Page 194 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Reactive Oxygen Species Generation on Nanoparticulate Material 179

The photochemistry and photophysics of quantum-sized nanoparticles
[16, 22, 24, 35] (1 nm D 10 nm) in contrast with the larger, bulk-phase
p
particles (D < 100 nm), can be quite different. Many nanoparticulate semi-
p
conductors, which are often described as “quantum-sized particles” or
“quantum dots” depending on their applications, exhibit a characteristic
blue shift in the UV or visible absorption spectrum. Along with the blue
shift in the absorption spectrum, there is a corresponding increase in the
bandgap energy, E which can be described in terms of a simple solution
g
to the Schrödinger equation with an appropriate Hamiltonian.
2
2
p h 1 1.8 e 2 (39)
b 2
E g . a 2
2R εR
where R is the particle radius and µ is the reduced mass of the exciton
or the electron-hole pair.
1 1 1 (40)
m 5 a ∗ 1 ∗ b
m e 2 m h 1
∗ ∗
where m e 2 is the effective mass of the electron and m h 1 is the effective
h
mass of the hole, and ε is the dielectric constant of TiO 2 , and is Planck’s
constant.
According to Eq. 39, as R decreases the bandgap energy, E , increases
g
(i.e., RT 5 E g c ). As an example, Kormann et al. [24] prepared Q-sized
TiO 2 with a characteristic blue shift from the bulk state bandgap of 385
nm for anatase TiO 2 down to 350 nm (i.e., E 3.2 to 3.35 eV). The steady-
g
state particle size ranged from 2.0 nm to 2.5 nm depending on the prepa-
ration conditions and the Ti(IV) reagent used in the synthesis (e.g., TiCl 4
or Ti(IV)-isopropoxide). The corresponding cluster size (oligomer) for the
nanoparticles ranged from 120 to 220 monomers. In an earlier study,
Bahnemann et al. [16] reported that Q-ZnO exhibited bandgap increases
4.2 eV).
as large as 1 eV or E g, Q2ZnO
An increase in E g often enhances the reactivity of the photocatalyst
by increasing its reduction/oxidation potential and thus the driving
force for electron transfer in the normal Marcus regime; thereby ROS
(reactive oxygen species) should also be a function of particle size.
In a subsequent study, Hoffman et al. [36] investigated the photo-
chemical production of H 2 O 2 on irradiated Q-ZnO over the wavelength
range of 320 370 nm in the presence of carboxylic acids and
oxygen. Steady-state concentrations up to 2 mM H 2 O 2 were formed.
Maximum H 2 O 2 concentrations were obtained only with added electron
donors (i.e., hole scavengers). The order of photochemical efficiency for

H 2 O 2 production with carboxylic acids as electron donors was HCO 2
2
C 2 O 4 CH 3 CO 2 citrate. Isotopic labeling of the electron acceptor,