Roles of Oxygen Vacancies on Metal Oxides

Introduction

Oxygen vacancies are conductive nature, optical specificity and magnetic characteristics of oxygen in compounds of silicon oxide SiO2, magnesium oxide MgO and titanium oxide TiO2. The vacancies occur due to the removal of an oxygen atom. According to research, the role of OV has less study despite its relevance in chemical properties. In this report, we calculate energies to form oxygen vacancies in stable compounds of TiO2 and SiO2 nanoclusters where the oxygen vacancy ranges from two to 100 nanometers. Mixed oxides applications are many despite that their understanding is lagging scientifically. A relevant surface constitution and structural properties are absent. Therefore, the concept of mixed oxides has deteriorated over a long period.

Background

The mixed oxides comprise of molybdates, spinels, perovskites, vanadates, among other mixed oxides. The perception of mixed oxides by scientists is an attribute of modern technologies. Metal oxides are oxides of important classification both scientifically and technologically. Silicon and titanium oxide compounds are available in large quantities and are useful geologically, in astronomy, environmentally, chemical sciences medical sciences and biology. The speed at which the oxides exchange oxygen determines their characteristics. For titania, the loss of oxygen results in the color change from white to blue, magnetic observation and photocatalytic characterization. Silica, on the other hand, has oxygen vacancies that undergo modification electronically and optically.

Oxygen in silica has a property of electric disintegration and deforms electrically in memory devices. Effects of forming an OV in large quantities and surfaces of silicon and titanium oxides are well informative despite their role having less information. Through DFT, computations, the energetic properties of oxygen vacancies in silica and titania displays their formation differences. The generation of energy clusters depends on the experiments and stabilization of landing techniques. The reactivity of nanoclusters is an estimate of reduction by removal of oxygen.

The computation of the energetic value of drawing an oxygen atom from a metal oxide compound uses the following equation:

MxO2x → MxO2x−1 + 1/2 O2

Evac = EMxO2x−1 + 1/2EO2 − EMxO2x

Titania is a reducible oxide while silica is reducible. Hence, silica tends to possess higher energy value for it to form an oxygen vacancy than titania. Energy values are measures of various systems and ranks of irreducibility. Distinctive measures for surface and bulk E values in titanium oxide and silicon oxide are 3.65-4.51eV and 5.2-7.8eV respectively. Moreover, electrons localization after the oxygen displacement gives information relating to the electron configuration of defective matter.

Mx+− O2—Mx+→ Mx+− 2e—Mx++ ½O2

The removal of neutral oxygen from titania forms Ti3+ ion in surface and bulk which is a paramagnetic site. Reduction of TiO2 forms an open shell electron configuration. In silica and quartz, reduction leads to the production of silicon-silicon single covalent bond, where the two silicon atoms will neutralize two electrons that leads to closed-shell electronic state.

The properties of a material are subject to change depending on the effects of reduction and the values of energy, an example is in ZrO2, TiO2, and CeO2 that may lead to change in the properties of a material. In this experiment, the focus will be on energy, ability to lose oxygen and electronic characteristics of silica, titania, and their combination. We will then compare the chemical properties with other oxides according to the compound size and the bulk.

Experimental Details

There are two procedures for computing energy values:

1. Remove an oxygen atom from a nanocluster whose structure has frizzed where the energy difference before and after the removal of neutral oxygen is E1.
2. The nanocluster structure after the removal of oxygen undergoes an optimization by use of energy difference before and after optimizing. We then determine the relaxation energy E3 to be the difference between E1 and E2.

We tested every oxygen site for small sizes between 2 and 10 nanometers but reported only the representatives. A comprehensive way for larger sizes between 11-100 was challenging during calculations. Therefore, we resorted to taking a sample of 10 sites. We made the DFT calculations, which involved reduced sites using polar spin formula. For most computations, there was no coercion on multiplicity spins systems, though in the case of titania, we coerced the multiplicity spin to a single shell to avoid convergence. There is a recent report on Zirconia particles ferromagnetism to be at room temperatures, which is an implication that, it can also affect titania in future studies.

