Photocatalysts use the energy from absorbed photons to accelerate the rate of chemical reactions. Many different semiconducting compounds display photocatalytic proprties under different conditions. TiO2 is the most commonly used photocatalyst but it is only active under UV irradiation.

Photocatalytic Water Splitting

In general, the water splitting reaction occurs by the following mechanism:

Photon Absorption:   2hv → 2e- + 2h+
Anode: H2O + 2h+1/2O2 + 2H+
Cathode: 2H+ + 2e- → H2
Overall: H2O + 2hv → H2 + 1/2O2

The products, oxygen and hydrogen gases, may be used as a fuel to undergo direct combustion or in a hydrogen fuel cell. They may also be cooled and compressed for use as cryogens. The use of hydrogen as a fuel and a coolant could have environmental advantages in a variety of potential applications.

In order for reaction to occur the half-cell reaction potentials must be bridged by the surface electron and hole energies. That is, the conduction band electrons must have sufficient energy to reduce protons whilst holes must capable of accepting electrons from surface-absorbed water molecules.

Figure 1: Simplified energy level diagram to illustrate photocatalytic watersplitting by an n-type semiconductor

Efficiency of Solar-Activated Photocatalysis

Figure 2: Reference solar spectrum (irradiance as a function of wavelength at air mass 1.5) ASTM G 170-03, ASTM International.

Only those photons with sufficient energy to promote an electron across the band gap can contribute energy that may be used to drive the water-splitting reaction. And, due to rapid energy loss by "hot" electrons, each photon only contributes the band gap energy to do useful work. For a standard solar spectrum (Figure 2) the Shockley-Queisser limit calculated from these assumptions predicts that the band gap capable of extracting the maximum amount of useful work (30% efficiency) is 1.1 eV.

Considering this reaction mechanism the properties of a high efficiency visible light photocatalyst for water splitting can be deduced:

  • Band gap approximately equal to 1.7 eV (approaching the optimum band gap but still bridging the water-splitting reaction potentials and overpotentials).
  • Excellent chemical stability and photocorrosion resistance under reaction conditions.
  • High internal quantum efficiency (implies low electron-hole recombination rate).

Unfortunately, the majority of known narrow band semiconductors show poor chemical stability against photocorrosion under illuminated aqueoues conditions rendering them unsuitable for applications as water-splitting photocatalysts. Amongst other compounds, Bi2O3, WO3 and their mixed oxides demonstrate both medium range band gaps (2.6 eV to 2.8 eV) and good chemical stability. Photocatalyst band gaps may be measured experimentally using UV-vis diffuse reflectance spectroscopy.

Other Applications of Photocatalysis

Other chemical reactions may be photocatalysed leading to alternative applications of photocatalytic technology.

  • The oxidation of organic molecules upon the surfaces of a colloidal photocatalyst or a photoelectrode can be used to clean waste water.
  • Redox reactions can change the state of heavy metal ions allowing mineralisation and their removal from contaminated ground water.
  • The generation of surface -OH groups upon photocatalytic TiO2 leads to superhydrophilicity. Windows and street lamps coated with this material exhibit self-cleaning properties- rain water and dirt run off without forming droplets.

Selected References

[1] Kaneko, M. and I. Okura, Photocatalysis: Science and Technology. Biological and Medical Physics Series. 2002, Tokyo: Kodansha.

[2] Tan, M.S. and et al., Progress in Inorganic Chemistry, 1994. 41: p. 21-144.

Related Publications

Showing 8 publications related to photocatalysis in date order.
Links to online copies of these papers may require a journal subscription (personal or institutional).
1 Melis Arin, Petra Lommens, Simon C Hopkins, Glenn Pollefeyt, Johan Van der Eycken, Susagna Ricart, Xavier Granados, Bartek A Glowacki and Isabel Van Driessche, Deposition of photocatalytically active TiO2 films by inkjet printing of TiO2 nanoparticle suspensions obtained from microwave-assisted hydrothermal synthesis
Nanotechnology, 23 (16) 165603 (27 April 2012) | DOI: 10.1088/0957-4484/23/16/165603 
2 I. Fasaki, K. Siamos, M. Arin, P. Lommens, I. Van Driessche, S.C. Hopkins, B.A. Glowacki, I. Arabatzis, Ultrasound assisted preparation of stable water-based nanocrystalline TiO2 suspensions for photocatalytic applications of inkjet-printed films
Applied Catalysis A: General, 411-412 60-69 (16 January 2012) | DOI: 10.1016/j.apcata.2011.10.020 
3 Melis Arin, Petra Lommens, Nursen Avci, Simon C. Hopkins, Klaartje De Buysser, Ioannis M. Arabatzis, Ioanna Fasaki, Dirk Poelman and Isabel Van Driessche, Inkjet printing of photocatalytically active TiO2 thin films from water based precursor solutions
Journal of the European Ceramic Society, 31 (6) 1067-1074 (June 2011) | DOI: 10.1016/j.jeurceramsoc.2010.12.033 
4 A. P. Finlayson and B. A. Glowacki, Theoretical and experimental investigation of N-doped WO3 for photocatalytic solar energy conversion
Proceedings of the 18th Workshop on Quantum Solar Energy Conversion, (March 2006) | View paper online 

QUANTSOL 2006: 18th Workshop on Quantum Solar Energy Conversion, Rauris, Salzburg, Austria, 19-24 March 2006

5 A. P. Finlayson, A. Ball and B. A. Glowacki, Sol-gel ink-jet printing of photocatalytic systems for hydrogen production
Proceedings of the Eurosun 2006 conference, (June 2006)

EuroSun 2006, Glasgow, Scotland, 26-29 June 2006

Proceedings available on CD from the Solar Energy Society, UK-ISES (more details)

6 A. Finlayson, V. N. Tsaneva, L. Lyons, M. Clark, B. A. Glowacki, Evaluation of Bi- W- Oxides for Visible Light Photocatalysis
Physica Status Solidi (a), 203 (2) 327-335 (February 2006) | DOI: 10.1002/pssa.200521129 
7 A.P.Finlayson, E.Ward, V.N.Tsaneva, B.A.Glowacki, Bi2O3-WO3 compounds for photocatalytic applications by solid state and viscous processing
Journal of Power Sources, 145 (2) 667-674 (18 August 2005) | DOI: 10.1016/j.jpowsour.2005.01.072 

Fuel Cells Science and Technology Meeting, 6-7 October 2004

8 Anna P. Finlayson, Vassilka N. Tsaneva, Bartek A. Glowacki, Leslie Lyons,Mike Clark, Optimisation of narrow band gap semiconductors for photocatalytic applications
Proceedings of the 17th Workshop on Quantum Solar Energy Conversion, (2005) | View paper online 

QUANTSOL 2005: 17th Workshop on Quantum Solar Energy Conversion, Rauris, Salzburg, Austria, 13-19 March 2005