In contact with redox molecules in the electrolyte, can the conduction band of a n-type degenerate semiconductor such as indium tin oxide (ITO), be bent to such a degree that oxidation reactions are stopped (or significantly slowed down) because they fall into the bandgap region?
I made a sketch detailing (a) the band-energy model of TCOs and (b) band-energy model meeting the density of states diagram of a redox couple with high Fermi level and (c) with low Fermi level relative to the bulk TCO electrode.
I understand that in the electron transfer equilibrium, the Fermi level of an electrode equals the Fermi level of the redox molecules in the electrolyte, (given a sufficient concentration of redox molecules). Furthermore I understand that at high density of states (metals and degenerate semiconductors), the Fermi level becomes pinned in the conduction band (or at surface defect states)* [Book1: Norio Sato, Electrochemisty at Metal and Semiconductor electrodes; Book2: P. Schmuki and S. Virtanen, Electrochemistry at the Nanoscale]
But I cannot find explicit information about the expected behaviour of band edges in the situation of Fermi level pinning (my drawings are guesses), and whether the Fermi level at the TCO surface can be moved into the bandgap, preventing low energy redox-reactions on a TCO electrode similar to normal semiconductors. Any insights and references would be much appreciated!
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* More detail: When the Fermi level is pinned, applied potentials are dropped across the Helmholtz layer in solution rather than a polarisation dependent change of interfacial charges (band edge level pinning).
If you dope the surface of the ITO to make it less n-type (more p-type), the Fermi energy may move as you described.Add your answer
How did you generate your figure? PowerPoint?
Hi Matthew, thank you for your answer. I do not intend to change anything about the doping of the commercially available and widely used ITO material. Instead, I wonder what happens to the bandgaps of the electrode, when it is immersed in a redox electrolyte for the purpose of electrochemistry (and the Fermi-levels equilibrate). ITO electrodes are frequently used in this context but usually treated like noble metal electrodes despite being a semi-conductor and apparently without explanation.
I make all my figures in Inkscape - my favorite (open source) vector image editing software.
I'd defer to someone with more experience, but I am happy to suggest the following:
If the concentration of redox couple in solution is sufficiently high (and the volume of liquid sufficiently big compared to the ITO), the redox potential of solution will ~not change. This means the fermi energy of the ITO will move as you have suggested (as electrons are transferred from the ITO to the redox couple, depleting negative charges at the interface of the ITO, leaving behind positively charged ions in the ITO lattice).
^ this is a very general statement. The interface is a place where conditions can change quickly and wildly. The chemical composition of the surface of the ITO might change since it is in contact with this particular solution (chemical reactions take place/ion exhanges, etc.), and through this process the electronic properties will change. If you want to know what your interface is really doing/looks like, you should do characterization on the interface. There are many papers (most papers) that do not do this, most of these papers make wild assumptions and are not accurate.
NOTE: if the ITO is going into accumulation, the length scale of the accumulation region may be sufficiently thin as to model the interface like a ~metal. If the interface is going into depletion, that is a different story.
Redox reactions are based on frontier's orbitals energetic. This energy gap with proper unit should match with the band gap energy for assisting any electron transfer. Therefore HOMO-LUMO orbital energies to be taken into account.
Hi Tobias, the Fermi level at surface will be pinned at Fermi stabilization level (Efs). This is positioned at ~4.9eV below the vacuum level. Please see this paper form more details: Mechanism of Fermi-level stabilization in semiconductors https://journals.aps.org/prb/abstract/10.1103/PhysRevB.37.4760
Thus if for the n-type TCO the Efs is positioned in the bandgap, the alignment of the Fermi level will cause the CBE to rise at surface. The surface of the TCO will be depleted of conduction electrons. If Efs is positioned above the CBE then the CBE at surface will be lowered to allow the alignment of the Fermi level and some amount of electron accumulation will take place.
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