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Unexpected Physisorption of NOx on ultrapure Carbon Nanotubes

The electronic detection of environmentally harmful or poisonous species and specific biomolecules using carbon nanotubes is one of the most attractive alternatives to traditional detection methods.  These structures are a natural choice as gas sensor components because they have a high surface to volume ratio, outstanding electronic transport properties, and simultaneously, the capability to mediate chemical reactions. However, one of the most challenging and remaining problems is the assessment of the interaction between nanotubes and the molecules that adsorb on their outer parts, particularly during gas phase reactions where new species form.  Most studies available in the literature have used nanotube material that is either highly defective or non-purified, which makes the understanding of the involved chemical pathways not a straightforward matter. As pilot experiment, we investigated the physicochemical effects in the interaction of NOx and the outer wall of SWCNTs, which are sorted according to metallic character.  Using ultrapure separated nanotube-material allowed us to identify that the adsorption reaction is charge transfer mediated physisorption, which directly depends on the metallic character of the samples. 

Figure 1.  Left:C 1s XPS spectra recorded on the metallic (bottom) and semiconducting (top) SWCNT-samples after exposure to the 260 L saturation limit of NO2. A spectrum recorded with 70 L on the metallic sample is shown for comparison with lower dosages.  The adsorbed atomic oxygen forms a ketene group (C=O) at the monovacancies in both cases. Right: N 1s core level photoemission spectra of sorted metallic and mixed SWCNTs after exposure to NO2 at the saturation limit of 260 L. The highest binding energy component corresponding to N2O4 and NO3 is not observed in any spectra corresponding to metallicity sorted tubes. 

We inspected the C 1s responses in high resolution photoemission during the exposure of the nanotubes to different doses of NOx. Core-level shifts are noticeably different from what had been previously reported for unsorted material.  Our general observations on the lineshape and shifts in the C1s strongly suggest that the reaction pathways of the NOx molecules are very different to what had been reported so far (see left panel on figure 1). Upon exposure to NO2, a component independent from the π plasmon satellite and the shakeups related to low energy π-π* interband transitions was observed. Its intensity appeared to be directly related to the hybridization of the system and clearly depends on the metallicity of the samples. In order to understand the nature of this feature, we performed DFT calculations. We showed that the experimentally observed feature can only be attributed to a C=O species within a ketene group. This oxygen is available via a new reaction pathway generated by the photoinduced interaction with synchrotron radiation. Looking at the N1s line in photoemission (figure 1 - right), its shape suggests that the reaction pathway has to be different when ultraclean nanotubes are used. The chemical reactions are different and take place at a different speed from the reported for unsorted material. NO2 is weakly hybridized with the SWCNTs, and the relative intensities of the reaction products as compared to the main NO2 contribution are much smaller than previously reported. The components that can be identified are NO (∼401.2 eV), N2O2 (∼403 eV), N2O4 and NO3 (∼407.1 eV). NO2 is by far the strongest component after it interacts with the nanotubes.

The core level N 1s XPS signal includes the response from NO and NO2 molecules. Experimental evidence of stoichiometric equilibrium of these monomers with their dimers (N2O2 and N2O4) at low temperatures was reported elsewhere. To further understand our data we performed XAS on the N 1s edge.  Franck Condon satellites were resolved for this gas-nanotube system, revealing a weak chemisorption, related to NO dimer molecules (see figure 2). The nicely revealed fine structure in the N 1s edge that we reported has not been investigated in studies with carbon nanotubes before. With the data obtained in this study, we could observe that the NO2 is transformed into NO dimers following the endothermic chemical reaction of 2NO2 → 2NO+ 2O. The oxygen from the chemical reaction with the SWCNTs may be physisorbed as O2 or chemisorbed on pristine SWCNTs forming epoxide groups or at defects as carbonyl or ketene groups.

Figure 2.  Franck Condon satellites observed on the main peak of N 1s XAS response for metallic and semiconducting SWCNTs. 

This research was conducted by the following research team:

  • Andrea Goldoni, Paolo Lacovig, Matteo Dalmiglio and  Silvano Lizzit, Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
  • Paola Ayala, Thomas Pichler, Georgina Ruiz-Soria and Markus Sauer,University of Vienna, Austria
  • Duncann J. Mowbray, Alejandro Pérez Paz and Angel Rubio, Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, University of the Basque Country, Spain 
  • Kazuhiro Yanagi, Tokyo Metropolitan University, Japan

 

Reference

 G. Ruiz-Soria,  A. Pérez-Paz, M. Sauer, D. J. Mowbray, P. Lacovig, M. Dalmiglio, S. Lizzit, K. Yanagi, A. Rubio, A. Goldoni,  P. Ayala, and T. Pichler, “ Metallicity-Sorted Carbon Nanotubes”ACS Nano, 8, 2 (2014)1375, DOI: 10.1021/nn405114z 


Last Updated on Thursday, 05 June 2014 17:04