Page 195 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

180 Principles and Methods

18
O 2 , with O verified that H 2 O 2 was produced directly by the reduction
of adsorbed oxygen by conduction band electrons. Quantum yields were
as high as 30 percent for H 2 O 2 production at low photon fluxes. At the
same time, the quantum yield was shown to vary with the inverse
square root of absorbed light intensity [i.e., f~s!I abs d 21 ], with the wave-
length of excitation f~sld 21 , and with the diameter of the Q-sized col-
loids (i.e., f~D 21 ). For example, d[H 2 O ]/dt is 100 to 1000 times faster
p
2
on Q-sized ZnO particles (D p 2 4 nm) than with bulk-phase ZnO par-
ticles (D p 100 nm).
Hydrogen peroxide production proceeds, after initial photoactivation,
by electron transfer from the conduction band to dioxygen adsorbed on
the surface of the excited-state metal oxide as follows:

2
2.
2[e cb 1 O 2 h O 2 ] (41)
4.8
pK a
→
⎯⎯⎯⎯⎯

.
⎯
O H ←⎯⎯⎯⎯ HO . 2 (42)
2
.
2HO 2 h H O 1 O 2 (43)
2
2
Hoffmann and coworkers [25, 36, 37] observed a tenfold increase in
the measure quantum yield for H 2 O 2 production upon reduction of the
mean particle diameter from 40 to 23 nm for ZnO, where O 2 was the elec-
tron acceptor and small molecular organic compounds (e.g., carboxylic
acids and alcohol) the electron donor. Similar effects were reported by
Hoffmann and coworkers [35, 38, 39] for photo-polymerization reac-
tions catalyzed by Q-sized CdS, Q-ZnO, and Q-TiO 2 and for SO 2 oxida-
tion in the aqueous phase.
In addition to ROS generated from surface hydroxyl species and from
adsorbed O 2 , there are other oxygen-containing free radical species
that are generated on the surface of photoactivated semiconductors. For
2
⋅ H O] [HSO ] [SO ]) is readily
example, S(IV) ( [S(IV)] # [SO 2 2 3 3
photooxidized [28] in the presence of colloidal suspensions of nanopar-
ticulate -Fe O .
2
3
hn#520 nm
2
22

1 2 HSO 3 h2 SO 4 1 2H 1
O 2 (44)
-Fe 2 O 3
Quantum yields ranged from 0.08 to 0.3 with a maximum yield found
at pH 5.7. The primary initiation pathway involved irradiation at wave-
lengths equal to or less than the nominal bandgap of hematite, which
is 2.2 eV or 560 nm. Upon bandgap illumination, conduction-band electrons
and valence-band holes are separated; the trapped electrons are trans-
ferred either to surface-bound dioxygen or to Fe(III) sites on or near the
surface, while the trapped holes accept electrons from adsorbed SO 2 to
3