Surface-Modified Halloysite Nanotubes as Electrochemical CO2 Sensors

The outer surface of halloysite nanotubes (HNTs) was selectively modified using (3-aminopropyl)triethoxysilane (APTES) via a self-assembled monolayer (SAM) method. The product of this reaction, amine-functionalized HNTs (HNTs/APTES), can selectively capture carbon dioxide (CO 2 ) gas at room temperature and then after being heated at a relatively low temperature can release the gas. The CO 2 adsorption capacity of HNTs/APTES was measured using thermogravimetric analysis conducted at 35 °C for 1 h and found to be ∼0.30 mmol CO 2 /g sorbent under a pure CO 2 atmosphere. Furthermore, plate electrodes of HNTs/APTES were fabricated for electrochemical impedance spectroscopy measurements involving the exposure of the material to different amounts of CO 2 . Upon the binding of CO 2 to the HNTs/APTES plate electrodes, the proton conductivity of the material was observed to fall. A correlation between the CO 2 concentration in the gas mixtures and proton conductivity of HNTs/APTES was thus established. Cycling measurements were then conducted to measure the CO 2 adsorption/desorption of the material and revealed that HNTs/APTES exhibit high stability and reversibility, without any significant decrease in efficiency. This work demonstrates that HNTs/APTES have promising potential as a material for the electrochemical sensing of CO 2 .