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Photocatalysis 'Hole' explained

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tinypeach:
I'm researching photocatalysis specifically TiO2 substances, and I'm a little confused about the use of a 'hole' when describing the mechanism.

I understand that an electron is excited from the valence band to the conduction band by light and that this process leaves behind a positively charged 'hole' in the valence band. However, I don't understand how this hole has a charge (Surely its just an absence of an electron) and I'm confused about how it is able to perform reduction reactions with molecules at the surface - I read that it is able to travel to the surface of the molecule and react with organic pollutants (Can a 'hole' move??)

Would somebody be able to explain this to me please?

Corribus:
In rough language, the valence band contains a sea of negatively charged electrons overlaid upon an ordered array of positively charged nuclei. For any given local area, the charges are balanced. If you remove a negatively charged valence electron and transport it somewhere else, the "somewhere else" has one additional electron, giving that "somewhere else" a net negative charge. That negative charge acts as a negatively charged electron against whatever dielectric background the "somewhere else" has. The original area now has a surplus of positive charge because an electron was removed. For all purposes, this hole in the electron field behaves like a positively charged particle. It's not really a particle but it acts that way because it is equivalent to an isolated charge unit. It behaves like an electron but opposite in character.

A common analogy is this: imagine a train with a long row of seats. To the left is the front of the train, to the right is the rear of the train. The seats are full. Everyone wants to sit as close as possible to the front of the train, but since all the seats are currently full, nobody can change position. Now imagine one person leaves their seat and exits the train, leaving behind an empty seat - a hole. Since everyone wants to sit as close as possible to the front of the train, people shift seats. The person to the right of the hole shifts one seat left, leaving behind a new empty seat to the right of the original one. Then the person to the right of this empty seat moves left, leaving behind their empty seat, and on and on. If you were an observer standing far away, you might say that the people are gradually moving left to the front of the train. But you may also properly observe that the empty seat is moving right toward the back of the train. Although the empty seat isn't really a thing, it behaves just like a person, but opposite. People move left, the empty seat moves right. This is only possible under the assumption that there is an implicit bias (people want to move toward the front of the train). In this analogy, this bias is equivalent to an applied voltage. But the important consideration here is that although the emptiness above the seat isn't a physical thing, it behaves like one (but opposite in many properties) because it is explicitly defined by the behavior of the people on either side of it.

Hunter2:
This takes place in a p-doted semiconductor.
Good explanation with the train.
Silicon has 4 electrons and these bond to other silicon electrons. If you dope the material with gallium, which has only 3 electons in outer shell, then one electron of the Silicon will not bond. A gap is left. With an electrical fiel electrons can jump into the gap and a new gap or hole is formed. The electrons travel from minus to plus. Therefore the gap ( holes) travel in the opposit plus to minus.

tinypeach:

--- Quote from: Corribus on March 01, 2024, 09:46:53 AM ---A common analogy is this: imagine a train with a long row of seats. To the left is the front of the train, to the right is the rear of the train. The seats are full. Everyone wants to sit as close as possible to the front of the train, but since all the seats are currently full, nobody can change position. Now imagine one person leaves their seat and exits the train, leaving behind an empty seat - a hole. Since everyone wants to sit as close as possible to the front of the train, people shift seats. The person to the right of the hole shifts one seat left, leaving behind a new empty seat to the right of the original one. Then the person to the right of this empty seat moves left, leaving behind their empty seat, and on and on. If you were an observer standing far away, you might say that the people are gradually moving left to the front of the train. But you may also properly observe that the empty seat is moving right toward the back of the train.

--- End quote ---

Thank you so much, this analogy made so much sense :)

Corribus:
If you really want something outlandish, since an electron excited into the conduction band is obviously a negatively charged particle, and the hole it leaves behind in the valence band acts like a positively particle, under the right conditions the two can bind each other and form a short-lived hydrogen-like "atom". This complex is called an exciton and plays a major role in the operation of LEDs over certain temperature ranges: when the exciton collapses, the electron fills the hole and emits a photon.

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