Electrochemical Transfer Doping of Diamond, Carbon Nanotubes, and Graphene
Authors: Chakrapani Vidhya 
Published: February 2020

The conductivity of semiconducting, single-walled carbon nanotubes (SWCNT) is a sensitive function of small concentrations of O2, NO2, NH3, and O3.1-5 The cause of this effect is not well understood. We present evidence that these observations are consistent with charge transfer between the SWCNT and the oxygen redox couple in an adsorbed water film.

Experimental: We observe an increase in conductance and a change in conductivity type from n-type to p-type when vacuum-annealed SWCNT are exposed to humid air. Exposure to the basic gas, NH3, restores n-type conductivity. See Fig. 1.
Exposure to the acidic gas, N2O, enhances the p-type conductance. Vacuum annealing of air-exposed SWCNT at temperatures up to 400C has little effect on the conductance or the Seebeck coefficient; however, when the annealing temperature is increased to 535C, the conductance decreases and the Seebeck coefficient becomes negative. Upon cooling and re-exposure to air, the original p-type conductance and positive Seebeck coefficient are restored. We observe similar behavior with graphene, and closely related effects have been observed with diamond.6

Electrochemical Transfer Doping The results are explained by charge transfer between the semiconductor and the oxygen redox couple in an adsorbed water film.
4H+ + 4e- + O2 = 2H2O
The direction of charge transfer depends on the relative positions of the Fermi energy of the SWCNT and the electron chemical potential (Fermi energy) of the redox couple, which can be estimated from the Nernst equation. At high pH the electron chemical potential of the redox couple is close to the conduction band edge; at low pH the electron chemical potential is slightly below the valence band edge. These positions are consistent with our observations of the changes in conductance and conductivity type with pH.

Discussion: The electron affinity of the oxygen redox couple ranges from 4.83 at pH =14 to 5.66 eV at pH = 0, which makes the energetics favorable for electron transfer to the redox couple from SWCNT at low pH. On the other hand, the electron affinities of molecular O2, N2O, NO2 and O3 are respectively 0.451, 0.22, 2.273 and 0.451eV, which mean that electron transfer directly to these molecular species is unlikely. Furthermore, the Fermi energy of graphene has been estimated to be 4.46 eV below the vacuum level,7which is close to the range of the oxygen redox couple and therefore may be influenced by changes in the ambient.
After electron transfer, there will be a positive space charge layer in the SWCNT and charge-compensating solvated anions in the adsorbed water film.8, 9 The resulting electrostatic attraction between the SWCNT and the solvated anions will enhance the stability of the water layer.

Summary: Electrochemically mediated charge transfer appears to be a very general phenomenon that has often been unrecognized.8-10 The effect has been found in GaN and ZnO where it affects the luminescence properties.9

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