11 | Summary
A set up for in situ electrochemical ATR-IR spectroscopy was developed and applied to
study the ORR on semiconductor electrodes, i.e. Ge and ZnO. Furthermore, the surface
transformation from OH- to H-termination was investigated on Ge(100) and Ge(111) surfaces.
In the ORR studies, surface-bound superoxide (GeOO• ) was detected as the main ORR
intermediate on the Ge(100) surface in 0.1 M HClO4 . It is also detected as intermediate
species during ORR at alkaline pH 10.5 in 0.1 M NaClO4 and NaCl electrolytes on the
Ge(100) surface. However, the ORR mechanism was found to be dierent in acidic and
alkaline electrolytes.
In aqueous HClO4 , a catalytic cycle was proposed, where the ORR proceeds via a stepwise
mechanism, transferring one electron at a time. The main reaction steps are a surface radical,
a Ge-bound superoxide and a Ge-bound peroxide. A surface radical species, generated by
the initial reduction from the Ge surface during the surface transformation of the OH- to
H-terminated surface or in the HER, reacts with O2 and generates a Ge-bound superoxide,
which is subsequently reduced to the Ge-bound peroxide. The actual initial reduction step
happens not at the molecule to be activated, but at the surface and is followed by a chemical
reduction of oxygen. Hence, the ORR mechanism on the Ge(100) surface was found to be
coupled with the HER and the surface transformation under acidic conditions.
Because of the coupling of the ORR with the surface transformation, the latter has been
studied in detail on both Ge(100) and Ge(111) in 0.1 M HClO4 . At negative potentials, the Ge
surface was found to consist of a mixture of GeH2 and H-Ge-Ge-H. The surface transformation
of the Ge surfaces was found to involve a complex mechanism with several intermediates and
surface species involved.
Under alkaline conditions, the surface transformation of Ge is shifted to more negative
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Chapter 11: Summary
potentials by ∼ 0.06 V per pH decade. The onset of ORR is found to shift substantially
less and therefore occurs on the OH-terminated surface in alkaline solution. Hence, ORR at
alkaline pH is neither coupled to the surface transformation, nor to the HER, as in case of
HClO4 . Dierence in the vibrational frequencies of the Ge-bound superoxide mode between
the two electrolytes (ClO4 and Cl ) indicates a direct interaction of the counter-ions with
the ORR intermediates. The main dierence observed in alkaline pH , is the role of Ge-bound
peroxide as compared to acidic pH . In acidic pH , the rate of formation of peroxide exceeds
the rate of reduction close to the onset potential of ORR and the absorbance decreases at
more negative potentials. On the other hand, under alkaline conditions, the peroxide band is
more prominent and H2 O2 was detected as a nal product of ORR unlike for acidic pH .
The TDM of the superoxide and peroxide intermediates is clearly oriented upright under
acidic conditions. The upright orientation of the surface-bound superoxide (GeOO• ) shows
that the oxygen is neither bridging between dierent Ge atoms nor bound to a single Ge atom
in a side-on position. This orientation was found to closely resemble the Pauling model ( [2]
p. 27). Under alkaline conditions, the orientations of the TDM were found to be more tilted
compared to acidic pH . This behaviour is attributed to the dierent surface termination
under acidic and alkaline pH s.
In addition to the studies on Ge surfaces, ZnO thin lms were deposited on a Ge(100)
ATR crystal by electrochemically triggered deposition. Surface-bound superoxide has also
been detected as an ORR intermediate on the ZnO surface. The frequency was shifted to
lower wavenumbers, indicating a dierent interaction with the surface compared to Ge.
As superoxide has been detected as the main ORR intermediate in this work, an understanding of the superoxide decomposition reactions is required. Hence, a non-electrochemical
decomposition of superoxide was studied under dierent humidity conditions. The decomposition of superoxide is found to undergo dierent reaction pathways forming dierent nal
products under dierent relative humidities.
The results add experimental evidence to the understanding on a molecular level of the
complex ORR mechanism. The work also opens the interesting perspective to investigate the
kinetics of dierent steps of a complex reaction as the ORR by the combination of electrochemical and spectroscopic experiments.
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