Why have you brought a burnt sausage into the lab?
- Well Dimitrios
we've all been there getting carried away at the barbecue and burn our sausages.
Although you may be a bad cook, you may actually be a great chemist.
- Great chemist? How can I be a great chemist by burning my sausages?
- Well, the thing is you might have created a new carbon material.
The carbon material is a material comprised exclusively of carbon atoms.
Interestingly though, depending on how the carbon atoms
are bonded, this gives rise to various different structures
which have various different properties.
- So can you give me some examples of these carbon materials?
- One of the most well-known carbon materials is diamond, okay, and another example
is that from the barbecue earlier, is charcoal.
Some more interesting carbon materials
are graphitic materials. So if we take this
graphite block, this book here, imagine this is graphite,
consists of multiple layers of stacked graphene sheets.
- So what is graphene? - Well graphene, being a single
layer of the graphite structure, consists of carbon atoms
that are interconnected in a hexagonal lattice structure.
Interestingly though these two
materials, graphene and graphite, have significantly different properties.
Whereas graphene is strong in every direction,
graphite may be strong in this direction but because it consists of the stacked
layers as you can see it can be
broken apart, it's quite brittle in this direction.
Interestingly though, graphene is known as the mother of all graphitic materials
and from this one graphene sheet you can for example wrap it
or roll it up to form a carbon nanotube, which again has different
properties, or, if you roll it into a sphere,
you get a C60 or fullerene
which is know as a bucky ball.
If you imagine these intersections are carbon atoms and it consists of 60
carbon atoms, this structure, this ball, is identical to that of a fullerene.
So the 2010 Nobel Prize in Physics was awarded to
Andre Geim and Konstantin Novoselov for the isolation of a single layer of graphene
using the Scotch tape method. - And what does this method involve?
- So we take this graphite and gently press it against the Scotch tape. You can see that as
we remove the graphite from the Scotch tape some of the graphite has been isolated
and transferred onto the Scotch tape. So now if we repeatedly
fold over the Scotch tape, pressing a clean part of the Scotch tape
each time onto the graphite that's been isolated
and if you gently prise them apart you can see that
each time the graphite flakes get transferred and effectively
we're splitting these up into smaller sizes of graphite flakes.
So eventually you'll end up with a single layer of graphene
which you can then transfer to a silicon wafer and locate using a microscope.
So did you know that graphene has some fascinating properties?
Graphene is a two-dimensional, one-atom-thin
crystal so it's the thinnest and lightest material known to man.
It's also one of the strongest and most flexible,
yet it's non-flammable, transparent and highly conductive.
- So what are all these properties useful for?
- One of the most promising properties of graphene is its use in circuits
using its conductivity. So for example here, if I take an ordinary pen,
you can see that this is not conductive,
there's no current flow between the two parts.
However, if I do exactly the same using a graphite pencil,
so we're depositing graphene layers as we write,
and we go from one point here to another point here
you can see that in this case it's conductive and a current
flows between the two. - Can you give me some more examples?
- One great example is a combination of graphene's transparency
and great conductivity in the creation of a solar cell. - So what is a solar cell? - Well a solar cell
is basically a converter. It converts light energy
into electrons or electricity. Smaller solar cells produce tiny amounts
of electricity,
just enough to power a small light or a calculator,
whereas larger arrays of solar cells are used
in earth-orbiting satellites, or in the home
to generate electricity. - So what does graphene have to do with solar cells?
- Well currently solar cells use titanium dioxide
to convert light into the electricity. Unfortunately though,
titanium dioxide can only be coated onto glass, and glass is both inflexible
and heavy so if we use graphene, which is
light and flexible, in place of titanium dioxide,
then solar cells can then be applied onto a wider variety of surfaces.
- So how does a solar cell work? - Well in a solar cell
you have two glass plates acting as electrodes, and these are conductive.
In the case of a graphene solar cell
the graphene coated on top of the glass plate
has a dye on top of it which when light passes through the glass
this excites the dye molecules and then electrons travel from this
to the graphene through an external circuit to a cathode
and the circuit is completed by the use of an
iodide electrolyte. So I'll now show you
how to make your own solar cell. Unfortunately
we have to use titanium dioxide rather than graphene
because at the present time it's not possible to coat
a single layer of graphene covering the whole surface of this glass plate.
So here we have two conductive glass plates.
The first step is to coat the
anode material with titanium dioxide
and a platinum paste for the cathode.
So once I've covered the
glass plate in its entirety, I will then leave this to dry
before removing the Sellotape.
Here's a plate that I made earlier. This has been fired to dry it and remove any contaminants.
So now it's time to dye the titanium dioxide.
Here we're using a squash containing
concentrated fruit juices. And we'll then add the anode
into the juice. We'll leave it there now for two hours to make sure that it
gets stained efficiently. We can now
remove the stained anode from the dye solution.
So after leaving the anode for five minutes
to dry it's now time to construct our solar cell. So the first stage is to place
a spacer which is going to be between the two electrodes,
and this is to stop short circuiting. Once that's in place
we place the two electrodes on top of each other with the coated sides together
so that we can effectively clamp these together and have room for connection.
So in the final stage we now fill
the solar cell with our electrolyte through a hole
in one of the plates. So having covered the holes with
Sellotape, there you have it, your homemade solar cell.
To test this out we've now connected it up to a multimeter
and as you can see when I apply a light
to the cell the output of the solar cell
increases and then again when I turn
the light off the output drops.
So as you can see using just a couple of pieces of glass
and some fruit juice you can take a plentiful
energy source like light and convert it into electricity
using this. So when large areas of graphene can be produced
and used in applications like this, its true potential
can be realised.