ZnO — Zinc Oxide

Zinc oxide (ZnO) is a white, amphoteric, inorganic compound that plays a vital role in various industries, from cosmetics to electronics. It is naturally found as the mineral zincite, though most commercial ZnO is produced synthetically. Known for its excellent UV-blocking, antibacterial, and catalytic properties, zinc oxide is used in sunscreens, rubber vulcanization, paints, ceramics, and even pharmaceuticals.

In addition to its protective and functional properties, ZnO is an important semiconducting material with a wide bandgap (~3.37 eV) and high exciton binding energy, making it valuable in photonics, LEDs, and solar cells. Its unique combination of optical transparency and electrical conductivity makes it a key material in modern nanotechnology and energy applications.

Zinc oxide has the empirical formula \( ZnO \), consisting of zinc ions (Zn²⁺) and oxide ions (O²⁻). The structure of ZnO depends on the conditions under which it is formed and can exist in three main crystalline forms: wurtzite (hexagonal), zinc blende (cubic), and rock salt (cubic, under high pressure).

• Wurtzite Structure: This is the most stable and naturally occurring form of ZnO at ambient conditions. It has a hexagonal close-packed structure with alternating layers of Zn and O atoms.
• Zinc Blende Structure: Found under specific conditions, this cubic form is metastable but converts to wurtzite when heated.
• Rock Salt Structure: Appears under extremely high pressures.

Each zinc ion in ZnO is tetrahedrally coordinated by four oxygen atoms, and vice versa, resulting in a strong ionic lattice with partial covalent character. This hybrid bonding provides ZnO with both chemical stability and semiconducting behavior.

Zinc oxide can be prepared by several industrial and laboratory methods. The two most common industrial methods are the French process and the American process.

• 1. French Process (Indirect Process): Metallic zinc is vaporized and then oxidized in air to produce zinc oxide.

This method yields very pure, fine white ZnO powder.

• 2. American Process (Direct Process): Zinc ores or residues containing zinc compounds are reduced by carbon to form metallic zinc vapor, which is then oxidized to ZnO.

• 3. Wet Chemical Methods: Zinc salts like zinc sulfate or zinc chloride are precipitated as zinc hydroxide using alkalis (NaOH, NH₄OH), followed by heating to yield ZnO.

The obtained zinc oxide can be tailored in particle size and morphology depending on the desired application, ranging from micro-powders to nanoscale ZnO.

Physical Properties:

• White crystalline powder that turns yellow when heated (due to oxygen loss) but regains its white color upon cooling.
• Insoluble in water but soluble in dilute acids and strong alkalis.
• Good conductor of heat and poor conductor of electricity at room temperature.
• Exhibits photoluminescence under UV light due to its wide band gap.

Chemical Properties:

• 1. Reaction with Acids: ZnO reacts with acids to form corresponding zinc salts and water.

\( ZnO + 2HCl \rightarrow ZnCl_2 + H_2O \)

• 2. Reaction with Alkalis: It reacts with strong bases like NaOH to form soluble zincates, showing its amphoteric nature.

\( ZnO + 2NaOH + H_2O \rightarrow Na_2[Zn(OH)_4] \)

• 3. Reduction: When heated with carbon, ZnO is reduced to metallic zinc.

\( ZnO + C \xrightarrow{heat} Zn + CO \uparrow \)

• 4. Photocatalytic Property: ZnO exhibits strong photocatalytic activity under UV light, decomposing organic pollutants and dyes.

Zinc oxide has a wide range of applications across multiple industries due to its chemical versatility, UV protection, and electrical properties.

• 1. Cosmetics and Sunscreens: Used as a physical UV filter in sunscreens, creams, and lotions, protecting skin from both UVA and UVB radiation.
• 2. Rubber Industry: Acts as an activator in rubber vulcanization, improving elasticity and durability.
• 3. Paints and Coatings: Provides whiteness, UV resistance, and anti-corrosive properties.
• 4. Electronics: Used in varistors, gas sensors, LEDs, and transparent conductive films due to its semiconducting nature.
• 5. Pharmaceuticals: Used in ointments, calamine lotions, and antiseptic creams for its soothing and healing effects.
• 6. Ceramics and Glass: Enhances thermal and mechanical strength in glass and ceramic glazes.
• 7. Food Industry: Acts as a zinc supplement and fortifying agent in animal feed and food products.

Zinc oxide is generally safe and non-toxic when used appropriately, but inhalation of ZnO fumes (e.g., during welding) can cause metal fume fever, a temporary flu-like illness characterized by fever, chills, and nausea. Prolonged exposure to ZnO nanoparticles may have respiratory or oxidative effects.

Safety Precautions:

• Handle ZnO powders in well-ventilated areas.
• Wear protective masks and gloves during industrial handling.
• Avoid inhalation or ingestion of fine ZnO dust.
• Store in a cool, dry place away from acids.

In topical applications and food-grade use, ZnO is safe and approved by health agencies such as the FDA for cosmetics, food fortification, and pharmaceutical formulations.