Particle Size and Surface Chemistry
J. Phys. Chem. B, Vol. 101, No. 14, 1997 2507
isopropanol as a hole scavanger and CO2 as an electron
acceptor.20,36 Henglein et al. found pinacol after illumination
of ZnS (11 mol % Zn2+ excess).20 On the other hand, Inoue et
al. studied reactions at ZnS colloids with different excess of
Zn2+ (25, 50, and 200 mol %) but did not observe any pinacol
formation.36 They only found acetate, formate, and hydrogen.
These differences may be due to slight differences in the
preparation of the ZnS colloids, which seem to be extremely
critical.37
German Minister for Education and Research (BMBF) under
Contract 0329580 and by the Volkswagen Foundation under
Contract I/71 902 is gratefully acknowledged.
References and Notes
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6. Conclusions
(7) Brus, L. In Nanotechnology; Timp, G., Ed.; AIP monography; in
The very different generation rates of electron/hole pairs in
nanometer and particles lead to different reaction mechanisms
for the light induced oxidation of ethanol. To our best
knowledge it is the first time that this phenomenon has been
studied systematically on semiconductor particles in the na-
nometer to micrometer size regime. It is concluded that on ZnS
particles radicals are formed in a primary photoinduced oxida-
tion step via a one hole process. The secondary reactions of
these intermediates now depends strongly on the chemical
composition of the particle surface as well as on the availability
of a second hole. In the case of defect free nanometer ZnS
particles it takes more than 50 ms before the next photon is
absorbed by the same particle and even a few seconds before
the next successful charge transfer occurs at this particle. Here
therefore the radicals have plenty of time to diffuse into the
electrolyte where disproportionation and recombination of
radicals can take place. However, in the case of a semiconductor
with surface states (anion vacancies) the radicals can inject
electrons into these surface states, which are located energeti-
cally in the band gap of the semiconductor.
On micrometer ZnS the formation of long-lived radicals plays
a much less important role and the oxidation of ethanol can
proceed here via a two hole process without forming dimer-
ization products. We cannot exclude that the very porous
surface of the self-synthesized micrometer ZnS particles also
supports the two hole process, because the initially produced
short-lived radicals may here be trapped within nanopores
leading to a second oxidation step. In order to investigate this,
particles with different porosities, including such with a smooth
surface, should be studied.
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In order to compare reactions on nanometer and micrometer
particles it was found necessary to control precisely the reaction
conditions for the differently sized particles including their
surface chemistry. Both surface chemistry and size strongly
control the formation of products on the particles. The surface
to volume ratio of small particles is by orders of magnitude
larger than for large particles and therefore any surface effects
will control the reactions on these particles, which play a less
important role in the case of large particles.
Acknowledgment. We thank Dr. Renate Hiesgen for sup-
(37) Mu¨ller, B. R. Ph.D. Thesis, University of Hamburg, 1993.
porting SEM and STM investigations. Financial support by the