Graphene Surprises By Decomposing

Graphene’s hallmark chemical stability has made this ultrathin carbon network an ideal support material in catalysis and energy studies. But that inertness is now being called into question by an investigation showing that the material can decompose when used in common applications (Chem. Mater. 2014, DOI: 10.1021/cm5026552).

Water-dispersible forms of graphene are easy to make and easy to handle via simple wet-chemistry methods. Large numbers of researchers use the materials to support nanoparticle catalysts for use in environmental remediation, solar cells, and fuel cells.

For such applications, scientists often use a solution-phase form of graphene called reduced graphene oxide. RGO is made by chemically reducing graphene oxide, a common starting material. RGO’s conductivity and bonding characteristics are credited with the high level of catalytic activity it promotes.

But this highly touted material may not stand up to repeated use. University of Notre Dame scientists have shown that aqueous suspensions of RGO-supported TiO₂ nanoparticles unexpectedly undergo decomposition. Specifically, James G. Radich, Prashant V. Kamat, and colleagues demonstrate that upon exposure to ultraviolet light, the nanoparticle surfaces generate hydroxyl radicals, which oxidatively attack RGO.

On the basis of spectroscopy and microscopy analyses and total-organic-carbon measurements, the group explains that UV radiation triggers a fragmentation process that breaks the RGO sheets into polyaromatic hydrocarbon (PAH)-like compounds. Prolonged irradiation eventually decomposes the organic compounds completely, leaving behind CO₂ and water.

The Notre Dame study was carried out in a photocatalysis setting but might apply to other graphene applications, the team notes. For example, oxygen reduction reactions on fuel-cell cathodes can also generate hydroxyl radicals and initiate RGO decomposition. And in cases in which TiO₂-RGO composites are used to destroy waterborne organic pollutants photocatalytically, the possibility that oxidation of RGO sheets will generate toxic PAHs may outweigh RGO’s potential environmental benefits.

Characterizing the study as “incredibly important,” the University of Alberta’s Jillian M. Buriak, a specialist in nanoscience and materials chemistry, says this study clearly shows that using graphene as a platform for photocatalysis requires careful evaluation. But she adds that the very same reactivity that causes unwanted degradation of RGO may lead to new approaches in environmental remediation of related carbon contaminants, including carbon nanotubes, fullerenes, and other nanostructured forms of carbon.