Recent
work has shown that heterojunction photoelectrodes can achieve
synergistically higher water splitting activity than their parent
materials. To optimize the performance in such layered systems, it
is necessary to develop new methods capable of assessing heterojunction
phase space. Herein, we explore WO3/TiO2 heterojunction
phase space as a model system. Using chemical vapor deposition, 71
unique photoanodes were grown (15 single-layer; 56 heterojunctions).
The materials were physically characterized using X-ray diffraction,
Raman spectroscopy, scanning electron microscopy, energy-dispersive
X-ray spectroscopy analysis, and ultraviolet–visible transmission
spectroscopy. Various key performance indicators were measured. Within
this WO3/TiO2 heterojunction phase space, the
onset potentials ranged from ∼0.45 to ∼0.81 V
RHE; the incident-photon-to-current efficiencies
at 350, 375, and 400 nm ranged from ∼0.6 to ∼50.9, ∼0.1
to ∼30.0, and ∼0 to ∼15.6%, respectively; and
the theoretical solar photocurrents ranged from ∼0.01 to ∼0.94
mA cm–2. Contour plots allowed us to identify regions
of heterojunction phase space with high activity and establish trends.
We identified an electronic barrier to charge transfer between the
heterojunction layers that required a sufficiently high applied potential
(≥1.0 V
RHE) to be surpassed for
synergetic improvements in activity to be observed. We recommend that
the methods developed herein, for assessing the performance of sample
libraries of heterojunction photoelectrodes, be used alongside combinatorial
synthesis methods and high-throughput photoelectrochemical measurements
to optimize promising heterojunction systems more rigorously and rapidly.