I've been working on graphene ink for a while now and have encountered some issues with its spreading on paper. I'm uncertain whether the problem lies in the ink formulations or the size of the graphene particles.

My typical process involves chemically intercalating #2 flake graphite and expanding it in the microwave, followed by ultrasonication in an ammonium carbonate solution. After sonication, the few-layer graphene sinks to the bottom, where it can be collected and dried in a desiccator or vacuum oven.

I've experimented with various ink formulations, but none of them seem to spread satisfactorily. The formulations I've tried so far are as follows:

First formula:

Solvent: 10 ml Ethyl acetate

Binder: 3 g Polystyrene

Plasticizer: 1 g Mineral Oil

Conductive Material: 800 mg Few-layer graphene powder

Second formula:

Solvent: 20 ml Water

Surfactants: 2 g PVP, 2 g Sodium Dodecyl Sulfate

Plasticizer: 0.5 ml Glycerin

Conductive Material: 1 g Few-layer graphene powder

Third formula:

Solvent: 10 ml Water

Binder: 5 g PVA

Plasticizer: 0.5 ml Glycerin

Conductive Material: 1 g Few-layer graphene powder

When I used the ink formulation with surfactants, it appeared to work better, although it still didn't spread as well as expected. The conductivity could be better but I've gotten around 40 ohms to 100 ohms so far between each formulation.

I suspect the issue may be related to the size of the graphene particles, as I initially used #2 flake graphite, which consists of rather large flakes. Despite attempting to intercalate and expand 5 and 44-micron graphite, I encountered no success. I have now ordered some 44-micron vein graphite in the hope that its structure will facilitate better intercalation and yield smaller particles suitable for ink. Does anyone have any suggestions regarding what might be going wrong here? I have attached an image of some test lines I've painted with the ink to get a better idea of what I'm talking about. As you can probably see in the image, the ink seems to bleed a bit. I'm unsure of the reason for this.

I've even attempted to use a high-shear mixer to break up the graphene particles into smaller ones however, the graphene continues to agglomerate, worsening the spreading quality. I even used a ball mill with 4mm ceramic balls in an attempt to mill the graphene into a finer powder but it to aglomerates and sticks to the ceramic balls. Any suggestions would be greatly appreciated!