Friday, February 20, 2009

Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide

Sasha Stankovich, Dmitriy A. Dikin, Richard D. Piner, Kevin A. Kohlhaas, Alfred Kleinhammes, Yuanyuan Jia, Yue Wu, SonBinh T. Nguyen, and Rodney S. Ruoff (2007). Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide Carbon, 45 (7), 1558-1565 DOI: 10.1016/j.carbon.2007.02.034

This is a mostly characterization-based paper looking in more depth at traditional hydrazine-reduced graphene oxide (GO). After oxidizing using Hummer's method (sulfuric acid, potassium permanganate) and reducing with hydrazine, the researchers characterized the before-and-after transformation with:

SEM: showed crumpled up sheets of the reduced GO
Surface area: 466 m2/g for reduced GO, rather high but still way below "real" graphene (2620 m2/g)
Elemental analysis: C/O ratio of 2.7 before reduction and 10.3 after, which still seems like a lot of oxygen after reduction. The C/N ratio after reduction was 16.1, meaning a good bit of the nitrogen from the hydrazine ended up in the final product.
Water content: 25 wt% before reduction; 2.8% after
TGA: for GO before reduction, mass loss started below 100 C, but main mass loss came around 200. Reduced GO was thermally stable up to 800 C.
C13 NMR (MAS): GO showed peaks for epoxides, hydroxyls and carbonyls, as expected; these peaks were absent in the reduced sample.
XPS: GO had 4 peaks for "oxygenated" components. These 4 peaks were smaller in the reduced sample, but a 5th peak for a C-N bond appeared.
Raman: Best shown with the picture below, borrowed under the auspices of fair use:

The first spectrum is pristine graphene showing a sharp "G" peak at 1581 cm-1. Second spectrum is GO, showing a much broader "G" peak in addition to a new "D" peak at 1363 cm-1, showing disorder. Third spectrum is reduced GO, which interestingly shows an even larger "D" peak, implying that the reduced film might have a higher disorder than the GO.
Conductivity: GO was least conductive (on the order of 10-3 S/m), with the reduced GO being much more conductive (around 10^2 S/m), a conductivity close to that of graphite (around 10^3 S/m).
The authors also briefly discuss the mechanism of hydrazine reduction, but essentially say that any explanation they can come up with doesn't fully explain their observations.

Moral of the story: As we knew, "graphene" made from reduced graphite oxide can be useful, but has so many defects that it's properties are very different from pristine graphene.