Results

Generally, SiO2 structures have between two to four oxygen sites that not saturated. The structure's centers have planar triple coordination involving oxygen terminals. The least amount of energy we used to remove oxygen atom from silica, nearly all the nanoclusters of silica. The energy used ranged from 2.5 to 2.9eV with energy increase as the size of the silica was increasing. The SiO2 of N=12 differ from others since it displays species with no bridging terminals, that is, silica has four free coordinated bonds. Removing the atom of oxygen from the non-bridging silica needs higher energy of 3.55eV to the silicon defects. We observed that the energy for removal of oxygen for non-bonding and silanone are lower than the ones for coordinated oxygen atoms. From the calculations we made, we found that quartz possesses energy values between 5.1eV and 5.4eV with concentration vacancies ranging from 0.04 to 0.33 per silicate unit.

The estimates are consistent throughout with other work where the amount of energy is 5.15V. For example, we compared the plane-wave method results with the results we obtained, and we found similar answers. We realized that it costed more energy to remove two oxygen atoms, which uses between 4.1eV and 5.7eV, these results were also similar with ones we obtained for bulk alpha quartz. The implication here was that the oxygen terminals are less energetic in SiO2 nanoclusters. Structurally, we note that, silanone reduction form 2 bond silicon site showing a silicon-oxygen bond of distance 1.71 angstroms which are longer than Pacchioni calculations with a value of 1.61 angstroms for Si-O bond. A further comparison of results with the cluster bulk calculations showed the bonding angle between silicon and two oxygen atoms as 87 to 111 degrees depending on defect type.

Titanium oxide

We observed that the titania nanocluster Structures are dense and are symmetrically less than SiO2 clusters for the range we used, with Ti atoms being in 4 to 6 fold coordinate setting. Many of these structures, especially for smaller sizes than N =20, have end oxygen sites attached to a titanium atom in a tetrahedral setting (≡Ti–O). In their presence, the terminals use the least energy to remove an atom. In some cases, where, N = 4, 9, 11, 12, the removal of a bonded oxygen do not have the lowest associated Eunrel value, removing a 2 fold oxygen atom nearby can cause a reduced atom with similar characteristics as the one that 58eV and 5.02 eV and display unclear dependent trend, unlike in the situation of silica nanoclusters.

The energy cost to reduce an oxygen neutral critically depends on the structure of the clusters. In the case of N = 17, 22, the energy range for TiO2 nanoclusters was found to have a comparison with that for silicon mixed oxide nanoclusters. In the same cases, after the easing, the titanium mixed oxide nanocluster configuration we found that it does not have either oxygen terminal sites or three-fold bonded Ti defective clusters, which is highly nonstable when left in open after reduction process. We were unable to get energy cost for bulky systems with the calculation environment for the nanoclusters because of extreme merging difficulties from titanium 3 ion clusters. Energy costs for titanium mixed oxide oscillate between twenty and thirty percent which is a value between 4.85eV and 5.4eV.

We also found out that the energy for an attached oxygen in an anatase was 3.75 eV, using the procedure we used here. The value is the same to the mean value of 3.75 eV, which we computed for the lowest energy cost over the fixed nanoclusters. For the titanium oxide with N=35, the reduction of three oxygen atoms resulted in a low energy value compared to that for reducing one atom. The property derives from the strong relaxation from the vacancy available. Relaxation of the titanium mixed oxide nanocluster after oxygen reduction comprise more Evac than in silica.

The average E3 value for the least energy cluster is 1.53 eV, and in sometimes it more than 2.45 eV for N = 21 and N= 23. With exceptional, like N = four, N= 17 and N=22, the reduction of coordinate oxygen generates a sensible amount of relaxation. However, removing 2-fold terminal oxygen atoms causes a suggestive structure displacement.Either the electronic configuration of the oxygen deficient centers is a triplet with two unpaired electrons filling two titanium atoms or a single open shell with unpaired electrons delocalized on three Ti clusters near the oxygen vacancy.

