Multicomponent structures are often needed when a special property needs to be optimized.
High-temperature superconductors — in which the oxide with the current best transition
temperature contains four metals (HgBa 2 CaCu 2 O6+δ, Tc = 125 K) — are a good example. A
similar number of components may be necessary for water photoelectrolysis because the
electrode must have structural stability, light absorption over much of the solar spectrum, and
catalytic activity for the multielectron reactions required to produce useful fuels. When three or
more components are needed, the number of possible combinations can be very large; therefore,
a high-throughput technique capable of screening many compositions at once is required,
because serial synthesis would not be fruitful on a reasonable timescale.
In the many years of research on light-induced water-splitting, researchers have not made use of
the significant advances offered by first-principles electronic structure theory for screening novel
semiconductors. Theoretical approaches have been developed to search for systems that satisfy
well-posed target electronic properties; these include prediction of band gap and band energy
positions, all based on alloy composition. Synergy between experiments and theory would
provide predictive power to reduce the search space and enable researchers to better understand
the properties of discovered phases that make them effective photoelectrolysis systems.
Configuration of Discovered Electrodes for Optimal Light Absorption, Carrier
Collection, and Electrocatalysis
Mesoscopic Electrode Configurations. The band gaps of many of the stable oxides that have
been used as photoelectrodes are so large that they do not absorb a large fraction of the solar
spectrum. To address this shortcoming, researchers have made photoelectrodes by adding
colored transition metal centers to high-band-gap oxides to absorb light in the visible region of
the solar spectrum, or they have tried lower-band-gap oxides such as Fe 2 O 3. The optical
transitions between the bands formed by the transition metal centers in these materials are
forbidden d-d transitions, resulting in low absorption coefficients in the visible region and
leading to a rather high penetration depth of the light into the material. The large penetration
depth of the light, and the fact that the carrier mobilities in oxides are generally lower than in
conventional solar cell semiconductors, results in recombination of photogenerated carriers
before they reach the semiconductor/electrolyte interface where the photoelectrolysis reactions
occur.
Similar problems of low light absorption and carrier mobilities are overcome in the
nanocrystalline TiO 2 photovoltaic cell. The light absorption is improved by creating a high-
surface-area porous TiO 2 electrode covered with a light-absorbing dye. The light traverses many
interfaces, so that when absorption occurs, the carriers are always near the interface; hence,
carrier diffusion lengths are no longer a limiting factor. Nanostructured films, either with ordered
domains (such as nanorods or nanowires) or with random domains of interpenetrating networks
(also sometimes denoted as bulk heterojunctions), offer the opportunity to circumvent this key
limitation. Similar approaches using either nanoporous films of active materials or particles of
photon-absorbing and/or catalytic materials supported on a nanocrystalline scaffold are expected
to yield high efficiencies for photoelectrolysis. Because no large crystals or crystallites are
required, very low costs can be anticipated. In addition, these configurations lower the local
current density for the electrolysis reactions, reducing overpotential losses. Back illumination