Page 170 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Chapter
5

Reactive Oxygen

Species Generation on

Nanoparticulate Material

Michael Hoffmann California Institute of Technology, California
Ernest M. Hotze Duke University, Durham, North Carolina
Mark R. Wiesner Duke University, Durham, North Carolina

Background
Photoactive nanomaterials can be grouped roughly into two different
classes: metal oxide or metal chalocogenide photocatalysts such as tita-
nium dioxide or cadmium sulfide, respectively; and materials that can
be photosensitized such as chromophores and certain types of fullerenes.
Semiconductor photochemistry behavior is governed by the band
gap that can be described as the energy difference between the valence
band (fermi level highest energy electrons) and the conduction band
(lowest energy unoccupied molecular orbitals). Light absorbed at wave-
) of a semi-
length less than or equal to the band-gap energy ( bg
conductor will result in the promotion of an electron to the conduction
band, and as a consequence, a hole or vacancy in the HOMO state of the
molecular orbitals is created. Both the promoted electrons and the holes
can migrate to the surface of the semiconductor and subsequently react
in aqueous solution to form reactive oxygen species such as superoxide
.
(O2 ) and hydroxyl radical ( OH).
Photosensitizers are sensitized by light, and their electrons are excited
within the molecular orbitals. Sensitized electrons can behave accord-
ing to two mechanisms: type I electron transfer involving a donor
molecule and type II energy transfer involving no reaction. Both of these

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