Titania photocatalysis through two-photon band-gap excitation with built-in rhodium redox mediator

Article abstract of DOI:10.1039/c4cc07049f

Titania particles modified with an extremely small amount (<0.01 mol%) of a rhodium species exhibited photocatalytic activity for the oxidative decomposition of acetaldehyde in air under visible-light irradiation. The reaction proceeded via a two-photon band-gap excitation mechanism resembling the Z-scheme with an external redox couple, using a built-in Rh(iii)-Rh(iv) redox couple.

Full text of DOI:10.1039/c4cc07049f

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ARTICLE TYPE
Titania photocatalysis through two-photon band-gap excitation with
built-in rhodium redox mediator
a
b
Joanna Kuncewicz* and Bunsho Ohtani
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
10-14
Titania particles modified with extremely small amount
< 0.01mol%) of rhodium species exhibited photocatalytic 40 photoexcitation from Rh(III) to the CB of titania or SrTiO
strontium titanate (SrTiO
3
).
It has been claimed that
leaves
Rh(IV) which oxidizes compounds adsorbed on the surface.
(
3
activity for oxidative decomposition of acetaldehyde in air
under visible-light irradiation through two-photon band-gap
excitation mechanism, resembling so-called Z-scheme with an
external redox couple, using a built-in Rh(III)/Rh(IV) redox
couple.
Thus, though the CB electrons can be used in a reduction step
same as that in the case of UV band-gap excitation, oxidation is
induced by localized species, Rh(IV), but not by VB positive
holes. Here we report the visible light-induced photocatalysis by
titania modified with extremely small amount of Rh, in which VB
positive holes, as well as CB electrons, participate through two-
photon band-gap excitation.
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It has been often stated in papers of studies on visible light-active
photocatalysts that titanium(IV) oxide (titania) has many
advantages as a photocatalyst, e.g., chemical stability both in the
dark and under photoirradiation, sufficient redox ability in the
excited state even inducing water splitting, non-toxicity, and high
availability, though titania can be excited only by ultraviolet light
In present work, Rh-modified titania (Rh-TiO
prepared by impregnation of commercial titania (Showa Denko
Ceramics FP6, anatase) with rhodium(III) chloride (RhCl ; Wako
Pure Chemical; 0.001–0.1mol%) followed by calcination in air at
23 K for 3 h to result in rutile-based materials as confirmed by
X-ray diffractometry (SI). A reference sample (0% Rh-TiO
rutile) was prepared by the similar calcination process without
adding RhCl . Representative photoabsorption spectra are shown
2
) samples were
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;
(UV). To overcome this sole disadvantage of titania, there are
two strategies. One is to find a photocatalyst absorbing visible
light keeping the advantages of titania, but such a "superman"
metal-oxide photocatalyst has not yet been discoveredin spite of
at least 40-years attempts by many researchers. The other
strategy is to modify titania to give visible-light absorption, by
3
in Fig. 1. In addition to the absorption at < 420 nm by rutile
titania, two shoulders at around 450 nm and 620 nm attributable
to Rh(III) and Rh(IV), respectively,
modification. The latter was evident for the sample with
relatively large amount of Rh (> 0.01%), i.e., loaded Rh was
predominantly in the valency of Rh(III).
Photocatalytic activity of samples was evaluated by the rate of
2
carbon dioxide (CO ) liberation by visible-light photoirradiation
(> 440 nm) of a sample (50 mg; spread in a 1 cm -area sample
holder) in a reaction chamber (357 mL) containing air and 224
ppm of acetaldehyde (AcH). As Fig. 2 shows, the activity higher
7, 15
appeared by Rh-
2
3
4
e.g., doping with nitrogen, sulfur or carbon or loading metal
5
oxide clusters to put filled and vacant electronic states,
respectively, in the band gap of titania. However, one of the
titania advantages, sufficient redox ability, may be lost, since, in
the former and the latter cases, positive holes in the valence band
2
(
VB) and photoexcited electrons in the conduction band (CB) of
titania, which induce redox reactions when titania is excited by
ultraviolet light, cannot be used, respectively. Therefore, a
remaining strategy to develop visible light-active titania is to
introduce a mechanism of two-photon band-gap excitation.
