On to the first review:
"Processable aqueous dispersions of graphene nanosheets"
Dan Li, Marc B. Müller, Scott Gilje, Richard B. Kaner & Gordon G. Wallace
Nature Nanotechnology 3, 101 - 105 (2007)
In the quest to find a way to make layers of graphene real cheap and real easy (so that even experimental physicists can make them), Wallace's paper seems a small but good step forward. Wallace takes a technique used before- oxidizing graphite and then breaking it into graphene sheets with sonication. The oxidation works to help break apart the sheets because the "graphene oxide" is coated with phenols and carboxylic acids, giving it a large number of negative charges in water which, naturally, repel each other and overcome the more tame Van der Waals forces that usually hold graphite together. At the end of the day you get an aqueous dispersion of graphene oxide sheets, which can then applied to a substrate in a variety of ways before or after reduction to graphene.
The main problem is that once the graphene oxide is reduced to a more graphene-like product, many of the negative charges go away and so the sheets naturally start to aggregate, sometimes even re-forming graphite. Even an aggregate of 10 graphene sheets gives a substance with more graphitic than graphenic (sp?) properties. Previous attempts (look the papers up yourself) have stabilized the chemically treated graphene with polymers or other surfactants; the triumph of this paper is that the graphene can be stabilized without either of those.
The key to Wallace's approach is realizing that the conventional method of reducing the graphene oxide (with hydrazine) doesn't fully reduce it, leaving at least some carboxylic acid to give a negative charge. Wallace thinks that, as long as those native charges aren't stabilized by some cofactor, they should be enough to give stable colloids in aqueous solutions. Turns out, he's right. He carefully controls the amount of extra hydrazine present and also makes sure no salts are still hanging around to stabilize those charges. He evens adds in some base (ammonia) to ensure that everything is deprotonated. He then shows through light scattering, UV/Vis, IR, and something called a particle analyzer, whatever that means, that he has nice dispersed graphene and not big bad graphite. Adding salt or other electrolytes to the solution (or even lowering the pH) results in aggregation, further confirming that the unstabilized charges are necessary for aggregate prevention. Great. Then, Wallace goes a little crazy.
The authors don't claim that this material will cure cancer, AIDS, poverty, hunger, and global warming; but let's just say they didn't explicitly rule any those out. They made graphene paper (I'll post a paper about that later) which was more flexible and metallic than previous graphene/ graphene oxide paper. Then they discuss putting the graphene-ish material onto surfaces, through drop-casting, air-brushing, and layer-by-layer assembly. To their credit, they get some (supposedly) nice films of graphene in these ways, but they go a little overboard describing the anti-static coatings, FETs, sensors, supercapacitors, and membranes that future generations will make with their materials.
All in all, a decent paper that describes a small but quite important step in furthering graphite oxide-derived graphene science.
Li, D., Müller, M.B., Gilje, S., Kaner, R.B., Wallace, G.G. (2008). Processable aqueous dispersions of graphene nanosheets. Nature Nanotechnology, 3(2), 101-105. DOI: 10.1038/nnano.2007.451
Tuesday, March 11, 2008
On to the first review: