In order to control the formation of metal carbide nanoparticles from biopolymers, it’s important to understand the mechanism.
We have studied several aspects of the mechanisms and two parts of this are now published. Firstly, we used in situ synchrotron X-ray diffraction at Diamond Light Source to study the synthesis of Fe3C nanoparticles from gelatin.
The data to the right came from that experiment (peaks for Fe3N are marked with + and Fe3C is marked with *). It shows one of the interesting features we learned, that the formation of Fe3C proceeds via an intermediate Fe3N phase. It’s also possible to see the transition from Fe3N to Fe3C via a mixed FeCxNy system.
Another key thing we were able to learn from this data was that the Fe3N formation is preceded by formation of an iron oxide phase. The particles of iron oxide were approximately a factor of 10 smaller than the Fe3N nanoparticles that they transform into (estimated from peak broadness). The fact that we have a substantial particle growth (~2 nm to ~20 nm diameter) during the oxide to nitride transition is important as if we can control this step we may be able to reduce the size of the nitride and subsequently the carbide nanoparticles.
For more information on this project check out our paper in Chemistry of Materials.
Another aspect of our mechanistic research has looked at the reason that biopolymers often form ‘foams’ when they are used as sol-gel precursors for metal nitrides and carbides. An example of the sort of ‘foam’ structures we can achieve is shown in the picture to the right. This was made by combining aqueous solutions of iron nitrate and gelatin and drying at 70 °C in an oven.
It’s important to understand the foaming process as there are a lot of catalytic applications that would benefit from ceramic foams. This is because the open foam structure allows easy flow of fluids through to the catalytic surfaces.
We used a range of techniques including small angle neutron scattering and rheology to show how iron and gelatin interact. We showed that the iron nitrate salt changes the structure of the gelatin biopolymer. The change in structure results in a change in the viscoelastic properties of the biopolymer. This stabilises bubbles as they form during drying and ‘traps’ those bubbles in a solid foam structure. This work was selected as a ‘Hot Paper’ by the Journal of Materials Chemistry A and you can read it here.