Why can nanoparticles be used as
effective catalysts in very small quantities? (A) They have a high
surface-area-to-volume ratio. (B) They are inflammable. (C) They are transition metals. (D) They have low reactivity. Or (E) they are lightweight.
Nanoparticles are very small
particles of matter. They usually contain only a few 100
atoms, and these atoms arrange themselves into spheres, 3D geometric shapes, or into
nano-tubes or fibers. The sizes of these particles vary
between one and 100 nanometers in diameter, length, or breadth. In certain cases, particles that
are smaller than 500 nanometers are also considered nanoparticles.
Nano- means one billionth. So, one nanometer is the same as a
billionth of a meter or one times 10 to the negative nine meters. A billionth of a meter is the same
as 0.000000001 meters. So, a nanoparticle of, say, 20 or
17 nanometers in diameter is a very small particle indeed. So, what is so special about
When particles are smaller than 100
nanometers in diameter, they exhibit very different properties to bulk
materials. In bulk materials, properties are
independent of and do not rely upon the size of the particles. For example, melting point, color,
and reactivity remains constant in a bulk material, regardless of the size of the
particles or the amount of bulk material present. Let’s use a specific example.
The element gold, Au, regardless of
the amount present, if it is in the form of a bulk material and not in the form of
tiny nanoparticles, it will have a melting point of 1064 degrees Celsius. It will be gold yellow in color and
will be inert or unreactive. For nanoparticles, the properties
depend on and are related to the size of the particles. Let’s take gold as an example
For gold nanoparticles,
approximately 1.4 nanometers in size, the melting point is somewhere between 23 and
25 degrees Celsius. Gold nanoparticles are often dark
red to black in color in solution depending on their size. And these nanoparticles are
reactive and can also be used as catalysts.
The graph shows us visually how a
property such as melting point depends on the particle radius. Though there are no values given,
we can see a general trend. The property, in this case, melting
point, is constant or almost constant for large particle sizes. And the property, in this case,
melting point, changes greatly at very small particle sizes. Let’s briefly investigate particle
size in terms of surface-area-to-volume ratio.
Let’s compare a large particle and
a small particle. We will use cubes just to make it
easier. The numbers represent the lengths
of each side, and we will leave out the units for now. If we calculate the surface area of
the large particle, there are six sides to the cube. The surface area of each side is
length times breadth. Putting in the values, we get 150
units squared for the total surface area of the large particle. Doing the same for the small
particle, we get a surface area of six units squared.
Let’s now calculate the volume of
each particle. Volume is length times breadth
times height. And putting in the values, for the
large particle, we get a volume of 125 units cubed and for the small particle, one
unit cubed. Let’s now put the surface area and
volume for each into a ratio. And we get for the large particle
150 as to 125 and for the small particle six as to one. Simplifying, for the large
particle, we get 1.2 as to one. And we can now compare the surface
area to volume ratio for each particle.
We can see that, for the large
particle, there is a small surface area as to volume ratio. We know this because there is very
little difference in size between 1.2 and one. However, for the small particle,
there is a large surface area as to volume. We know this because six is much
bigger than one. It is this large or high surface
area as to volume which give nanoparticles their very interesting properties.
And the reason why nanoparticles
can be used as effective catalysts in very small quantities is answer (A) because
they have a high surface-area-to-volume ratio.