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Charge Transfer Equilibria Between Diamond and an Aqueous Oxygen Electrochemical Redox Couple

Vidhya Chakrapani et al.

In 1989, Maurice Landstrass and K. V. Ravi observed a curious phenomenon—undoped diamond, known to be an exceedingly good insulator, showed substantial conductivity when exposed to air (1). The source of the conductivity has been uncertain and a matter of controversy since that time, which is surprising for such an important and long-studied material. Resolution of the problem is of inherent scientific interest and could be important for applications of diamond and in other contexts as well. Subsequent studies confirmed that the conductivity was confined to a near-surface region, carried by positive charge carriers (holes); and that the change occurred only when the diamond was covered with chemically bound hydrogen. Numerous proposals were made to explain the phenomenon; none has received wide acceptance. One recent proposal was that the effect occurred when electrons were transferred from the diamond to an electrochemical couple in an adsorbed water film (2). This proposal has received limited support, in part because it posited an adsorbed water film on an extremely hydrophobic substrate, and also because the energetics and dynamics of the proposed electrochemical couple were problematic.

In this paper we describe a series of controlled experiments to explore this effect in which the presence of an aqueous phase is unambiguous. We hydrogenated diamond particles and measured the changes in pH and oxygen concentrations when the particles were added into aqueous solutions. These experiments show that electron exchange systematically occurs between diamond and the aqueous redox couple O₂ + 4H⁺ + 4e⁻ ⇌ 2H₂O, which results in the consumption or formation of O₂. This electron exchange influences other properties—both the contact angle of water with the diamond surface and the amount and sign of the charge on diamond particles change in a predictable way by changing the extent of reaction that takes place. Adhesion of water to diamond is enhanced by electrostatic attraction after the charge transfer, which enhances the ability of water films to adsorb on otherwise hydrophobic surfaces.

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Electron energies of various solids. The vertical bars show the band gap. The continuous blue line is the electron energy of the oxygen couple in humid air (pH = 6); the lower dashed line is the value for pH = 0, the upper dashed line for pH = 14.

These results imply that this process is a more general, unrecognized phenomenon that can influence a wide range of materials and processes. The key components are all derived from normal humid air: The water film provides both a medium for the electrochemical reaction as well as the O₂ and the H⁺ (the protons arise from acidity generated by CO₂ that is present in air). This means that the effect can occur whenever semiconductors or other solids are exposed to humid air.

The atmosphere thus provides a source of electrons whose electrochemical potential (Fermi energy) is fixed by the oxygen redox couple. If the atmosphere is in contact with a solid phase, electrons will transfer between the adsorbed water film and the solid in a direction that tends to bring the Fermi energy of the solid equal to that of the ambient film. The process is similar to that at a metal-semiconductor contact, except that a water film replaces the metal.

The electron energies of several solids and the oxygen redox couple in humid air are shown in the figure. In equilibrium in air, the couple fixes the Fermi energy of diamond at the top of the valence band, which generates positive charge carriers (holes). The energy range of the couple spans the band gap of semiconducting single-walled carbon nanotubes (s-SWNTs). For GaN, the electron potential of the redox couple lies near the states in the middle of the energy gap responsible for its ubiquitous “yellow band” luminescence (see the figure). Other experiments suggest that the conductivity of carbon nanotubes can be changed from that based on electrons to that based on holes, and that the intensity of luminescence from GaN can be modulated by changing the electrochemical potential of the ambient air (3).

Several caveats are in order. For the ambient air to fix the Fermi energy of a solid, there must be a large reservoir of reactants and facile reaction kinetics at the interface. The air provides quasi-infinite sources of O₂, H₂O, and CO₂; however, several factors can inhibit equilibration of the Fermi energies. An oxide layer, e.g., on silicon, or a thin film of adsorbed hydrocarbon can block electron exchange. The reduction of O₂ in the aqueous redox couple may require trace metallic impurities on the surface. Chemical ionization of oxidized surfaces, not involving electron transfer, and intrinsic conductivity in small–band gap semiconductors and metals can both mask the effect.

It is highly likely that the effects of electrochemically mediated charge transfer have remained unrecognized in many common situations. In this light, it will be of great interest to reexamine the literature on mechanical sliding friction and contact electrification, both of which depend in complex ways on relative humidity and impurities in the ambient air. Even more speculative is the possibility that certain animals and insects have evolved the capability of modulating the electrochemical potential in their feet to change the adhesive force to solid surfaces. Charge transfer to nanostructures not only can affect their properties, but also will vary with the size of the structure because of quantum confinement effects.

Summary References

1. M. I. Landstrass, K. V. Ravi, Appl. Phys. Lett. 55, 975 (1989).
2. F. Maier, B. Riedel, J. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett. 85, 3472 (2000).
3. V. Chakrapani, J. C. Angus, A. B. Anderson, G. Sumanasekera, Mater. Res. Soc. Symp. Proc. 956, paper J15-01 (2007).

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