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Photocatalysts for hydrogen generation and organic contaminants degradation 221

Table 10.2 Typical timescale of various processes

Process Timescale
Charge generation Few femtoseconds
Charge trapping 100 ps to 10 ns
Charge recombination 10–100 ns
Charge transfer 100 ns to few milliseconds

Adapted from M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor
photocatalysis. Chem. Rev. 95 (1995) 69–96.

10 –15 10 –12 10 –9 10 –3
Seconds
Timescale

CB

VB
Generation Trapping Recombination Charge transfer and separation

Fig. 10.5 Four different processes involving electron and hole charge carrier: generation
(~femtoseconds), trapping (pico to nanoseconds), recombination (nanoseconds), and transfer
(milliseconds). Whereas trapping can occur at surface defects or at a compositionally different
site, recombination can occur at surface or in bulk. The charge transfer is made effective by
using co-catalysts [13].

10.1.4 Fundamental approach to hydrogen generation and
organic contaminants’ degradation using semiconductors

To generate hydrogen from water, the stable water molecule needs to be split into
hydrogen and oxygen. Thermodynamic studies reveal that about 237 kJ of energy per
mole of water or 1.23 eV per electron is required to carry out the reduction of a wa-
ter molecule. The intermediate photochemical reactions governing the photocatalytic
process is shown in Eqs. (10.1)–(10.4).

-
hn+ Photocatalyst ® e + h + (10.1)
+
+
h + HO ® H + OH • (10.2)
2
h + OH ® OH • (10.3)
-
+
e + O ® O 2 - (10.4)
-
2