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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 .

Keywords:Oxygen storage material Double perovskite oxide Yttrium barium manganese oxide Oxygen non-stoichiometry a b s t r a c tThe A-site ordered double-perovskite oxide, YBaMn 2 O 5 þ δ , has been of recent interest for possible application as an oxygen storage material. In the present study, the oxygen non-stoichiometry of YBaMn 2 O 5 þ δ has been determined as a function of pO 2 at 650, 700 and 750°C by Coulometric titration at near-equilibrium conditions. The results confirm that this perovskite oxide has three distinct phases on oxidation/reduction with δE0, 0.5 and 1. The stabilities of the YBaMn 2 O 5 þ δ phases span a wide range of oxygen partial pressures (∼10 À 20 rpO 2 (atm) r ∼1 ) depending on temperature. The phases interconvert at higher pO 2 values at higher temperatures. The partial molar free energies (Δμ O ) corresponding to the oxidation of YBaMn 2 O 5 to YBaMn 2 O 5.5 and of YBaMn 2 O 5.5 to YBaMn 2 O ∼6 were determined. The value of Δμ O in both oxidation steps becomes less negative with increasing temperature. At some T and pO 2 conditions, YBaMn 2 O 5 þ δ is unstable with respect to decomposition to BaMnO 3 À δ and YMnO 3 . This instability is anticipated from the previous studies of the synthesis of YBaMn 2 O 5 þ δ but is more apparent in the present experiments which are necessarily slow in order to achieve equilibrium with respect to the oxygen content.

Amine-surface-functionalized halloysite nanotubes are used for electrochemical sensing TNT.