Phosphorus and phosphorus–nitrogen doped carbon nanotubes for ultrasensitive and selective molecular detection Eduardo Cruz-Silva, * a Florentino Lopez-Urias, b Emilio Munoz-Sandoval, b Bobby G. Sumpter, a Humberto Terrones, ac Jean-Christophe Charlier, c Vincent Meunier ad and Mauricio Terrones e Received 19th July 2010, Accepted 27th October 2010 DOI: 10.1039/c0nr00519c A first-principles approach is used to establish that substitutional phosphorus atoms within carbon nanotubes strongly modify the chemical properties of the surface, thus creating highly localized sites with specific affinity towards acceptor molecules. Phosphorus–nitrogen co-dopants within the tubes have a similar effect for acceptor molecules, but the P–N bond can also accept charge, resulting in affinity towards donor molecules. This molecular selectivity is illustrated in CO and NH 3 adsorbed on PN-doped nanotubes, O 2 on P-doped nanotubes, and NO 2 and SO 2 on both P- and PN-doped nanotubes. The adsorption of different chemical species onto the doped nanotubes modifies the dopant-induced localized states, which subsequently alter the electronic conductance. Although SO 2 and CO adsorptions cause minor shifts in electronic conductance, NH 3 , NO 2 , and O 2 adsorptions induce the suppression of a conductance dip. Conversely, the adsorption of NO 2 on PN-doped nanotubes is accompanied with the appearance of an additional dip in conductance, correlated with a shift of the existing ones. Overall these changes in electric conductance provide an efficient way to detect selectively the presence of specific molecules. Additionally, the high oxidation potential of the P-doped nanotubes makes them good candidates for electrode materials in hydrogen fuel cells. 1. Introduction It is well known that the intrinsic electronic properties of single- walled carbon nanotubes (SWCNTs) can be modified by the adsorption of molecules on their surface. 1 The adsorption usually proceeds in parallel with a significant modification of the nano- tube electronic structure. For instance, the exposure of SWCNTs to NO 2 and NH 3 is known to induce two orders of magnitude changes in the nanotube conductivity, namely a conductivity increase when exposed to NO 2 and a conductivity decrease when exposed to NH 3 . 2 Molecular oxygen can also increase the conductivity of SWCNTs, 3 due to charge transfer and strong interactions between the nanotube and the substrate. 2 Chemical and structural modifications of nanotubes can therefore improve the nanotube sensitivity and selectivity, as shown for chemical functionalization, 4 structural defects, 5,6 and chemical doping. 7,8 It was recently found that phosphorus can effectively dope carbon nanotubes, either as a single substitutional dopant 9 or as a co-dopant with nitrogen. 10 Theoretical investigations showed that both P and PN defects are characterized by the presence of a highly localized state close to the Fermi level, a promising premise for notorious chemical reactivity and sensing capabilities. 11 In this paper, we analyze in detail how phosphorus (P) and phosphorus–nitrogen (PN) doped carbon nanotubes can be used as ultrasensitive molecular sensors, based on the chem- ical reactivity of the phosphorus atoms and the localized states it induces in P- and PN-doped carbon nanotubes. Additionally, we note that these doped nanotubes could hold tremendous poten- tial for applications for cathode materials in fuel cells, where the oxygen contamination of the Pt based cathodes currently used is a major problem. 12 Specifically, doped carbon nanotubes could therefore catalyze a four-electron oxygen reduction reaction (ORR) process with a much higher electrocatalytic activity, more resistance to CO-poisoning, and better long-term operation stability even than those of commercially available or similar platinum-based electrodes. 13 As demonstrated in this paper, P-doped nanotubes provide a viable material for fuel cells, thanks to the presence of a highly localized electronic state that should allow for even more efficient electrocatalytic activity than N-doped nanotubes. 2. Computational details Ab initio calculations were performed using the density func- tional theory 14,15 implementation of the SIESTA code 16 within the local spin density approximation (LSDA). Norm-conserving pseudopotentials are used to represent the core electrons, 17 and the wave functions are expanded in terms of a numeric pseudo- atomic double-z basis set with polarization orbitals, as described by Junquera et al., 18 using an energy shift of 25 meV to confine the basis functions, and a cutoff radii in the 5–8 a.u. range. A real space mesh equivalent to an energy cutoff of 200 Ry was used for the electrostatic potential integrals. In order to simulate the doping of the nanotube, a single P or PN doping site was created a Oak Ridge National Laboratory, P.O. Box 2008, MS6367, Oak Ridge, Tennessee, 37831-6367, USA. E-mail: cruzsilvae@ornl.gov b Advanced Materials Department, IPICyT, Camino a la Presa Sn. Jose 2055, San Luis Potosi, Mexico 78216 c Universit e Catholique de Louvain, Institut de la Mati ere Condens ee et des Nanosciences, Place Croix du Sud 1 (PCPM-Boltzmann), B-1348 Louvain-la-Neuve, Belgium d Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, NY, 12180-3590, USA e Research Center for Exotic Nanocarbons (JST), Shinshu University, 4-17-1 Wakasato, Nagano City, 380-8553, Nagano, Japan 1008 | Nanoscale, 2011, 3, 1008–1013 This journal is ª The Royal Society of Chemistry 2011 PAPER www.rsc.org/nanoscale | Nanoscale Downloaded on 25 March 2011 Published on 09 December 2010 on http://pubs.rsc.org | doi:10.1039/C0NR00519C View Online