Angewandte
Chemie
DOI: 10.1002/anie.201406324
Photocatalysis
Tuning the Selectivity of Photocatalytic Synthetic Reactions Using
Modified TiO Nanotubes**
2
Jyotsna Tripathy, Kiyoung Lee, and Patrik Schmuki*
Abstract: Differently modified TiO nanotubes were used to
achieve a drastic change in the selectivity of a photocatalytic
reaction. For the photocatalytic oxidation of toluene, depend-
based photocatalysts the real aggregate size in solution is of
a very critical role. In the present work, to circumvent the
2
aggregate issue, we use self-organized TiO nanotube layers
2
ing on the electronic properties of TiO (anatase, rutile, Ru-
fixed on their substrate as a highly geometry defined photo-
catalyst. Such anodic nanotube layers additionally allow an
easy and straightforward modification of the electronic
properties of the photocatalyst. This is important as the
electronic properties of the semiconductor, namely its band-
edge position and the presence and position of carrier
trapping states relative to the environment directly determine
2
doped), a strong change in the main reaction product (namely
benzoic acid versus benzaldehyde) can be achieved, and
certain undesired reaction pathways can be completely shut
down.
P
hotocatalytic reactions on semiconductive TiO are widely
2
[
3,13,14]
used and investigated for 1) water splitting in view of H2
production, and 2) the destruction of environmental pollu-
the thermodynamic feasibility of reactions.
Addition-
ally, kinetic effects of the catalyst and its selectivity can
strongly be altered by “adding” charge transfer co-catalysts or
by introducing charge carrier recombination or trapping
centers. In the present work, we use nanotube layers and show
that in these systems, modification of structure (rutile vs.
anatase) or even more the incorporation of a doping element
such as ruthenium can have an extreme effect on the
selectivity of a photocatalytic synthesis reaction. As an
example we use the photocatalytic oxidation of toluene to
carbonyl and carboxyl compounds. This toluene photocata-
lytic oxidation is known to yield a wide range of reaction
[1–8]
tants from water and air.
A much less explored application
is the use of the photogenerated electron–hole pairs in redox
or radical based organic synthetic reactions. In comparison
with the large potential of this approach, only a small number
[
9,10]
of successful attempts have been reported.
The main
reason that photocatalytic synthesis is still in its infancy may,
to a large extent, be ascribed to the fact that a multitude of
reaction pathways become accessible when a photoinduced
electron or hole transfer from TiO to an organic species
2
occurs. For example, a wide range of radical species can be
initiated at the TiO2 valence or conduction band, which
generally leads to a high degree of non-selectivity and thus
a wide product distribution. For the majority of photocatalytic
[15–18]
products.
Oxidation approaches of toluene using a range
[
19]
of classical routes, as well as some photocatalytic attempts,
typically lead to a mixture of compounds such as benzyl
alcohol, benzaldehyde, benzoic acid, and carbon dioxide;
additionally, if benzoic acid and benzyl alchohol are simulta-
neously formed they may react to benzyl benzoate (Support-
ing Information, Scheme S1). Usually the target product in
toluene oxidation is either benzaldehyde or benzoic acid;
both are used in the pharmaceutical, agricultural, and
industrial applications.
experiments, commercial TiO nanoparticles such as P25 are
2
used as suspensions, not only for photocatalytic decontami-
nation reactions (pollutant degradation) but also for con-
structive organic synthesis. In the latter case, the main
approaches to enhance the reaction selectivity are based on
optimizing solvents to steer lifetime and speciation of radicals.
Nevertheless, examples where a high selectivity in organic
[
9–11]
synthesis is reached are relatively scarce.
Herein we report the use of TiO2 nanotubes for the
photocatalytic oxidation of toluene, and demonstrate how
specific modification of the tubes electronic properties leads
to a significant change in selectivity (Figure 1).
Ordered Ti-oxide based nanotubes were grown on their
metallic substrates by self-organizing anodization to a length
of 1–2 mm (as shown in Figure 2a) and then crystallized by an
adequate heat treatment. In our work we annealed the
nanotube layers in air to a partially crystalline anatase at
A conceptually entirely different approach to shift the
reaction selectivity is altering the photocatalyst, namely its
electronic properties and its nanoscopic geometry. Geometry
control seems particularly crucial, as recent work by Bahne-
[
12]
mann et al. demonstrated that for practical nanoparticle-
[
*] Dr. J. Tripathy, Dr. K. Lee, Prof. Dr. P. Schmuki
Department of Materials Science and Engineering
University of Erlangen-Nuremberg
3
008C, to fully crystallized anatase at 4508C, and rutile-rich
nanotubes at 6508C and full rutile tubes (see XRD in
Figure 2b; Supporting Information, Figure S1d). Addition-
Martensstrasse 7, 91058 Erlangen (Germany)
E-mail: schmuki@WW.uni-erlengen.de
ally we used Ru-doped TiO nanotubes produced as described
2
[
20]
in previous work (compositional information is also given
in the Supporting Information, Section S1l). To distinguish
effects distinct to Ru from other metal addition, reference
[
**] We would like to express their gratitude to the DFG and the Cluster
of Excellence at the University of Erlangen-Nuremberg (Engineering
of Advanced Materials; EAM) for financial support.
experiments were carried out with TiO -Pt and TiO -Pd tubes
produced (see the Supporting Information).
2
2
Angew. Chem. Int. Ed. 2014, 53, 12605 –12608
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
12605
Tripathy, Jyotsna
