Page 83 - Multifunctional Photocatalytic Materials for Energy

72 Multifunctional Photocatalytic Materials for Energy

4.6 Concluding remarks

Overall, metal oxides are earth-abundant and physically and chemically stable, but
their photocatalytic applications are limited because of their poor visible light absorp-
tion capability due to their wide band gap. So improving the visible light absorption
for metal oxide semiconductors has priority in this field (increasing η abs ). At the same
time, charge carrier behaviors (increasing η sep )—that is, charge separation, migration,
and recombination—are of equal importance in achieving high solar energy conver-
sion efficiency, which always competes with light absorption η abs . Engineering the
electronic energy band and geometric structures is the key to balancing and decou-
pling η abs and η sep and to achieving an acceptable value for η abs ×η sep .
Doping with alien atoms has long been employed for wide band gap metal oxides
in order to enable visible light absorption, but the introduced shallow and deep energy
levels act as recombination centers leading to decreased η sep . It is worth noting that
doping-induced band gap narrowing without introducing internal band energy levels is
of particular interest. However, this is a very unique case, and no other doping exam-
ples have been found so far. More insight and understanding of the origin and cause of
this band gap narrowing is needed. Fortunately, through an alloying or solid solution
technique, multiple metal-cation oxides (≥2) can be treated as intrinsic narrow band
gap semiconductors to improve light absorption η abs , while apparently not decreasing
η sep . Tuning of the light absorption range also is possible by varying the electronic
energy structure in the valence band with different electronic configurations.
In a photosensitizer system, traditional sensitizers like dyes and QDs still face the is-
sue of photochemical stability in an aqueous electrolyte. By contrast, plasmonic nano-
structures are more flexible and can take on this responsibility in the future. Plasmonic
nanoparticles cover the whole sunlight spectrum and transfer the absorbed solar energy
to an adjacent semiconductor through variable pathways, i.e., photonic enhancement,
hot electron injection, and PIRET. More important, thermodynamic band alignment
and intimate contact between a semiconductor and plasmonic nanostructures are no
longer required as in traditional photosensitizer/semiconductor systems, which opens
up new directions and offers more designing opportunities to improve the light absorp-
tion of metal oxides. The fact is that plasmonic energy transfer efficiency remains very
low in most cases; therefore a more fundamental understanding of the energy barriers
that compete with the plasmonic energy transfer process is in urgent demand.
Finally, the multijunction system is highly desirable for unbiased solar fuel gen-
eration. Metal oxide photoanode materials have been studied extensively and used
either in half cells or in a PV-PEC cell for water oxidation. Stable p-type metal oxide
photocathodes are still in high demand. A typical p-Cu 2 O photocathode is stable for a
very short term, even with a multiple film-protecting layer, for example, AZO-TiO 2 .
Long-term stability is still problematic. More affordable techniques for depositing
protection layers are also required. Cu-based ternary oxides are potential candidates
as photocathodes for H 2 evolution or CO 2 reduction, not only because of their im-
proved tuning flexibility on the light absorption, but more importantly because photo-
stability is highly possible owing to the new d/s energy levels in the conduction band