Nanoscience: nanoparticles are only a few hundred atoms across
Definition
Diameter
The straight-line distance across a particle through its centre.
- Nanoscience is the study of structures with sizes between 111 and 100 nm100\ \text{nm}100 nm.
- One nanometre, written nm\text{nm}nm, is equal to 1×10−9 m1 \times 10^{-9}\ \text{m}1×10−9 m.
- A nanoparticle is a particle whose dimensions fall between 111 and 100 nm100\ \text{nm}100 nm.
- A nanostructure of this size typically contains only a few hundred atoms.
- A single atom has a diameter of roughly 0.1 nm0.1\ \text{nm}0.1 nm.
- A 1 nm1\ \text{nm}1 nm structure is therefore about ten atoms across, and a 100 nm100\ \text{nm}100 nm structure about one thousand atoms across.
- Many small molecules measure less than 1 nm1\ \text{nm}1 nm across.
- Nanoparticles are therefore larger than individual atoms and many simple molecules.
Note
- Course boundary: this is separate (triple) chemistry only and is not needed for the Combined Science course.
- Unit check: 100 nm100\ \text{nm}100 nm is equal to 1×10−7 m1 \times 10^{-7}\ \text{m}1×10−7 m.
Particle sizes: nanoparticles are smaller than fine particles and dust
Definition
Particulate matter
Tiny solid particles or liquid droplets suspended in the air.
- Nanoparticles have diameters between 111 and 100 nm100\ \text{nm}100 nm.
- They are the smallest category of airborne particle described here.
- Fine particles, labelled PM2.5\text{PM}_{2.5}PM2.5, have diameters between 100100100 and 2500 nm2500\ \text{nm}2500 nm.
- This range is equivalent to 1×10−7 m1 \times 10^{-7}\ \text{m}1×10−7 m up to 2.5×10−6 m2.5 \times 10^{-6}\ \text{m}2.5×10−6 m.
- Coarse particles, labelled PM10\text{PM}_{10}PM10, have diameters between 250025002500 and 10 000 nm10\,000\ \text{nm}10000 nm.
- This range is equivalent to 2.5×10−6 m2.5 \times 10^{-6}\ \text{m}2.5×10−6 m up to 1×10−5 m1 \times 10^{-5}\ \text{m}1×10−5 m.
- Coarse particles are often called dust.
- Fine and coarse particles are both forms of particulate matter in the air.
- The size order from smallest to largest is nanoparticles, then fine particles, then coarse particles.
Common Mistake
- Do not read the number in PM2.5\text{PM}_{2.5}PM2.5 as 2.5 nm2.5\ \text{nm}2.5 nm, because 2500 nm2500\ \text{nm}2500 nm is equal to 2.5×10−6 m2.5 \times 10^{-6}\ \text{m}2.5×10−6 m.
- To convert nanometres to metres, multiply the value by 1×10−91 \times 10^{-9}1×10−9.
Smaller cubes: shrinking the side raises the surface area to volume ratio
- The surface area to volume ratio compares an object's total surface area with its volume.
- For a cube with side length xxx, the surface area is 6x26x^26x2 and the volume is x3x^3x3.
- Its surface area to volume ratio is therefore 6x2x3=6x\frac{6x^2}{x^3}=\frac{6}{x}x36x2=x6.
- Because xxx is in the denominator, a smaller side length gives a larger ratio.
- Decreasing the side of a cube by a factor of 101010 increases its surface area to volume ratio by a factor of 101010.
- Smaller particles therefore expose more surface area for the same total volume of material.
Example
- A cube of side 10 nm10\ \text{nm}10 nm has surface area 6×102=600 nm26 \times 10^2 = 600\ \text{nm}^26×102=600 nm2 and volume 103=1000 nm310^3 = 1000\ \text{nm}^3103=1000 nm3.
- Its surface area to volume ratio is 6001000=0.6 nm−1\frac{600}{1000} = 0.6\ \text{nm}^{-1}1000600=0.6 nm−1.
- A cube of side 1 nm1\ \text{nm}1 nm has surface area 6 nm26\ \text{nm}^26 nm2 and volume 1 nm31\ \text{nm}^31 nm3.
- Its ratio is 6 nm−16\ \text{nm}^{-1}6 nm−1, which is ten times greater than 0.6 nm−10.6\ \text{nm}^{-1}0.6 nm−1.
High surface area: nanoparticles can behave differently from bulk materials
Definition
Bulk material
A material in a large piece or with normal particle sizes rather than as nanoparticles.
- Nanoparticles may show different properties from the same substance as a bulk material.
- Their high surface area to volume ratio means much more of the material is exposed at the surface for a given volume.
- This exposed surface is where the material meets and reacts with other substances.
- Changing the particle size can therefore change how a material behaves.
- A smaller quantity of nanoparticles may achieve the same effect as a much larger quantity of the bulk material.
- Use the word may here, because nanoparticles do not always differ from the bulk material.
Self review
- What range of sizes does nanoscience study?
- How does the size of a nanoparticle compare with the typical diameter of an atom?
- What are the diameter ranges of fine particles and coarse particles?
- What happens to a cube's surface area to volume ratio when its side decreases by a factor of 101010?
- Why might a smaller quantity of nanoparticles be as effective as more of the bulk material?
