its proximity to the spectral cutoff region and is thus strongly affected by any traces
of H 2 O incorporated in the cell.
In the absence of temporal resolution, the kinetic relevancy or position of these
species in the photocatalytic cycle could not be addressed. Yet, knowledge of
individual events in the four-electron cycle of water oxidation on the catalyst surface
through observation of structurally identified species and their temporal behavior is
required to uncover kinetic bottlenecks and to provide insights regarding design
changes to overcome them. Isotope-labeling experiments using
18 O-labeled and
16 Olabeled water were used for the peak assignments to distinguish Fe ¼ O and Fe–O–
OH groups with overlapped vibration frequency in the range of 750–900 cm
À1 and
740–950 cm
À1 , respectively (Fig. 2.12). The
16 O/
18 O shift for Fe ¼ O is expected to
be $40 cm
À1 . The frequency shift associated with O–O isotope exchange is between
20 and 30 cm
À1 for partially labeled (
16 O–
18 O) and between 44 and 61 cm
À1 for
fully labeled (
18 O–
18 O) groups. Operando spectra collected in contact with D 2
18
O
exhibited absorption peaks at 743 and 857 cm
À1 . The peak at 857 cm
À1 in D 2
18 O is
shifted by 42 cm
À1 compared with the spectrum taken in D 2
16
O. This shift is
consistent with either Fe ¼
16 O to Fe ¼
18 O or Fe–
16 O–
16 O to Fe–
18 O–
18 O, both
of which are possible intermediate species of water oxidation on hematite. Further
study performed in a 1:1 ratio of D 2
16
O/D 2
18 O produced an equally weighted
doublet with peaks at 898 and 857 cm
À1 , which are thus assigned as Fe ¼
16 O and
Fe ¼
18 O, respectively, since the peroxide is supposed to produce three different
vibrations that correspond to
16 O–
16 O,
16 O–
18 O, and
18 O–
18 O stretching modes with
a 1:2:1 ratio of intensities [22].
Fig. 2.12 Plot of the effect
of oxygen isotopic variation
on the infrared absorption
spectra during PEC water
oxidation with hematite
electrodes. The spectra were
measured at an applied
potential of 1.63 V versus
RHE under illumination in
contact with D 2
16
O, D 2
18 O,
or a 1:1 ratio of D 2
16
O/D 2
18
O. (Reprinted from Ref.
[22], with kind permission
from Springer Science
+Business Media)
32
2 In Situ Characterization of Photocatalytic Activity
of H 2 O incorporated in the cell.
In the absence of temporal resolution, the kinetic relevancy or position of these
species in the photocatalytic cycle could not be addressed. Yet, knowledge of
individual events in the four-electron cycle of water oxidation on the catalyst surface
through observation of structurally identified species and their temporal behavior is
required to uncover kinetic bottlenecks and to provide insights regarding design
changes to overcome them. Isotope-labeling experiments using
18 O-labeled and
16 Olabeled water were used for the peak assignments to distinguish Fe ¼ O and Fe–O–
OH groups with overlapped vibration frequency in the range of 750–900 cm
À1 and
740–950 cm
À1 , respectively (Fig. 2.12). The
16 O/
18 O shift for Fe ¼ O is expected to
be $40 cm
À1 . The frequency shift associated with O–O isotope exchange is between
20 and 30 cm
À1 for partially labeled (
16 O–
18 O) and between 44 and 61 cm
À1 for
fully labeled (
18 O–
18 O) groups. Operando spectra collected in contact with D 2
18
O
exhibited absorption peaks at 743 and 857 cm
À1 . The peak at 857 cm
À1 in D 2
18 O is
shifted by 42 cm
À1 compared with the spectrum taken in D 2
16
O. This shift is
consistent with either Fe ¼
16 O to Fe ¼
18 O or Fe–
16 O–
16 O to Fe–
18 O–
18 O, both
of which are possible intermediate species of water oxidation on hematite. Further
study performed in a 1:1 ratio of D 2
16
O/D 2
18 O produced an equally weighted
doublet with peaks at 898 and 857 cm
À1 , which are thus assigned as Fe ¼
16 O and
Fe ¼
18 O, respectively, since the peroxide is supposed to produce three different
vibrations that correspond to
16 O–
16 O,
16 O–
18 O, and
18 O–
18 O stretching modes with
a 1:2:1 ratio of intensities [22].
Fig. 2.12 Plot of the effect
of oxygen isotopic variation
on the infrared absorption
spectra during PEC water
oxidation with hematite
electrodes. The spectra were
measured at an applied
potential of 1.63 V versus
RHE under illumination in
contact with D 2
16
O, D 2
18 O,
or a 1:1 ratio of D 2
16
O/D 2
18
O. (Reprinted from Ref.
[22], with kind permission
from Springer Science
+Business Media)
32
2 In Situ Characterization of Photocatalytic Activity