In 2014 we visited the I11 beamline at Diamond Light Source to study the formation of iron carbide (Fe3C) nanoparticles. Iron carbide is a fascinating material. It is perhaps most famous for being found in Damascus Steel blades. More recently, iron carbide has been investigated for its remarkable properties as a catalyst.
One of the most exciting possibilities for iron carbide is as a catalyst in fuel cells. There are several different types of fuel cell and iron carbide is being investigated in hydrogen fuel cells. These react hydrogen with oxygen to create water. The process releases lots of energy and fuel cells allow this to be released in the form of electricity. This means they could be used to power cars or homes using hydrogen as a fuel and the only waste would be water.
Existing fuel cells typically use platinum as a catalyst – a material to drive the chemical reactions inside the fuel cell. Platinum is extremely good at this. However, it is also scarce and very expensive. Iron carbide could provide a cheaper and more abundant alternative.
A few years ago, our research group showed that very small particles of iron carbide (nanoparticles) could be made using a biopolymer called gelatin. This material shows really promising activity as a fuel cell catalyst. To improve this material, we applied to Diamond (the UK Synchrotron) to study the mechanism of formation of the iron carbide nanoparticles.
The big challenge in studying the formation of iron carbide nanoparticles is the temperature. The mixture has to be heated up to 800 °C to form iron carbide and this is normally achieved inside a furnace. Materials like iron carbide can be studied using X-rays but the thick walls of a furnace would easily stop X-rays. At Diamond, we were able to heat the sample inside a tiny quartz capillary. The X-ray beam of the Synchrotron could easily pass through this capillary, allowing us to probe the sample as it was heated.
The other advantage of the Diamond beamline is that it is very powerful. It is possible to study materials using X-rays in a normal laboratory. However, it can take 30 minutes to perform one scan. If you want to study a reaction happening in a furnace, it could be over in just a couple of minutes! At Diamond, the very bright X-ray beam is combined with a really powerful detector. This means that you can record an X-ray pattern every few seconds. Therefore you can capture very fast changes in your reaction.
Using the data from this experiment we were able to see all of the stages involved in heating the gelatin starting-material to form iron carbide. We were able to observe several intermediates including compounds called iron oxide and iron nitride. What was most interesting was that the data gave us information about the size of the particles in the system. This can be inferred from the broadness of the peaks in the X-ray pattern. From this information, we learned where in the heating process most of the particle growth occurred. Since we want our nanoparticles to be as small as possible, this means we can focus on controlling that stage where most growth occurs.