While Dr Richard Tilley of Victoria University of Wellington explains how surfactant chemicals are used to control size and shape of nanocrystals, we see John Watt making gold nanoparticles in a flask.
DR RICHARD TILLEY
A surfactant is a soap molecule with two different types of end. So when you do a chemical reaction and you are making a material in solution, you start by forming small crystals, and then these will grow and grow and grow and grow and grow, and eventually you will have very large crystals, which are not nanoparticles at all. So when we put the surfactant into our reaction, the surfactant molecules will bind to the particle surface and prevent them from bumping into each other and joining together, and growing into big particles. The fun thing about the chemistry we can do with the surfactants is not only when we put a surfactant into the solution can we control the particle size, but we can also, with some nice chemistry, try and control the particle shapes as well. So when our particles first form as very small crystals in the solution, they have lots of different crystal faces. The surfactant will actually stick to different faces of the particle with different strengths. So if we have the surfactant binding very strongly onto one crystal face, that face can't grow at all. But if they don't bind to other faces then they can grow. And through this we can have certain faces growing very long arms and then other faces not growing at all. So we can make some particles which look like star shapes, or tripod shapes, or other shapes as well.