Rhodium (Rh)-doping, modifying with small amount of Rh
species without changing the original crystal structure, has been
2
than that of TiO was observed for the sample modified with
6-9
already employed to develop visible light-active titania or
a
Faculty of Chemistry, Jagiellonian University, ul. R. Ingardena 3, 30-
0
11 Kraków, Poland, Fax: +48 126340515; Tel: +48 126632005;
E-mail: kuncewic@chemia.uj.edu.pl
b
Catalysis Research Center, Hokkaido University, Sapporo 001-0021,
Japan, Fax: +81 11 706 9133; Tel: +81 11 706 9132; E-mail:
ohtani@cat.hokudai.ac.jp
†Electronic Supplementary Information (ESI) available: Preparation
procedure and experimental details, example of the time-course curve of
photocatalytic test reaction, dependency of photocatalytic reaction rate
on Rh-concentration determined for the materials calcined at 973 K. See
DOI: 10.1039/b000000x/
Fig. 1. Diffuse reflectance spectra of Rh-TiO
without rhodium species.
2
and a reference sample
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Fig. 2
2
Rates of CO liberation in first two hours of the process of
acetaldehyde oxidative decomposition in the presence of materials
with various Rh concentration (black line – CO
presence of bare titania).
2
generation rate in the
extremely small amount of Rh (<0.01%). Although the origin of
visible-light activity of TiO (non-modified) may be very small
2
undetectable absorption due to surface states and is still
ambiguous, the loading-amount depending activity suggests that
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the TiO
Rh species of relatively high content hindering intrinsic
photoabsorption of titania. A possible reason for lower
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-originated activity might be diminished by overlaying of
photocatalytic activity of samples with relatively large amount of
Rh might be also higher content of Rh(IV), which was negligible
for extremely low Rh-loaded samples as seen in Fig. 1, which
may act as an electron trap to enhance electron–hole
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Fig. 3
2
Differential absorption spectra of 0.01% Rh-TiO (with
reference to the spectrum of the sample kept in a dark) under
photoirradiation with light of various wavelength ranges (upper) in
air, (middle) in air or under argon, and (lower) in the presence of
AcH in air.
7, 12
recombination.
Thus, extremely small amount of Rh gives
titania visible-light activity. The optimal Rh concentration
should be around 0.0005–0.001%, what was confirmed also by
studies on photocatalytic activity of less active samples calcined
at slightly higher temperature (973 K) (SI). Characterization of
the sample with Rh in such low concentration was rather difficult
due to small intensity of photoabsorption of Rh species, therefore,
a 0.01% sample was used for the following experiments. It is
suggested that active Rh species were small and attached on the
surface because repeated rinsing of the samples with water caused
appreciable, not complete, decrease in the photoabsorption but
almost complete loss of photocatalytic activity under visible-light
irradiation. Considering extremely small amount, i.e., ~0.008–
original concentration, since transition of Rh(III) to Rh(IV)
cannot be induced by this longer-wavelength irradiation.
Photoinduced regeneration of Rh(III) under the irradiation with
light from the range 590–730 nm, seems to be slower than the
reversed process. In-situ photoabsorption measurements shown
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in Fig.
3 (lower) confirmed light wavelength-controlled
reversibility of this redox processes during the photocatalytic
reactions.
The photoinduced absorption changes depended strongly on
the presence/absence of electron acceptors and donors in the
system (Fig. 3). In the absence of oxygen, photoinduced
generation of Rh(IV) was not observed or even original Rh(IV)-
concentration decreased upon irradiation, indicating that the
shorter-wavelength light does not work, i.e. electrons excited
from Rh(III) to the titania CB must be consumed by oxygen
adsorbed on the surface. On the other hand, the presence of AcH
enhanced photoinduced regeneration of Rh(III); higher and lower
concentration of Rh(III) and Rh(IV), respectively, were observed
in all the cases of wavelength ranges. These facts suggested that
efficiency of photoinduced reduction of Rh(IV) may be improved
by enhancement of consumption of the VB positive holes by AcH.
It was also observed that the Rh(IV)/Rh(III) ratio was decreased
slowly in the dark in the presence of AcH, indicating that AcH
reduces Rh(IV) slowly.
2
2
0.08 Rh atoms per 1 nm of titania of specific surface area 10 m
-
1
g , isolated Rh ions might be dispersed on the titania surface in
the form of oxide clusters or even be embedded in titania crystal
lattice near to the surface, which is supported by the observed
strong interaction of these species with titania.
