| Summary: | Understanding the toxicity of carbon nanomaterials, such as carbon nanotubes
and graphenes, is important for the development of nanotechnology. Studies have shown
that surface redox capability is an important factor for toxicity of carbon nanomaterials.
We have measured the surface reductive capacity for a number of carbon nanomaterials
in previous studies, but the effects of various engineering processes on surface redox
capability have not been investigated until this study.
In this study, commercially available carbon black, carbon nanotubes, standard
reference materials, fullerenes, graphenes and acetylene soot generated in the lab were
used. The carbon nanomaterials were subjected to heating at various temperatures in
various atmospheres up to 500 ˚C, and soaking in water at room temperature under
various atmospheres, and weathering in the powder form at room temperature under
various atmospheres. The redox capability of the carbon nanomaterials was quantified in
terms of the reductive capacity towards Fe3+ ions (RCFI). The RCFI values of the asreceived
nanomaterials and that of the nanomaterials after various treatments were
compared. The carbon nanomaterials were also characterized using x-ray photoelectron
spectroscopy (XPS), for understanding the surface chemistry mechanisms of RCFI and
the effects of various treatments.
In general, heating induced a significant increase in RCFI, regardless of the
atmosphere under which the nanomaterials were heated. On the other hand, aging in O2-
containing atmospheres brought about significant decrease in RCFI, either in water
suspension or in the powder form. Water vapor enhanced the aging effect of O2. CO2
was found to affect the RCFI and the aging of carbon nanomaterials. The extent of RCFI
change due to heating or aging was dependent on the type of material.
According to the XPS results, the RCFI of some carbon nanomaterials such as
carbon black may be correlated with the C-O surface functional groups. However, the
definitive correlation between the oxygen-containing surface functional group and RCFI
for all carbon nanomaterials couldn’t be determined by the XPS result. This indicates
that the RCFI changes of carbon nanomaterials after treatments mainly derived from the
factors such as the active sites of edges other than the oxygen-containing surface
functional group changes as other studies show. This suggests that the RCFI
measurement cannot be replaced by XPS analysis.
The effects of heating and aging on RCFI, and more generally the surface redox
capability of carbon nanomaterials, reveals that various engineering and environmental
processes may significantly change the toxicity of carbon nanomaterials. The findings of
this study suggest that it is important to take into account the effects of engineering and
environmental processes when assessing the toxicity of carbon nanomaterials.
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