DFT Calculations of Hydrogen Interactions with Pd and Pd/Ni Chain Functionalized Single-Walled Carbon Nanotubes for Sensor Applications

Density functional theory is employed to study Pd and Pd/Ni alloy monatomic axial and circular chain-functionalized metallic single walled carbon nanotube SWNT(6,6) and semi-conducting SWNT(10,0), and their interactions with hydrogen molecules. The stable geometries and binding energies have been determined for both isolated chains and chains on SWNT surface. We found that both axial and radial continuous Pd and Pd/Ni axial chains form on SWNTs with a geometry close to the stable geometry in the isolated chains. Ni alloying improves stability of the chains owing to a higher binding energy to both Pd and C atoms. The physical properties of SWNTs are significantly modified by axial chain-functionalization. SWNT(10,0) is transformed to metal by either Pd or alloy chains, as well as to a smaller gap semiconductor, depending on the Pd binding site. From calculations for H2 interactions with the optimized axial chain SWNT systems, the adsorption energy per H atom is found to be about 2.6 times larger for Pd/Ni axial chain-functionalized SWNTs than for pure Pd chain-functionalized SWNTs. Band structure calculations show that the SWNT(10,0) reverts back to semi-conductor and SWNT(6,6) has reduced density of states at Fermi level upon H2 adsorption. This result is consistent with the experimentally observed increase of electrical resistance when Pd coated SWNTs are used as H2 sensing materials. Our results suggest that Pd/Ni-SWNT materials are potentially good H2 sensing materials. The behavior of H2 interaction with circular chain-functionalized is currently being studied to compare with that of axial chain. Because of it discontinuity along the tube axis, the study of circular can be used to elucidate the major conduction path of Pd functionalized SWNTs, therefore it helps understand the sensing mechanism of electrochemical hydrogen senors based on conductivity change.