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Chapters / Chapter 8, Photoelectrochemistry / Lesson 8.1

8.1 The semiconductor–electrolyte interface

Band gaps and band positions, band bending at the semiconductor–electrolyte interface, the flat-band potential and Mott–Schottky analysis, and common photoelectrode materials, with a calculator.

About 30 minutesOpen to everyoneQuiz at the end

Using light to drive a reaction

In a photoelectrode, a semiconductor absorbs light and creates electron–hole pairs. If they are separated before they recombine, the holes can oxidise water and the electrons can reduce protons, so part of the energy needed for water splitting comes from the light instead of a power supply.

  • Band gap. The semiconductor only absorbs photons with an energy above its band gap Eg. Splitting water needs at least 1.23 eV per electron, and with real overpotentials a single absorber needs about 1.6 to 2 eV or more.
  • Band positions. For unassisted water splitting, the conduction band must lie above the H+/H2 level and the valence band below the O2/H2O level. Many stable oxides only meet one of these, so they need a bias or a second absorber.

Band bending at the interface

When a semiconductor touches an electrolyte, electrons flow until its Fermi level matches the redox level of the solution. In an n-type semiconductor this empties a thin region near the surface of electrons, the space-charge or depletion layer, and the bands bend upward toward the surface. The electric field in this layer pushes photogenerated holes to the surface, where they can oxidise water, and electrons into the bulk and to the back contact.

Band bending at an n-type semiconductor in contact with an electrolyte under lightElectrolyten-type semiconductorConduction bandValence bandFermi levelredox level of the electrolytespace-charge layerlightelectrons to the back contactholes to the surface,oxidising water
An n-type photoanode under light. Upward band bending sends holes to the surface and electrons to the back contact. In a p-type photocathode the bands bend downward and electrons go to the surface.

The applied potential at which the bands are flat is the flat-band potential Efb. For an n-type photoanode, a photocurrent can only flow at potentials above Efb, and in practice the onset is later because of slow surface kinetics and recombination.

The Mott–Schottky plot

The space-charge layer behaves like a capacitor whose width changes with potential. In depletion, its capacitance follows the Mott–Schottky equation, so a plot of 1/C² against potential is a straight line.

1/Csc² = [2 / (e ε ε0 ND)] (E − Efb − kT/e)Positive slope for n-type, negative for p-type. The x-intercept gives Efb, and the slope gives the carrier density ND if the permittivity ε is known.
Use it with care. The capacitance from impedance is only the space-charge capacitance if the equivalent circuit is right. Rough and nanostructured films, surface states and frequency dispersion can all distort the plot. Hankin and co-workers (2019) found widely different flat-band potentials for the same material from different methods, so measure at several frequencies, and confirm Efb with another method such as the photocurrent onset or the open-circuit potential under strong light.

Common photoelectrode materials

MaterialTypeBand gapNotes
TiO2nabout 3.0 to 3.2 eVVery stable, absorbs only UV
WO3nabout 2.6 to 2.8 eVStable in acid
BiVO4nabout 2.4 eVGood visible absorption, needs co-catalysts and protection
α-Fe2O3, hematitenabout 2.1 eVCheap and stable, very short hole diffusion length
Cu2Opabout 2.0 eVPhotocathode, needs protective layers
Sip or n1.1 eVExcellent absorber, used with protection or in tandems

A co-catalyst on the surface, such as an OER catalyst on a photoanode, speeds up the reaction and lets holes be used before they recombine. Many of the OER catalysts from chapter 2 are used this way.

Check your understanding

Quiz for lesson 8.1

Pick one answer per question. You see the explanation straight away.

References and further reading

  1. M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori, N. S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473.
  2. K. Gelderman, L. Lee, S. W. Donne, Flat-band potential of a semiconductor, using the Mott–Schottky equation, J. Chem. Educ. 84 (2007) 685.
  3. A. Hankin, F. E. Bedoya-Lora, J. C. Alexander, A. Regoutz, G. H. Kelsall, Flat band potential determination, avoiding the pitfalls, J. Mater. Chem. A (2019). doi:10.1039/c9ta09569a
  4. T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014) 7520–7535. doi:10.1039/c3cs60378d