Photoelectrochemistry

Photoelectrochemistry is a subfield of study within physical chemistry concerned with the interaction of light with electrochemical systems.^[1]^[2] 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 70-80s because of the first peak oil crisis. Because fossil fuels are non-renewable, it is necessary to develop processes to obtain renewable ressources and use clean energy. Artificial photosynthesis, photoelectrochemical water splitting and regenerative solar cells are of special interest in this context.

H. Gerischer, H. Tributsch, AJ. Nozik, AJ. Bard, A. Fujishima, K. Honda, PE. Laibinis, K. Rajeshwar, PV. Kamat, N.S. Lewis, R. Memming, JOM. Bockris are researchers which have contributed a lot to the field of photoelectrochemistry.

Semiconductor's electrochemistry

Introduction

Semiconductor material has a band gap and generates a pair of electron and hole per absorbed photon if the energy of the photon is higher than the band gap 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 but the problem is that the electron-hole pair suffer from fast recombinations. 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 a downward band bending in a n-type semiconductor for n-type semiconductor/liquid junction (Figure 1(a)) and an upward 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.

Figure 1(a) band diagram of n-type semiconductor/liquid junction
Figure 1(b) band diagram of p-type 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...

Applications

Photoelectrochemical splitting of water

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. Recently water a splitting membrane concept has been developed. This method to split water is very similar to the principle of fuel cells but in a reverse way.

Artificial photosynthesis

Photosynthesis is the natural process that converts CO₂ using light to produce hydrocarbon compounds such as sugar. The depletion of fossil fuels encourages scientists to find alternatives to produce hydrocarbon compounds. Artificial photosynthesis is a promising method mimicking the natural photosynthesis to produce such compounds. The photoelectrochemical reduction of CO2 is much studied because of its worldwide impact. Many researchers aim to find new semiconductors to develop stable and efficient photo-anodes and photo-cathodes.

Regenerative cells or Dye-sensitized solar cell (Graetzel cell)

Dye-sensitized solar cells or DSSCs use TiO₂ and dyes to absorb the light. This absorption induces the formation of electron-hole pairs which are used to oxidize and reduce the same redox couple, usually I⁻/I₃⁻. Consequently a differential potential is created which induces a current.

References

↑ IUPAC Compendium of Chemical Terminology
↑ Electrochemistry Encyclopedia

External links

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