Functional Materials | Sahithyan's Notes

Classification of materials based on whether most important function they perform is either mechanical, structural, biological, electronic, magnetic, or optical.

These materials have one or more properties that can be significantly changed in a controlled fashion with an external stimuli.

List of properties

Electrical properties
- Piezoelectricity
- Ferroelectricity
- Superconductivity
- Capacitance
Optical properties
- Photoluminescence
- Photochromism
- Fluorescence
- Photoelectricity
Magnetic Properties
- Diamagnetism
- Ferromagnatism
- Ferrimagnetism
- Paramagnetism
- Anti-ferri magnetism
Chemical Properties / Reactivity properties
- reactivity with other substances
- catalytic properties
Others
- Photocatalytic - a material that uses light energy to catalyze a chemical reaction.
- Electroluminescent - a material that emits light under an electric current or strong electric field

Examples

Gas sensing

Graphene based field effect transistor is used as the sensor. When gas molecules get absorbed into the graphene surface, graphene’s charge carrier concentration and resistance changes. Large surface means higher sensitivity. Can be done for $\ce{NO},\ce{CO},\ce{NH3}$ .

Ethanol sensing

Metal oxide nanoparticles are used as the sensor. Nanoparticles are used because of high surface area, which means more sensitive.

$\ce{O2}$ gas is adsorbed on the metal oxide. It turns into $\ce{O^{2-}}$ as shown below. Number of free electrons will decrease as this series of reactions is happens. Hence conductivity will decrease.

\ce{O2 + e^{-} -> O2^{-} }

\ce{O2^{-} + e^{-} -> 2O^{-} }

\ce{O^{-} + e^{-} -> O^{2-} }

When ethanol gas is present, it will also be adsorbed.

\ce{CH3CH2OH + 6O^{2-} -> 2CO2 + 3H2O + 6e^{-}}

The above reaction will release the electrons back into the conduction band, and conductivity will be increased.

2 types of metal oxides are used.

n-type
- $\ce{TiO2}$
- $\ce{ZnO}$
p-type
- $\ce{NiO}$
- $\ce{Cu2O}$

Li-ion battery

Positive electrode is made of $\ce{LiCoO2}$ . During discharge, $\ce{Li^{+}}$ are released from cathode to the electrolyte. Connected using $\ce{Al}$ .

Negative electrode is made of graphite. $\ce{Li^{+}}$ are stored in graphite layers during charging. Connected using $\ce{Cu}$ .

The electrolyte is either a liquid or gel, containing $\ce{Li}$ salts. The anode and cathode are separated using a porous membrane which only allows $\ce{Li^{+}}$ to pass through.

While charging:

Anode - Graphite
Cathode - $\ce{LiCoO2}$

While discharging:

Anode - $\ce{LiCoO2}$
Cathode - Graphite

The reactions are:

\ce{LiC6 -> C6 + Li^{+} + e^{-}}

\ce{CoO2^{-} + Li^{+} + e^{-} -> LiCoO2}

Self-cleaning surfaces

The surface is coated with nano-scale layer of $\ce{TiO2}$ which is photocatalytic. It causes a series of chemical reactions, which loosen the dust particles from the surface. The glass surface turns superhydrophilic because of $\ce{TiO2}$ .

Superhydrophilic material is a material that attract water strongly which causes water to spread out into a thin layer on its surface.

When raining, a thin layer of water will be formed on the surface of $\ce{TiO2}$ -coated glass. This layer of water washes the dust particles away.

LEDs

LEDs are electroluminescent. Electrons and holes recombine at the p-n junction, and emit light.

The color of a LED depends on the energy band gap of the semiconductor material.

Blu-ray disk

Writing media of the blu-ray disc is a phase-changable material. The material can transition between crystalline and amorphous when hit by laser.

Example for phase-changable material: $\ce{Ge8Sb2Te11}$ .