islands. Water and acetic acid were deaerated by repeating a freeze-and-thaw cycle
for a gas-phase reaction. The vapor of water or acetic acid was introduced into the
cell from a gas handling system equipped with a Baratron pressure gauge. A 1:1
mixture of gas-phase water and acetic acid was prepared by mixing their vapor at the
same pressure in a glass reservoir. The light source was a high-pressure Hg lamp
(Ushio UIV-570) that was filtered through a band-pass filter (Toshiba UV-D33S,
240–400 nm) and a water filter (10 cm long) to remove heat.
Primary intermediates of oxygen photoevolution (water photooxidation) reaction
in the interface of TiO 2 (rutile) and aqueous solution were investigated by in situ
multiple internal reflection infrared (MIRIR) absorption and photoluminescence
(PL) measurements [19]. UV irradiation of TiO 2 in the presence of 10 mM Fe
3+ in
the solution caused the appearance of a new peak at 838 cm
À1 and a shoulder at
812 cm
À1 . Detailed investigations of the effects of solution pH, the presence of
methanol as a hole scavenger, and isotope exchange in water between H 2
16 O and H 2
18 O on the spectra have shown that the 838 and 812 cm
À1 bands can be assigned to
the O–O stretching mode of surface TiOOH and TiOOTi, respectively, produced as
primary intermediates of the oxygen photoevolution reaction. The results give strong
support to our previously proposed mechanism that the oxygen photoevolution is
initiated by a nucleophilic attack of a H 2 O molecule on a photogenerated hole at a
surface lattice O site, not by oxidation of surface OH group by the hole. The
conclusion is supported by PL measurements. A plausible reaction scheme is
proposed for the oxygen photoevolution on TiO 2 (rutile) in aqueous solutions of
pH less than about 12.
Semiconductor electrodes capable of using solar photons to drive water-splitting
reactions have been the subject of tremendous interest over recent decades. The
surface has been found to play a significant role in determining the efficiency of
water oxidation. However, previous works have only allowed hypotheses to be
formulated regarding the identity of relevant surface species. The first observation
of a surface intermediate of oxygen evolution at an Ir oxide multi-electron catalyst
was reported by Sivasankar et al. [20], where a surface hydroperoxide intermediate
has been detected upon oxidation of water at an Ir oxide nanocluster catalyst system
under pulsed excitation of a [Ru(bpy) 3 ]
2+ visible light sensitizer by recording of the
O–O vibrational mode at 830 cm
À1 . Rapid-scan FTIR spectroscopy of colloidal
H 2 O, D 2 O, and D 2
18 O solutions in the attenuated total reflection mode allowed
spectral assignment of IrOOH on the basis of an observed D shift of 30 cm
À1 and
18
O shifts of 24 cm
À1 (
16 O
18 O) and 46 cm
À1 (
18 O
18 O) (Fig. 2.10). The laser pulse
response of the infrared band is consistent with the kinetic relevancy of the
intermediate.
For Co 3 O 4 , two surface intermediates of visible light-sensitized water oxidation
were detected by rapid-scan FTIR spectroscopy. The agreement of the
18 O isotopic
composition of a surface superoxide species and the final O 2 gas product provides
evidence for the kinetic competency of the three-electron oxidation intermediate. At
a fast catalytic site absorbing at 1013 cm
À1 , the superoxide intermediate grows and
O 2 evolves within a 300 ms photolysis pulse. By contrast, a slow site marked by a Co
(IV) ¼ O group does not advance beyond the one-electron intermediate absorbing at
2.2 Infrared Spectroscopy
29
for a gas-phase reaction. The vapor of water or acetic acid was introduced into the
cell from a gas handling system equipped with a Baratron pressure gauge. A 1:1
mixture of gas-phase water and acetic acid was prepared by mixing their vapor at the
same pressure in a glass reservoir. The light source was a high-pressure Hg lamp
(Ushio UIV-570) that was filtered through a band-pass filter (Toshiba UV-D33S,
240–400 nm) and a water filter (10 cm long) to remove heat.
Primary intermediates of oxygen photoevolution (water photooxidation) reaction
in the interface of TiO 2 (rutile) and aqueous solution were investigated by in situ
multiple internal reflection infrared (MIRIR) absorption and photoluminescence
(PL) measurements [19]. UV irradiation of TiO 2 in the presence of 10 mM Fe
3+ in
the solution caused the appearance of a new peak at 838 cm
À1 and a shoulder at
812 cm
À1 . Detailed investigations of the effects of solution pH, the presence of
methanol as a hole scavenger, and isotope exchange in water between H 2
16 O and H 2
18 O on the spectra have shown that the 838 and 812 cm
À1 bands can be assigned to
the O–O stretching mode of surface TiOOH and TiOOTi, respectively, produced as
primary intermediates of the oxygen photoevolution reaction. The results give strong
support to our previously proposed mechanism that the oxygen photoevolution is
initiated by a nucleophilic attack of a H 2 O molecule on a photogenerated hole at a
surface lattice O site, not by oxidation of surface OH group by the hole. The
conclusion is supported by PL measurements. A plausible reaction scheme is
proposed for the oxygen photoevolution on TiO 2 (rutile) in aqueous solutions of
pH less than about 12.
Semiconductor electrodes capable of using solar photons to drive water-splitting
reactions have been the subject of tremendous interest over recent decades. The
surface has been found to play a significant role in determining the efficiency of
water oxidation. However, previous works have only allowed hypotheses to be
formulated regarding the identity of relevant surface species. The first observation
of a surface intermediate of oxygen evolution at an Ir oxide multi-electron catalyst
was reported by Sivasankar et al. [20], where a surface hydroperoxide intermediate
has been detected upon oxidation of water at an Ir oxide nanocluster catalyst system
under pulsed excitation of a [Ru(bpy) 3 ]
2+ visible light sensitizer by recording of the
O–O vibrational mode at 830 cm
À1 . Rapid-scan FTIR spectroscopy of colloidal
H 2 O, D 2 O, and D 2
18 O solutions in the attenuated total reflection mode allowed
spectral assignment of IrOOH on the basis of an observed D shift of 30 cm
À1 and
18
O shifts of 24 cm
À1 (
16 O
18 O) and 46 cm
À1 (
18 O
18 O) (Fig. 2.10). The laser pulse
response of the infrared band is consistent with the kinetic relevancy of the
intermediate.
For Co 3 O 4 , two surface intermediates of visible light-sensitized water oxidation
were detected by rapid-scan FTIR spectroscopy. The agreement of the
18 O isotopic
composition of a surface superoxide species and the final O 2 gas product provides
evidence for the kinetic competency of the three-electron oxidation intermediate. At
a fast catalytic site absorbing at 1013 cm
À1 , the superoxide intermediate grows and
O 2 evolves within a 300 ms photolysis pulse. By contrast, a slow site marked by a Co
(IV) ¼ O group does not advance beyond the one-electron intermediate absorbing at
2.2 Infrared Spectroscopy
29