Mixed silicon and titanium oxides

To study the impact of mixing silica and Titania on energy values, we chose y titanium atoms, y-1 silicon atoms and two oxygen nanoclusters and a titanium y content between 0 and 1. The nanoclusters we used shows difference in isomeric properties for pure and mixed configurations. The configurations exhibit ≡Ti-O bonds for oxygen atoms, except the pure (TiO2)10 structure. The pure silica with N=10 nanocluster shows only silanone coordinates, while mixed system shows two bonded oxygen clusters. The availability of titanium allows for more stable bonded oxygen atoms than silicon.. The mixed structure with y = 0.1 has two terminal oxygen sites joined to a Silicon and titanium atoms.

The removal of oxygen requires more energy when it is bound to a silicon atom than a titanium atom. When the amount of TiO2 increases, the Evac cost increases gradually up to y = 0.30, then start to decrease with an obvious level of oscillation. For the span of (TixSi1−xO2)10 nanoclusters, relaxation impacts are not clear for centers with Titania contents of y = 0.0, 0.1, 0.3. In the first two cases, the less energetic oxygen removed is from a silica or a Si-NBO bond, and therefore, the configuration relaxation effects are indifferent from those of pure silica.

Discussion

The outcomes of the experiment aimed at nanosize reliance of the oxygen vacancies formation in SiO2 and TiO2 structures. The removal of a one coordinate terminal oxygen atoms requires low energy values. The energy for breaking a metal oxygen bond explains the concept behind the low Evac value compared with the energy needed to disintergrate two metal oxygen bonds for oxygen atoms in 2-fold coordinated bonds. The formation of coordinates between oxygen and vanadia has an association with chemical characteristics. The lower Evac value of nanoclusters with comparison to bulk costs shows higher reducible property of the nanosized structures as seen in zirconium oxide. However, the lowest electronic energy for silica was, 2.4–3.5 eV, which is lower than for titania clusters, 2.48–5.02 eV.

The cost of computations in a direct convergence was high, and therefore we never managed to find out a direct relationship between bulk silica and bulk titania.

However, scientist have come up with studies on oxygen vacancy formation in mixed metal oxides that associate a higher reducible property to titania. Our outcomes showed that silica has high reducibility property than titania; this is a contradiction for bulk structures. The reason behind the characteristic has its association to the presence of bonded silanone oxygen atoms in low energy SiO2 nanoclusters. The levels of the two oxygen atoms in titania have a range of 3eV energy distribution. In titania with N=11, the electronic energy level of the hanging energy is close to Fermi levels ranging from -8 to -11eV.The ones which associates to two-fold coordinate sites are between -8 to -13eV.The electronic configuration of the dangling oxygen in silicon oxide is related to the reactivity of the site and has low energy.

Conclusion

In our experiment, we used DFT-based computations applying the PBE0 functions to investigate oxygen vacancy formation optimized titania, silica, and their mixture for the range between 2 and 100 units. The characteristics we calculated are critically dependent on the nature of the metal oxide. The property of silica is relatively constant and has the presence of hanging oxygen atoms in silanone sites. In titania and titanosilicate nanoclusters, we found a high dependence of the characteristics according to the local geometry.

When unsaturated oxygen clusters are in the structure, the removal requires less energy than removing two oxygen atoms. Since the centers have natural presence in low energy environments, unlike in bulk and surfaces, we can determine that the reduction of silica clusters requires less energy. Moreover, we realized that the oxygen vacancy creation in SiO2 nanoclusters is also more satisfactory than from TiO2 nanoclusters. The occurrence of an unpredicted chemical property is due to the small size of the structure. The results of our experiment has applications in the fields of science and technology.