Cleanly Removable Surfactant for Carbon Nanotubes

Surfactants, molecular surface-active agents, are widely used to stabilize single-walled carbon nanotubes (SWCNTs) and many other nanomaterials in water to facilitate solution processing and device integration. However, it is notoriously difficult to cleanly remove surfactants when they are no longer needed. Even high-temperature thermal annealing leaves residues that can degrade the otherwise remarkable electrical and optical properties of SWCNTs. Moreover, thermal annealing damages smaller-diameter nanotubes (<1 nm) due to aggressive oxidation at elevated temperatures. To address this challenge, we report the synthesis of ammonium deoxycholate (ADC) that can be cleanly removed at a relatively low temperature that preserves the SWCNT structure. We find that ADC is as efficient as sodium deoxycholate (a commonly used surfactant featuring the same anion) at individually dispersing SWCNTs in water. However, replacing the metal cation (Na+) of sodium deoxycholate with ammonium (NH4+) to form ADC reduces the peak thermal decomposition temperature by nearly 70 °C. Furthermore, unlike sodium deoxycholate, thermal annealing of ADC in Ar leaves behind only a small amount of carbonized residue that can be cleanly decomposed in the presence of 5% O2 at 400 °C, a condition that preserves SWCNTs even with a small diameter of just 0.76 nm. This work uncovers the chemical origin of residues from the thermal annealing of surfactant-processed carbon nanomaterials and provides an unexpectedly simple solution to this persistent challenge.