PNNL-13501 - Pacific Northwest National Laboratory

Development of Novel Photo-Catalysts Using Interfacial Engineering
Yong Liang, John L. Daschbach, Scott A. Chambers, Yali Su, Yong Wang, H. Luo
Study Control Number: PN98023/1269
This project focuses on developing novel photocatalysts with enhanced efficiency and using these photocatalysts to
produce clean fuels with zero carbon emission.
Project Description
The focus of this project is to use a novel approach to
enhance the performance of photocatalysts. The project is
designed to address two issues critical to photocatalyst
development: 1) increasing the efficiency of
photocatalysts, and 2) enabling the use of visible light to
facilitate reactions on catalysts. We are using interfacial
engineering to tailor the catalysts to simultaneously
achieve these properties. We conducted two main tasks
including 1) fabrication and characterization of molecular
beam epitaxial grown Cu2O quantum-dot based
heterostructures, and 2) synthesis and reaction tests of
engineered nano-clusters fabricated through chemical
routes.
Introduction
Photocatalysis has become increasingly important in
chemical processes. It can be used to produce fuel with
zero carbon emission, such as hydrogen production via
photolysis of water. It can also be used to reduce CO2 via
activation and methanation processes. Although the
potential of photocatalysis is tremendous, the use of
photocatalysis in chemical processing has been limited,
due largely to low efficiency and requirement of an
ultraviolet light source. This project proposes to use
interfacial engineering to address these two issues
simultaneously.
Although interfacial engineering has been widely used in
microelectronic applications, use of such a technology for
chemical applications has been limited. Because
interfacial engineering allows one to specifically tailor
material properties, the potential of such technology in
chemical processes is tremendous. One of the limiting
factors that controls the efficiency of photocatalysis is the
fast recombination between photoexcited electrons and
holes. Another major limiting factor is the requirement
for ultraviolet irradiation because most stable
photocatalysts have band gaps in either ultraviolet or near
ultraviolet regimes. For example, TiO2, the most
common photocatalyst, has an energy gap of 3.2 eV (390
mn). As a result, less than 7% of the light in the solar
spectrum can excite electron-hole pairs in TiO2. Efforts
to overcome these limitations include organic dyesensitization
and mixed colloidal systems. With all of
these approaches, the sensitizing moiety is not physically
separated from the heterogeneous reaction interface and
the major limitation in these schemes is the degradation of
the sensitizer through parasitic reactions. By creating thin
films with the desired properties, it is possible to separate
the more reactive sensitizing layer from the surface,
presumably resulting in a significant increase in the
stability and efficiency of the system.
The essence of interfacial engineering is to tailor the
electronic structure using thin film technology. The key
factor that controls separation of photo-induced electronhole
pairs is the relative position of conduction and
valance bands of films with respect to those of the
substrates. Separation of electrons and holes between
substrate and film can be accomplished upon photon
excitation if both the conduction and the valance bands of
the films are either lower (for reduction reactions) or
higher (for oxidation reactions) than those of the
substrates. In both cases, photoexcited electrons and
holes will be separated at the interface via injection of
either electrons or holes in the thin film. Such a scheme
also enables the generation of photo-induced electrons
and holes in smaller band-gap substrates and the injection
of either electrons or holes into larger band-gap thin films,
consequently realizing the use of the visible portion of
sunlight to facilitate processes in catalysts with ultraviolet
band-gaps.
Results and Accomplishments
We demonstrated that synthesis of engineered oxide
materials can be accomplished. These materials have the
appropriate band gaps and offsets for spatial charge
separation and enhanced optical adsorption. We also
Micro/Nano Technology 339