ZINC OXIDE (ZNO): STRUCTURE, PROPERTIES, USES, RISKS - CHEMISTRY - 2024

The zinc oxide is an inorganic compound with the chemical formula ZnO. It is composed solely of Zn ²⁺ and O ^2- ions in a 1: 1 ratio; however, its crystalline lattice can present O ^2- vacancy, which gives rise to structural defects capable of altering the colors of its synthetic crystals.

It is commercially acquired as a powdery white solid (lower image), which is produced directly from the oxidation of metallic zinc by the French Process; or subjecting zinc ores to carbothermic reduction, in such a way that their vapors then oxidize and end up solidifying.

Watch glass with zinc oxide. Source: Adam Rędzikowski

Other methods of preparing ZnO consist of precipitating its hydroxide, Zn (OH) ₂, from aqueous solutions of zinc salts. Likewise, morphologically varied thin films or nanoparticles of ZnO can be synthesized using more sophisticated techniques such as the chemical deposition of their vapors.

This metal oxide is found in nature as the mineral zincite, whose crystals are usually yellow or orange due to metallic impurities. ZnO crystals are characterized by being piezoelectric, thermochromic, luminescent, polar, and also by having a very wide energy band in their semiconductor properties.

Structurally, it is isomorphic to zinc sulphide, ZnS, adopting hexagonal and cubic crystals similar to those of wurzite and blende, respectively. In these there is a certain covalent character in the interactions between Zn ²⁺ and O ^2-, which causes a heterogeneous distribution of charges in the ZnO crystal.

Studies of the properties and uses of ZnO extend to the fields of physics, electronics and biomedicine. Its simplest and most everyday uses go unnoticed in the composition of facial creams and personal hygiene products, as well as in sunscreen.

Structure

Polymorphs

ZnO crystallizes under normal pressure and temperature conditions in a hexagonal wurzite structure. In this structure, the Zn ²⁺ and O ^2- ions are arranged in alternating layers, in such a way that each one ends up surrounded by a tetrahedron, with ZnO ₄ or OZn ₄, respectively.

Also, using a "template" or cubic support, ZnO can be made to crystallize into a cubic zinc blende structure; which, like wurzite, correspond to isomorphic structures (identical in space but with different ions) of zinc sulfide, ZnS.

In addition to these two structures (wurzite and blende), ZnO under high pressure (around 10 GPa) crystallizes in the rock salt structure, the same as that of NaCl.

Interactions

The interactions between Zn ²⁺ and O ^2- present a certain character of covalence, for which there is partially a Zn-O covalent bond (both atoms with sp ³ hybridization), and due to the distortion of the tetrahedra, they manifest a moment dipole that adds to the ionic attractions of ZnO crystals.

Blende (left) and wurzite (right) structure of ZnO. Source: Gabriel Bolívar.

You have the upper image to visualize the tetrahedra mentioned for the ZnO structures.

The difference between the blenda and wurzite structures also lies in that seen from above, the ions are not eclipsed. For example, in wurzite, the white spheres (Zn ²⁺) are seen just above the red spheres (O ^2-). On the other hand, in the cubic blende structure this is not the case because there are three layers: A, B and C instead of only two.

Nanoparticle morphology

Although ZnO crystals tend to have hexagonal wurzite structures, the morphology of their nanoparticles is another story. Depending on the parameters and synthesis methods, these can take such varied forms as rods, plates, leaves, spheres, flowers, belts, needles, among others.

Properties

Physical appearance

Odorless, white powdery solid with a bitter taste. In nature it can be found crystallized, with metallic impurities, such as zincite mineral. If such crystals are white, they show thermochromism, which means that when they are heated they change their color: from white to yellow.

Likewise, its synthetic crystals can present reddish or greenish colors depending on their stoichiometric oxygen composition; in other words, the gaps or vacancies caused by the lack of O ^2- anions directly affect the way in which light interacts with ionic networks.

Molar mass

81.406 g / mol

Melting point

1974 ° C. At this temperature it undergoes thermal decomposition releasing zinc vapors and molecular or gaseous oxygen.

