This document describes a combinatorial approach to developing thin film materials for photocatalytic water splitting. Researchers created composition spreads using reactive co-sputtering and characterized photocatalytic activity using a three-electrode half cell. They measured current under illuminated and dark conditions to determine photocurrent as a function of film composition and identified promising materials for further optimization. The goal is to discover efficient visible light photocatalysts for hydrogen production through water splitting.

1. Combinatorial method for photocatalytic water
splitting materials development
Bowen Dong, Ian M. Fuller, R. B. van Dover
Department of Materials Science and Engineering
Goal
We are developing a combinatorial
approach to identify thin film materials
that might serve as visible light
phtotocatalysts (photoanode or
photocathode) for hydrogen production
by splitting water. We create composition
spreads using 90º off- axis reactive co-
sputtering followed by ex-situ annealing.
A three-electrode electrochemical half
cell is employed for characterization of
activity as a function of composition.
Background
Photocatalytic splitting of water into
hydrogen and oxygen was discovered on
TiO2 photoanode under UV radiation in
the pioneering work of Fujishima in
1969.[1] Photocatalytic water splitting
represents a potentially effective method
to convert abundant solar energy into
stored chemical energy that can be used
in fuel cells or combustion engines.
If a semiconductor’s conduction band
and valence band edges are located
appropriately with respect to the redox
potential for water reduction and
oxidation half reactions, electrons that
have been photoexcited across the energy
gap will drive the the water splitting
process. (Figure. 2[2] )
TiO2 has a bandgap of 3.2 eV, so only
UV photons will be absorbed. These
represent cover a small portion of solar
energy (Figure. 3). Development of a
reliable and effective photocatalyst
effective over a wider range of the visible
spectrum would be very desirable.
Figure 1. Solar hydrogen alternative energy
Picture source: http://www.ecofriend.com/drawing-from-the-power-of-the-
elements-solar-hydrogen-energy.html
Figure. 2 Mechanism and Band location of photocatalyst for water splitting
Figure. 3 Solar Energy distribution
Graph courtesy of Berkeley Lab Heat Island Group
Method
Characterize in electrochemical half cell
using chopped laser as photon source
Anneal to achieve good crystallinity
RF reactive magnetron cosputter
composition spread sample (Figure 5)
E-beam evaporate Pt and Ti layers
Single material spread
Binary combination
A
B
C
Linear (single and binary) spreads Ternary combinatorial spread
Composition spread25nm Pt
layer for
electric
contact
11 nm
thermally
grown SiO2
insulation
layer
10 nm Ti adhesion
layer
3-inch Si
substrate
Figure. 4 Sketch of a thin-film oxide sample deposited on 3-inch Si wafer
Figure. 5 Composition spread synthesis. Cosputtering guarantees intimate mixing of
the constituents at the atomic scale. The photo of the W-Hf-O composition shows
the location of the magnetron sputter guns and indicates the W-rich and H-rich
regions of the film.
Characterization
The measurement platform allows arbitrary spots on the
wafer to be illuminated with chopped light. The entire
wafer is immersed in an electrolyte solution at neutral
pH. The current measured at each potential (applied
using a potentiostat) is measured as a function of time
using a high-speed A/D converter. Typical time traces are
shown below. The laser-off signal is subtracted from the
laser-on signal to obtain the photocurrent. The maximum
photocurrent (at -0.4 V < E <0.5 V) is then plotted as a
function of position in the false-color plot below.
Equipment
Reference
[1] K. Fujishima, Akira; Honda, “Electrochemical
Photolysis of Water at a Semiconductor Electrode,”
Nature, vol. 238, 1972.
[2] T. Hisatomi and K. Domen, “Recent progress in
photocatalysts for overall water splitting under visible
light,” Proceedings of SPIE, vol. 7408, pp. 740802–
740802–11, 2009.
[3] I. Fuller, “a combinatorial approach to identifying
candidate materials for photocatalytic water
splitting,” May, 2012.
Photocatalysis measurement system with blue laser installed
RF magnetron reactive
Vacuum deposition system for cosputtering[3]
Sputter gun configuration [3]
Future work
1. Extend this work to a wide range of binary and
ternary composition spreads.
2. Systematically determine optical band gap using
reflectometry.
3. Calibrate incident laser power to allow
calculation of absolute photocurrent efficiency.
4. Develop system to measure gas generation in
downselected compositions.
Electrode Area
Z
WO3 @700˚C sample; E=0.5V
Photocurrent analysis for specific point on a particular sample
WO3 @700˚C sample; E=0.3V
WO3 @700˚C sample; E=0.1V
Acknowledgements
This work was performed in part at the Cornell
Nanoscale Facility, a member of the National
Nanotechnology Infrastructure Network, which is
supported by the National Science Foundation (Grant
ECS-0335765).
Figure 6. Time series showing current observed as the incident light is chopped.
Measurements are shown for three values of the electrolyte potential (referenced to
Ag/AgCl), illustrating the increased photocurrent at high potential.
W Hf
Not used in
this
experiment
Hf-rich
W-rich
Figure 7. False color plot showing maximum photocurrent as a function of the
position on the wafer. The sample was a thickness spread of WO3, annealed at 600 ºC.
The thickness increases monotonically from the left (x=0). The measurement system
can address a two dimensional array of points, though only a single line is explored in
this experiment.
Conclusions
1. Demonstrated effectiveness of codeposited thin
films for the identification of superior
photocatalytic materials
2. Successful measurement of photocatalytic
activity as a function of position on a gradient
sample