It should be noted that photoabsorption of the active samples
changed during the photoirradiation and the behavior depended
on the photoirradiation-wavelength range. Upon irradiation with
light of the wavelength range 440–550 nm, overlapping the
absorption band assigned to Rh(III), decrease in the band
intensity along with the increase in the intensity of band assigned
to Rh(IV) could be observed, while subsequent irradiation with
full wavelength range light (440–730 nm) caused slight increase
and decrease in the intensity of the 450-nm band and the 620-nm
band, respectively, i.e., the longer wavelength part of the
irradiation (550–730 nm) recovered Rh(III) from once produced
Rh(IV) (Fig. 3, upper). Irradiation with light of the longer
wavelength ranges (590–730 nm) recovered Rh(III) to the
Thus, it was clarified that the photocatalysis by Rh-TiO
2
contains at least two independent photoexcitation processes;
excitations from Rh(III) to the titania CB by 440–550-nm light
and from the titania VB to Rh(IV) by irradiation of wavelengths
2
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from 590 to ca. 730 nm. In order to confirm the synergy of these
two photoreactions, the following experiments were performed
SI), as shown in Fig. 4, in which full output (440–730 nm) of a
(
xenon arc was divided by wavelength ranges into two parts,
440–550 nm and 590–730 nm (provided by two light sources
assuring similar intensities of both radiations (SI)), corresponding
to photoabsorption by Rh(III) and Rh(IV), respectively.
Negligible photocatalytic activity under the irradiation of only the
longer-wavelength region was attributed to lower concentration
of Rh(IV) to be excited, which could not be generated under such
irradiation conditions, and/or accumulation of electron in Rh(III)
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without transferring to oxygen. However, the rate of CO
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Fig. 4
Amounts of CO
2
generated in the process of AcH
liberation when irradiated in both wavelength ranges was higher
than that under irradiation with light of the shorter-wavelengths,
2
oxidative decomposition in the presence of 0.01% Rh-TiO under
irradiation with various light ranges ([440–550 nm and 590–730
nm] - black squares, [440–550 nm] - blue triangles, [590–730 nm]
- red circles).
i.e., synergetic effect was observed for 0.01% Rh-TiO
such synergetic effect of full range irradiation was seen for 0%
Rh-TiO . Analysis of the absorption spectra of 0.01% Rh-TiO
2
, while no
2
2
recorded during the full-range irradiation in the presence of AcH
showed negligible detectable change in the Rh(III)/Rh(IV) ratio,
while shorter-wavelength irradiation caused accumulation of
Rh(IV) due to slow reduction by AcH without photoinduced
reduction, which might induce the slow deactivation of the
sample in such conditions (Fig. 3, lower). Slightly higher
enhancement of the reaction upon full-range irradiation observed
at the beginning of the reaction, might be explained by the
recapturing of active Rh(III) through reduction of Rh(IV)
intrinsically present in the sample.
On the basis of above-mentioned results and discussion,
mechanism of the present photocatalytic reaction system with Rh-
2
TiO can be schematically shown in Fig. 5. Photons of relatively
higher energy (440–590 nm) excite electrons in Rh(III) into the
titania CB to leave Rh(IV) while photons of lower energy (590-
Fig. 5
Schematic representation of mechanism of
photocatalytic reaction with Rh-TiO (D - electron donor).
2
730 nm) excite electrons in the titania VB to Rh(IV) to leave
Rh(IV) and Ti(IV). Study on this line is now in progress.
The present mechanism of "two-photon band-gap excitation"
of titania is, within the authors' knowledge, sole possible solution
for overcoming the disadvantage of a titania photocatalyst, not
excited by visible light, without losing the advantages and gives a
new insight on visible light-induced photocatalysis by metal-
oxide particles.
This work was supported by Ministry of Science and Higher
Education in Poland within Iuventus Plus grant (No.:IP
012030572).
positive holes in the VB and to recover Rh(III). As a result, CB
electrons and VB positive holes are generated by two visible-light
photons, i.e., two-photon band-gap excitation proceeds using Rh
species, a built-in redox mediator. Such mechanism is supported
by theoretical calculations which showed that Rh-doping of rutile
titania should induce intermediate bands within titania band
6
0
6
5
15-17
gap.
This photocatalysis has, at least, three advantages. First,
the two different excitation processes occur in each titania
particle because of the built-in redox mediator, which is
contrastive to so-called Z-scheme photocatalysis using two kinds
2
18-21
of photocatalyst particles.
of titania is kept by the ensuant band-gap excitation. Third, two
excitations require different range of light wavelength which are 70 1. A. Fujishima, X. Zhang and D. A. Tryk, Surf. Sci. Rep., 2008, 63,
Second, the sufficient redox ability
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