(267e) First Principles Modeling of TiO2 Rutile/Anatase Interfaces

Metal
oxides, such as TiO₂, are often used as
photocatalysts in pollutant removal, water-splitting, or chemical
synthesis.
Multi-component photocatalyst materials can be more photoactive than
single-component catalysts, such as for hydrocarbon
decomposition[1][2]. Mixed-phase
TiO₂ is an important multi-component
photocatalyst, and there are
several explanations for its increased activity. One hypothesis is that
photoexcited
electrons prefer one phase over the other due to a conduction band
offset,
which separates the electrons and decreases the recombination rate of
holes and
electrons. Another possibility is that sites at or near the interface
between
rutile and anatase may serve to trap charge and slow down the
recombination
process[3]. Four-coordinated Ti (Ti_4c) atoms at
the interface have
recently been suggested as key to the catalytic activity[4].

Motivated
to explain the rationale of mixed-phase TiO₂,
we used molecular dynamics simulations and density functional theory
(DFT) to
model interfaces between rutile and anatase. Empirical models with
molecular
dynamics were used to optimize feasible interface structures between
several
different low-index surfaces. These interfaces involve thousands of
atoms, so
modeling at a quantum level is time-consuming, and use of empirical
potentials
saves computational time for the optimization. Several potential
interfacial
structures have been identified, and the structure of the interfacial
region,
which enables the adhesion of the two phases, is elucidated. With
viable interfaces
identified from the molecular dynamics simulations, we modeled the
interfaces
at the DFT and DFT+U level in order to obtain information on the
electronic bands.
We computed the valence and conduction band offsets for an interface
with
approximately 1500 atoms. Several alignment techniques were used, and
all show
that rutile’s conduction band lies higher than anatase’s conduction
band, in
agreement with previous theoretical estimates[5]. We observed the
formation of
Ti_4c atoms at the interface, and these Ti_4c
atoms can trap
electrons within gap states, suggesting an important role of Ti_4c
atoms during photocatalysis. We further modeled nanoscale interfacial
systems
with a high concentration of Ti_4c in order to
assess how increasing
the Ti_4c concentration may affect
photocatalysis. Such systems were
created by placing metastable phases of TiO₂,
with Ti_4c
as building blocks, in contact with rutile and anatase. Our results
provide
important insights on TiO₂ composite
photocatalysts that may guide
experimental synthesis of new composite photocatalyst systems.

References

[1]
T. Ohno, K. Sarukawa, K. Tokieda,
and M. Matsumura. “Morphology of a TiO2 Photocatalyst (Degussa, P-25)
Consisting of Anatase and Rutile Crystalline Phases.” Journal
of Catalysis
203, 2001, 82-86.

[2]
C. Wu, Y. Yue, X. Deng, W. Hua,
and Z. Gao. “Investigation on the synergetic effect between anatase and
rutile
nanoparticles in gas-phase photocatalytic oxidations.” Catalysis
Today
93-95, 2004, 863-869.

[3]
G. Li, and K. Gray. “The solid–solid interface: Explaining the high and
unique
photocatalytic reactivity of TiO2-based nanocomposite materials.” Chemical
Physics 339, 2007, 173-187.
[4]
G. Li, N. M. Dimitrijevic, L.
Chen, J. M. Nichols, T. Rajh, and K. Gray. “The important role of
tetrahedral
Ti⁴⁺ sites in the phase transformation and
photocatalytic activity
of TiO₂ nanocomposites.” Journal of
the American Chemical Society
130, 2008, 5402-3.

[5]
P. Deák, B. Aradi, and T.
Frauenheim. “Band Lineup and Charge Carrier Separation in Mixed
Rutile-Anatase
Systems.” The Journal of Physical Chemistry C 115,
2011, 3443-3446.