Question: how exactly do you split the hydrogen and oxygen with sunlight? thanks

Suze Kundu answered on 13 Jun 2011:

Hello 🙂

OK, thinking caps on? Are you sure? Then here we go 🙂

Light is normally seen as a wave, but each wave also has a packet of energy associated with it. These are called photons. Keep them in mind, as we’ll come to those in a moment.

Now, think for a moment about a rubbery length of plastic, and a metal wire. All of these things are made up of atoms and molecules – tiny little building blocks of the whole material. On an even smaller scale, each building block, or atom / molecule, has a set number of electrons (negatively charged bits) and protons (positively charged bits) – think of them as noughts and crosses pieces, with the noughts as the electrons, and the crosses as the protons. The number of electrons and protons are the same, and because these are oppositely charged, the overall charge is neutral.

Let’s think about electricity for a second. This is basically a flow of electrons (our noughts, in this case). They carry energy around a circuit. This energy is then used to power something, like a motor or a lightbulb. The circuit needs to be complete for the energy to flow around. Now, let’s compare the rubbery plastic to the metal wire. If you were to use a length of this rubbery plastic in a circuit with a battery, nothing would happen. This is because it takes a LOT of energy to make one of the electrons from the building blocks of this material to move around – more energy than is in the battery. This big energy gap makes a material an insulator.

If we were to attach a metal wire instead, the energy would be able to flow around the circuit easily, because there is a very small energy gap to make the electrons move around. This makes the material a conductor.

The material that we are using to catch the sunlight is in between these two types of material, and is called a semiconductor. The energy that it needs to make the electrons move around is not huge, but it is not really small either. Now think back to the packets of sunlight energy that we mentioned at the beginning. When a packet of energy that is the right size hits the semiconductor, this pushes one electron up into the place where it can move about. This is negatively charged electron moves to the surface and meets a water molecule. When it is there, it creates a reaction to ‘reduce’ the water molecule into hydrogen. It is called a reduction because the negative electron is involved, which is minus, or less, therefore it is reduced. As the charges are out of balance when the electron moves away, a positively charged ‘hole’ is left behind. This can also migrate to the surface, to oxidise (the opposite of reduce – they never make these things simple!) water to make oxygen.

Water molecules themselves can’t catch the packets of sunlight energy, because their energy gap doesn’t match the packets of sunlight energy coming from the sun, which is why when you have a glass of water outside in the sunlight, the water doesn’t start making bubbles of gas. Instead, we use this material to speed up the reaction. It is like a catalyst that makes something go faster, and is actually called a photocatalyst, because it is something that speeds up a light reaction (photo just means light, like our photons of light energy).

Now, please tell me whether this has made any sense or not! If not, I’ll try and explain it in a better way – trust me, it’s good practice for me too 😉