Photoelectrochemistry

Key takeaways

Photoelectrochemistry belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Photoelectrochemistry to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Photoelectrochemistry from memory before moving on to harder problems.

Reference excerpt

Photoelectrochemistry is a subfield of study within physical chemistry concerned with the interaction of light with electrochemical systems. It is an active domain of investigation. One of the pioneers of this field of electrochemistry was the German electrochemist Heinz Gerischer. The interest in this domain is high in the context of development of renewable energy conversion and storage technology.

Historical approach Photoelectrochemistry has been intensively studied in the 1970-80s because of the first peak oil crisis. Because the use of fossil fuels is not sustainable, it is necessary to develop processes to obtain renewable resources and use sustainable energy. Artificial photosynthesis, photoelectrochemical water splitting and regenerative solar cells are of special interest in this context. The photovoltaic effect was discovered by Alexandre Edmond Becquerel. Heinz Gerischer, H. Tributsch, AJ. Nozik, AJ. Bard, A. Fujishima, K. Honda, PE. Laibinis, K. Rajeshwar, TJ Meyer, PV. Kamat, N.S. Lewis, R. Memming, John Bockris are researchers which have contributed a lot to the field of photoelectrochemistry.

Semiconductor electrochemistry

Introduction Semiconductor materials have energy band gaps, and will generate a pair of electron and hole for each absorbed photon if the energy of the photon is higher than the band gap energy of the semiconductor. This property of semiconductor materials has been successfully used to convert solar energy into electrical energy by photovoltaic devices. In photocatalysis the electron-hole pair is immediately used to drive a redox reaction. However, the electron-hole pairs suffer from fast recombination. In photoelectrocatalysis, a differential potential is applied to diminish the number of recombinations between the electrons and the holes. This allows an increase in the yield of light's conversion into chemical energy.

Semiconductor-electrolyte interface When a semiconductor comes into contact with a liquid (redox species), to maintain electrostatic equilibrium, there will be a charge transfer between the semiconductor and liquid phase if formal redox potential of redox species lies inside semiconductor band gap. At thermodynamic equilibrium, the Fermi level of semiconductor and the formal redox potential of redox species are aligned at the interface between semiconductor and redox species. This introduces an upward band bending in a n-type semiconductor for n-type semiconductor/liquid junction (Figure 1(a)) and a downward band bending in a p-type semiconductor for a p-type semiconductor/liquid junction (Figure 1(b)). This characteristic of semiconductor/liquid junctions is similar to a rectifying semiconductor/metal junction or Schottky junction. Ideally to get a good rectifying characteristics at the semiconductor/liquid interface, the formal redox potential must be close to the valence band of the semiconductor for a n-type semiconductor and close to the conduction band of the semiconductor for a p-type semiconductor. The semiconductor/liquid junction has one advantage over the rectifying semiconductor/metal junction in that the light is able to travel through to the semiconductor surface without much reflection; whereas most of the light is reflected back from the metal surface at a semiconductor/metal junction. Therefore, semiconductor/liquid junctions can also be used as photovoltaic devices similar to solid state p–n junction devices. Both n-type and p-type semiconductor/liquid junctions can be used as photovoltaic devices to convert solar energy into electrical energy and are called photoelectrochemical cells. In addition, a semiconductor/liquid junction could also be used to directly convert solar energy into chemical energy by virtue of photoelectrolysis at the semiconductor/liquid junction.

Experimental setup Semiconductors are usually studied in a photoelectrochemical cell. Different configurations exist with a three electrode device. The phenomenon to study happens at the working electrode WE while the differential potential is applied between the WE and a reference electrode RE (saturated calomel, Ag/AgCl). The current is measured between the WE and the counter electrode CE (carbon vitreous, platinum gauze). The working electrode is the semiconductor material and the electrolyte is composed of a solvent, an electrolyte and a redox specie. A UV-vis lamp is usually used to illuminate the working electrode. The photoelectrochemical cell is usually made with a quartz window because it does not absorb the light. A monochromator can be used to control the wavelength sent to the WE.

Main absorbers used in photoelectrochemistry

Semiconductor IV

C(diamond), Si, Ge, SiC, SiGe

Semiconductor III-V BN, BP, BAs, AlN, AlP, AlAs, GaN, GaP, GaAs, InN, InP, InAs...

Semiconductor II-VI CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe, MoS2, MoSe2, MoTe2, WS2, WSe2

Metal oxides TiO2, Fe2O3, Cu2O

Organic dyes

Methylene blue...

Organometallic dyes

Perovskites

Very recently scalable all-perovskite based PEC photoelectrochemical system as solar hydrogen panel has been developed with >123 cm2 area.

Applications

Photoelectrochemical water splitting

Photoelectrochemistry has been intensively studied in the field of hydrogen production from water and solar energy. The photoelectrochemical splitting of water was historically discovered by Fujishima and Honda in 1972 onto TiO2 electrodes. Recently many materials have shown promising properties to split efficiently water but TiO2 remains cheap, abundant, stable against photo-corrosion. The main problem of TiO2 is its bandgap which is 3 or 3.2 eV according to its crystallinity (anatase or rutile). These values are too high and only the wavelength in the UV region can be absorbed. To increase the performances of this material to split water with solar wavelength, it is necessary to sensitize the TiO2. Currently Quantum Dots sensitization is very promising but more research is needed to find new materials able to absorb the light efficiently.

Photoelectrochemical reduction of carbon dioxide

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Illustrations

Worked examples

Example 1 — a first encounter with Photoelectrochemistry

Start with the simplest possible case. Write down what Photoelectrochemistry claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Photoelectrochemistry before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Photoelectrochemistry ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Photoelectrochemistry

In research: Photoelectrochemistry appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Photoelectrochemistry in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Photoelectrochemistry is common in secondary-school and first-year university syllabi. It links to neighbouring topics Photoelectrochemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Photoelectrochemistry outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Photoelectrochemistry in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Photoelectrochemistry means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Photoelectrochemistry out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Photoelectrochemistry in simple terms?

Photoelectrochemistry is a subfield of study within physical chemistry concerned with the interaction of light with electrochemical systems. It is an active domain of investigation.

Why does Photoelectrochemistry matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Photoelectrochemistry?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Photoelectrochemistry.