Density

5.1 g / cm ³

Water solubility

ZnO is practically insoluble in water, hardly giving rise to solutions with a concentration of 0.0004% at 18ºC.

Amphotericism

ZnO can react with both acids and bases. When it reacts with an acid in aqueous solution, its solubility increases by forming a soluble salt where Zn ²⁺ ends up complexing with water molecules: ²⁺. For example, it reacts with sulfuric acid to produce zinc sulfate:

ZnO + H ₂ SO ₄ → ZnSO ₄ + H ₂ O

Similarly, it reacts with fatty acids to form their respective salts, such as zinc stearate and palmitate.

And when it reacts with a base, in the presence of water, the zinc salts are formed:

ZnO + 2NaOH + H ₂ O → Na ₂

Heat capacity

40.3 J / K mol

Direct energy gap

3.3 eV. This value makes it a broadband semiconductor, capable of operating under intense electric fields. It also has characteristics of being an n-type semiconductor, which has not been explained why there is an extra supply of electrons in its structure.

This oxide is distinguished by its optical, acoustic and electronic properties, thanks to which it is considered a candidate for potential applications related to the development of optoelectronic devices (sensors, laser diodes, photovoltaic cells). The reason for such properties is beyond the realm of physics.

Applications

Medicinal

Zinc oxide has been used as an additive in many white creams to treat skin irritations, acnes, dermatitis, abrasions and cracks. In this area, its use is popular to relieve irritation caused by diapers on the skin of babies.

Likewise, it is a component of sunscreens, because together with the titanium dioxide nanoparticles, TiO ₂, it helps to block the sun's ultraviolet radiation. Likewise, it acts as a thickening agent, which is why it is found in certain light make-up, lotions, enamels, powders and soaps.

On the other hand, ZnO is a source of zinc used in dietary supplements and vitamin products, as well as in cereals.

Antibacterial

According to the morphology of its nanoparticles, ZnO can be activated under ultraviolet radiation to generate hydrogen peroxides or reactive species that weaken the cell membranes of microorganisms.

When this happens, the remaining ZnO nanoparticles furrow the cytoplasm and begin to interact with the compendium of biomolecules that make up the cell, resulting in their apoptosis.

That is why not all nanoparticles can be used in sunscreen compositions, but only those that lack antibacterial activity.

Products with this type of ZnO are designed, coated with soluble polymeric materials, to treat infections, wounds, ulcers, bacteria and even diabetes.

Pigments and coatings

The pigment known as white zinc is ZnO, which is added to various paints and coatings to protect metal surfaces where they are applied from corrosion. For example, coatings with added ZnO are used to protect galvanized iron.

On the other hand, these coatings have also been used on window glass to prevent heat from penetrating (if it is outside) or entering (if it is inside). Likewise, it protects some polymeric and textile materials from deterioration by the action of solar radiation and heat.

Bioimages

The luminescence of the ZnO nanoparticles has been studied for use in bioimaging, thus studying the internal structures of cells by means of the blue, green or orange lights that radiate.

Additive

ZnO also finds use as an additive in rubbers, cements, dentifrices, glasses and ceramics, due to its lower melting point and, therefore, behaving as a fluxing agent.

Hydrogen sulfide remover

ZnO removes the unpleasant gases of H ₂ S, helping to desulfurize some gas fumes:

ZnO + H ₂ S → ZnS + H ₂ O

Risks

Zinc oxide as such is a non-toxic and harmless compound, so the prudent handling of its solid does not represent any risk.

The problem, however, lies in its smoke, because although it decomposes at high temperatures, the zinc vapors end up contaminating the lungs and causing a kind of “metal fever”. This disease is characterized by symptoms of cough, fever, a feeling of tightness in the chest, and a constant metallic taste in the mouth.

It is also not carcinogenic, and creams containing it have not been shown to increase zinc absorption into the skin, so ZnO-based sunscreens are considered safe; unless there are allergic reactions, in which case its use should be stopped.

Regarding certain nanoparticles designed to combat bacteria, these could have negative effects if they are not transported correctly to their site of action.

References

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5. A. Rodnyi and IV Khodyuk. (2011). Optical and Luminescence Properties of Zinc Oxide. Recovered from: arxiv.org
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