Fig.1: Schematic representation of the pure Zirconium dioxide ZrO2 using the Xcrysden package [46].
Results and discussions
The total and partial density of states (DOS) of the pure zirconium dioxide ZrO2 have been plotted in order to explore the electronic behavior of the studied system.
The values of the electronegativity between the Zr and O elements are very different, while such values are: 1.33 for Zr atoms and 3.44 for O atoms. The difference between such values is equal to 2.11 which is larger than 1.7 on the Pauling scale. Additionally, the studied material is characterized by the ionic bonding. This ionic chemical bonding behavior can be explained by using the partial DOS from shown in Fig. 2 (a). From this figure, the 2p orbitals of O and 4d of Zr are different below and above Fermi level. In fact, the 2p states of O dominate below the Fermi level, while in the above Fermi level, the 4 d states of Zr are dominating.
Moreover, Fig.2 (b) shows that this material does not present any magnetic behavior, since a perfect symmetry is present between up and down density of states. Also, this compound is a semiconductor with a direct band gap value ≈ 3.1 eV. On the other hand, Fig.2 (c) presents the band structure of the pure ZrO2. This figure presents a direct band gap at the G point and confirms the value of the band gap of the pure ZrO2 found in Figs.2 (a) and 2 (b).
Such results are in good agreement with other theoretical calculations such as the value 3.09 eV using VASP program [58] while for the monoclinic phase the obtained value is located between 3.4 and 5.4 eV using (SIESTA) package [59].
On the other hand, Figs. 3 (a), 3 (b) and 3 (c) provide the obtained results of doping the dioxide ZrO2 with 5% of sulfur. In fact, Fig.3 (a) illustrates partial density of states showing the orbitals
Zr-(4d), O-(2p) and S-(3p). This figure shows also that the contribution of the partial density of states orbital O-(2p) is dominating in the valence band, while the contribution of the partial of density of states of Zr-(4d) is important in the conduction band region.
Moreover, Fig.3 (b) shows that the doping with 5% of sulfur make appearing a band gap value of 2.3 eV. Also, a perfect symmetry between spin-up and spin-down confirms the non-magnetic character of the doped zirconium dioxide ZrO2.
In addition, the band structure shown in Fig.3 (c) confirms the band gap value (2.3 eV) predicted in Figs.3 (a) and 3 (b). Similarly, to the Fig.2 (c), a direct band is found at the G point in Fig.3 (c).
Moreover, the existing experimental works showed that the formation of Zr–S bonds causes a decrease of the band gap from 3.38 eV to 2.46 eV [60]. When increasing the doping concentration values of sulfur atoms, the band gap decreases as it is shown Figs. 4 (a), 4 (b) and 4 (c). Such figures are plotted for 9% of sulfur concentration and predict a band gap value: 1.2 eV. This band gap decreases to achieve the value 0.6 eV as it is illustrated in Figs. 5 (a), 5 (b) and 5 (c) when doping with for 14% of sulfur. This band gap disappears when the doping concentration reaches value 18 % of the doping with sulfur as it is presented in Figs. 6 (a), 6 (b) and 6 (c).
Our results of band gap values of pure and doped zirconium dioxide ZrO2 are summarized in table 1 for the concentration values: 5%, 9%, 14% and 18%. This table reproduces the already mentioned results from different figures.
To complete this study, we present in Fig.7 the energy and band gap of the doped ZrO2 as a function of different concentrations (%) of sulfur atoms. From this figure, it is clear that when increasing the doping concentration of sulfur the band gap energy decreases. While the total energy of the studied system increases when increasing the sulfur concentration.
It is worth to note that the band gap value 1.2 eV, which useful for photovoltaic applications, which is reached for 9% of sulfur concentration.