropetrovsk National University
QUANTUM-CHEMICAL MODELING OF THE FORMATION AND GROWTH OF HYDRATED COPPER CLUSTERS
The research of submicron materials has gained rapid development in the last few years due to its existing or potential application in many technological areas such as electronics, catalysis, magnetic storage of information and so forth. There are many ways of creation of nanomaterials that can be divided into the so-called «from-top-to-bottom» methods (i. e. grinding the substance) and «from-bottom-to-top» methods (i. e. formation of nuclei, then clusters and nanocrystals). Electrodeposition belongs to the latter methods. It is a quite low-temperature process. It gives possibility of very precise control of the amount of received substance. Electrocrystallization process compared with the vapor deposition allows allocating the composition of the metals that are significantly different in their boiling point.
A quantum chemical modeling experiment should be used in the research of such objects along with classical electrochemical methods. DFT-methods of quantum chemical modeling are widely used for theoretical investigations of the structure and properties of metal clusters, particularly of copper. The combination of DFT-modeling with electrochemical research methods helps to explain the specific effects that accompany the process of electrocrystallization of copper from aqueous solutions, as well as to investigate the structure and particular properties of individual clusters, which are the proto-nuclei of metal phase.
Modeling was performed using the hybrid B3LYP method of DFT. This method provides sufficient accuracy in the description of the electronic transitions involving complex ions of d-metals. Energies of complex structures were specified considering solvating effects using continuum polarization model. The following basic sets were chosen for the calculations: the atoms of copper were described by Wachters+f base, and the atoms of hydrogen and oxygen were described with 6-311G ** base set. As the comparison of the electronic energy of different structures is correct only if they have the same content of each type of atoms, we have considered systems [Cuz+(H2O)n]aq·m(H2O)aq in the calculation of energy systems with different geometries of inner coordination sphere. The structure with the least amount of energies of the hydrated copper complex, as well as of energies of water molecules in m-quantity in the outer sphere was considered as the most energetically favorable, provided that (n + m) = const.
Initial stages of copper electrocrystallization were investigated using chronopotentiometry. Galvanostatic η,t-dependencies were recorded at the cathodic current densities of 2÷8 mA/sm2. The parameters of nucleation process were calculated according to the Hutsov model of galvanostatic nucleation, assuming two-dimensional and three-dimensional nucleation.
At the initial stages of copper electrocrystallization quantum-chemical modeling of cluster systems with the number of copper atoms from 2 to 14, having water molecules as ligands, was carried out in order to identify the regularities of formation of proto-nuclei of metal phase, as well as to analyze energy effects of the formation and growth of such clusters. Simulation of the successive addition of atoms to clusters showed that the particle which consists of seven copper atoms is the maximum cluster size, which entirely consists of hydrated atoms. Further joining of atoms occurs along with detachment of water molecules. The atoms in this cluster are arranged in a pentagonal bipyramid. Such structure corresponds to the D5h type of symmetry. The structure is repeated as a part of all larger-sized clusters. According to the published data, this type of symmetry is characteristic for copper microcrystals obtained by electrochemical crystallization on the foreign surface.
As it was observed, energy effects of the formation of copper clusters are strongly dependent on their size. Changes in energy effects are oscillatory in nature. Ionization potentials of clusters change similarly. The formation of clusters with an even number of atoms releases more energy than the formation of clusters with an odd number of atoms. Ionization of clusters with an odd number of atoms is thermodynamically more favorable than that with an even number of atoms. In our opinion, this happens due to the pairing of electrons in the formation of clusters with an even number of atoms.