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 1970-80s because of the

.

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 (

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...

Organometallic dyes

Perovskites

Very recently scalable all-perovskite based PEC photoelectrochemical system as solar hydrogen panel has been developed with >123 cm2 area. ^[3]

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

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)

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

External links

• Complete review about semiconductor's photoelectrochemistry
• Review about semiconductor's photoelectrochemistry
• Electrochemistry Encyclopedia at the Library of Congress Web Archives (archived 2001-11-25)