Room temperature methane detection using palladium loaded single-walled carbon nanotube sensors

https://doi.org/10.1016/j.cplett.2004.05.029Get rights and content

Abstract

Single-walled carbon nanotubes (SWNTs) loaded with palladium (Pd) nanoparticles are used for detection of methane ranging from 6 to 100 ppm in air at room temperature. The Pd-SWNT nanosensors show advantages over conventional catalytic beads and metal oxide sensors for methane detection in terms of reduced size and power consumption by a factor of 100 and sensitivity by a factor of 10. A charge transfer sensing mechanism in which Pd attracts electrons from SWNTs to form a weakly bound complex Pdδ+(CH4)δ− is discussed. In addition, the elevated temperature and ultraviolet light effects on the sensor characteristics are also studied.

Introduction

The electronic and structural properties of SWNTs enable a wide variety of applications [1], [2], [3], [4], [5], [6] including hydrogen storage [5], catalysis [3], DNA detection [7], and chemical sensing [2], [4], [8], [9]. In chemical sensor applications, SWNTs form conducting channels between two metal electrodes for gas detection through a sensitive electronic response of the nanotubes due to their interaction with gas molecules. Because SWNTs consist of a single layer of carbon, all the atoms are exposed to the environment and therefore can readily adsorb gas molecules [9]. While the adsorption interactions can be very specific, they generally result in some degree of charge transfer between gas molecules and the nanotubes causing a change in conductivity that can be detected [8]. Electron donating and withdrawing molecules will either transfer electrons to or attract electrons from SWNTs, thereby giving them more charge carriers or holes, respectively, that changes the SWNT conductance [2], [9]. Organic compounds containing these donating and accepting groups such as phenol and nitrotoluene can also be detected as demonstrated in [8].

For gas molecules that show no or very small electron donor or acceptor properties such as methane and carbon oxides, SWNTs can still be used for gas detection when loaded appropriately, particularly with transition metals that interact with these molecules. It should be noted that the experiment results of pure SWNTs do not show any sensor response to methane at all. In this Letter, we report Pd loaded SWNTs for methane detection at room temperature. In this demonstration, Pd attracts electrons from SWNTs, when methane molecule is adsorbed in Pd-SWNT matrix, to form a weakly bound complex Pdδ+(CH4)δ−. As a result, the current through p-typed SWNTs increases with CH4 concentration.

Methane and carbon oxides are green house gases and environmental monitoring requires chemical sensors for ppb to ppm level detection of these gases. Catalytic beads and metal oxides sensors have been developed for hundreds of ppm level methane detection while infrared and flame ionization detectors are suitable for instrumental analysis of sub ppm and ppb level hydrocarbons including methane [10], [11], [12]. These detection approaches are based on chemical oxidation or decomposition of methane requiring high energy or temperature (>500 °C) and therefore, the power consumption is relatively large (hundreds of mW and higher). The Pd-SWNT nanosensor presented in this work shows capability of ppm level methane detection at room temperature and power consumption of several mW. The orders of magnitude improvement in size, sensitivity and power consumption of the nanosensors over conventional catalytic beads and metal oxides sensors can be attributed to: (1) nanoscale enhanced charge transfer mechanism at room temperature between Pd-loaded nanotubes and methane molecules, and (2) a large surface area of SWNTs which effectively adsorb methane molecules to enhance the efficiency of dispersed Pd nanoparticles interact with methane molecules.

Section snippets

Experimental

The sensing platform is an interdigitated electrode (IDE) reported previously [8], which consists of two electrodes connected by interdigitated fingers over which Pd-SWNTs solution was dispensed. The loaded SWNTs serve as resistors, changing the current as their conductance changes with gas adsorption or desorption. In this work, pure SWNTs (98%, Carbon Nanotechnology, Inc., Houston, TX) were used as supporting material. A layer of 10nm thick metallic Pd was sputter coated onto a pile of SWNT

Results and discussion

Test results exhibit an increase in current upon the introduction of small concentrations of methane. Fig. 2 shows a typical sensor response to exposures of methane at 6, 15, 30, and then 100 ppm in air at room temperature. Based on the results from the four sensors tested, calibration curves for the relative sensor response versus the methane concentration were developed by comparing the change in conductance with the initial conductance. A logarithmic fit was found to most closely match the

Concluding remarks

In summary, Pd loaded SWNT sensors have been demonstrated for the detection of trace amounts of methane in air at room temperature, showing an order of magnitude improvement in size, sensitivity and two orders of magnitude improvement in power consumption over conventional sensors. These sensors give reproducible trends and consistently respond to heat and UV light for speed recovery.

Acknowledgments

The authors would like to thank Laura Q. Ye for making the interdigitated electrodes and Harry Patridge, Charles Bauchlicher and James Arnold for valuable discussion and comments. Y. Lu, J. Li, J. Han, and H. Ng are with the University Affiliated Research Center (UARC) at NASA Ames Research Center operated by the University of California, Santa Cruz. C. Binder and C. Partridge were summer interns from Rensselaer Polytechnic Institute and Palo Alto High School, respectively.

References (16)

  • C.K.W Adu et al.

    Chem. Phys. Lett.

    (2001)
  • J Au-Yeung et al.

    J. Catal.

    (1999)
  • P Fau et al.

    Sensors Actuat. B

    (2001)
  • R.J Chen et al.

    Appl. Phys. Lett.

    (2001)
  • J.Z Luo et al.

    Catal. Lett.

    (2000)
  • J Zhao et al.

    Nanotechnology

    (2002)
  • W Du et al.

    Nanoletters

    (2002)
  • I.W Chiang et al.

    J. Phys. Chem. B

    (2001)
There are more references available in the full text version of this article.

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