Saturday, March 15, 2008

Evaluation of solution-processed reduced graphene oxide films as transparent conductors

Evaluation of solution-processed reduced graphene oxide films as transparent conductors

Héctor A. Becerril, Jie Mao, Zunfeng Liu, Randall M. Stoltenberg, Zhenan Bao, and Yongsheng Chen

ACS Nano 2008; Still ASAP

DOI: 10.1021/nn700375n

Another excellent paper dealing with graphene oxide, it's reduction, and the properties of the films made before and after reduction.

As you might recall from half the posts I've done so far, one method of making graphene layers is to oxidize highly ordered graphite. This oxidation introduces all manner of oxygen-bearing functional groups (phenols, carboxylic acids, epoxides, other carbonyls) to the graphite layers, and in water some of these are deprotonated, giving an electrostatic repulsion between the resulting negative charges on adjacent layers
. After sonication, the graphite layers separate to give a graphene oxide solution, which can be put on a substrate and then reduced, or reduced and then put on a substrate. These authors choose the first option.

They begin by making graphene oxide (GO) in the normal manner and putting it on a surface; they find that spin-coating gives them the best quality films (which are about 3nm thick), whereas previous reports have used spray-coating. They found that the conventional reduction with hydrazine solutions pretty much destroyed their precious films, but that they could get some mediocre results putting hydrazine vapor and their substrate together into a petri dish.

Where this paper really gets original is that they decide to play with heating the GO. Previous reports have shown that GO is not thermally stable, and that heating it releases oxygen-containing compounds (this can also be used as an exfoliation method, which I'll post on some day). Releasing oxygen, reasoned the authors, sounds a lot like reducing something; so by heating GO to anywhere between 400 and 1100 C (with or without previous hydrazine reduction), they found that reduction did take place. They proved this with XPS measurements, which show the amount of C-C, C-O, C=O, or C-N bonds in a certain sample. The initial GO films have about 53% C-C bonds, with the rest taken up by C-O or C=O bonds. Treatment with hydrazine increased the proportion of C-C bonds, with C=O and C-N bonds (from incomplete reduction to hydrazone groups) making up the balance. The trend continues after annealing at 400 C, but the most graphitic surfaces were made by heating the GO at 1100 C with no previous hydrazine treatment, and had 88% C-C bonds. Catch that? They reduced most completely when they did not use hydrazine, but instead just heated the material.

After getting the most graphitic layer they could (from heating), the authors turn to examining the transparency and conductivity of their films. They find that conductivity and transparency are inversely proportional to each other, a fact which may be important to people other than me.

The authors give more questions and suggestions than hard answers in this paper, which I think is a good thing. They show that the current hydrazine-based method of GO reduction might have to be abandoned, particularly for applications where one is looking for no dopants (nitrogen and phosphorous are dangerously close to each other on the periodic table). They found a way to get nice films, but admit that their method is incomplete, and state that functionalizing graphite with things other than oxygen might be necessary to be able to fully reduce (or de-functionalize) back down to graphene. Kudos to them for asking the right questions and coming up with the first steps to answer them.

As an added bonus- this paper is the result of a collaboration between a chemical engineering group at Stanford and a chemistry group at Nankai University in China. Interdisciplinary work makes for some strange bedfellows.

Becerril, H.A., Mao, J., Liu, Z., Stoltenberg, R.M., Bao, Z., Chen, Y. (2008). Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors. ACS Nano DOI: 10.1021/nn700375n


Ψ*Ψ said...

Zhenan Bao does a lot of pretty cool research--I might have to read that.