APPLIED PHYSICS LETTERS 97, 063111 共2010兲
Tunnel barrier photoelectrodes for solar water splitting
Lian Guo,1 David Hung,1 Weigang Wang,2 Weifeng Shen,3 Leyi Zhu,2 Chia-Ling Chien,1,2
and Peter C. Searson1,2,a兲
1
Department of Materials Science and Engineering, Johns Hopkins University, Baltimore,
Maryland 21218, USA
2
Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA
3
Department of Physics, Brown University, Providence, Rhode Island 02912, USA
共Received 17 June 2010; accepted 23 July 2010; published online 12 August 2010兲
Tunnel barrier photoelectrodes, with a thin inorganic tunnel barrier layer that isolates the
semiconductor electrode from the electrolyte while allowing current flow across the interface, are a
possible solution to the problem of photocorrosion in solar water splitting. In this approach,
selection of the semiconductor for the light absorber is decoupled from selection of the tunnel
barrier material that provides chemical stability. Here we demonstrate a proof-of-principle of this
approach with TiO2 / MgO tunnel barrier photoelectrodes. © 2010 American Institute of Physics.
关doi:10.1063/1.3479055兴
The development of a practical photoelectrochemical
system that can use sunlight to convert water into hydrogen
and oxygen remains a major challenge.1 Photoelectrochemical water splitting utilizes a semiconductor photoanode to
absorb light and generate electron hole pairs such that the
holes at the surface of the photoanode oxidize water to oxygen and the photogenerated electrons reduce protons to molecular hydrogen at a conventional metal cathode. As a result
of the reactions involved, the photoanode operates under
strongly oxidizing conditions, at highly acidic or basic pH,
and with vigorous gas evolution. Despite the fact that semiconductors with appropriate band gaps for efficient water
splitting are well known, photocorrosion has been a fundamental limitation for technologically viable photoelectrochemical cells. Wide band gap semiconductors have excellent photostability but only harvest a small fraction of the
solar spectrum.2 To overcome this problem, three main strategies have been explored: 共1兲 decreasing the band gap of
wide band gap semiconductors, 共2兲 tandem cells,3 or 共3兲
searching for new materials.4–6
An alternative approach to the photocorrosion problem
is to isolate the light absorbing semiconductor from the aqueous environment by introducing a surface layer. However,
there are several major challenges associated with this approach: the surface layer must allow current flow across the
interface, it must be chemically stable under operating conditions, and it must be defect free to avoid exposing the light
absorbing semiconductor to the aqueous environment. Here
we investigate tunnel barrier photoelectrodes with a thin inorganic tunnel barrier layer that isolates the semiconductor
electrode from the electrolyte. In this way, selection of the
semiconductor is decoupled from selection of the tunnel barrier material that provides chemical stability.
Over the past 30 years there has been remarkable
progress in the fabrication of inorganic tunnel junctions, beginning with the development of Josephson junctions and
more recently with the development of magnetic tunnel
junctions.7,8 As a result of these technological developments,
Al2O3 or MgO tunnel junctions are routinely deposited on
a兲
Author to whom correspondence should be addressed. Electronic mail:
searson@jhu.edu.
0003-6951/2010/97共6兲/063111/3/$30.00
the wafer scale without defects and with layer thicknesses of
0.5–2 nm with a precision of better than 0.5 nm. These advances in processing make it possible to consider efficient
tunnel junction photoelectrodes for solar energy conversion.
The chemical deposition of thin wide band gap oxide layers
has been explored as a method to inhibit the back electron
transfer reaction in dye sensitized solar cells.9,10 However, in
this system it is not necessary to eliminate nonuniformities
and defects since photocorrosion is not a problem.
In this paper, we report a proof-of-principle demonstration of this approach with MgO tunnel barriers on single
crystal TiO2. The rationale for selecting this system is: 共1兲
although TiO2 is a wide band gap semiconductor, its photoelectrochemical properties have been widely studied and
共2兲 the deposition of wafer scale, high-quality MgO tunnel
barriers is routinely used in the fabrication of magnetic tunnel junctions.11 MgO is a good insulator with a band gap of
7.8 eV.12
The MgO films deposited on the single crystal 共100兲
rutile TiO2 wafers were characterized by x-ray photoelectron
spectroscopy 共XPS兲.13 Figure 1 shows the Ti 2p and Mg 2s
spectra for as-deposited TiO2 and TiO2 / MgO tunnel barrier
photoelectrodes with 1 nm or 2 nm MgO. The dramatic decrease in the Ti 2p peak after the deposition of a 1 nm MgO
layer is due to the attenuation of photoelectrons from the
TiO2 substrate. When the MgO thickness is increased to 2
nm, the Ti 2p peak is barely detected. A well-defined Mg 2s
peak is only seen after MgO deposition.
The thickness and uniformity of the tunnel barriers were
determined by angle resolved XPS. The film thickness is
related to the intensity ratio between the film and the substrate 共I f / Isub兲 measured at different take-off angles by:14
冉
ln 1 +
冊
If 1
d
= sec ␪ ,
Isub K
␭
共1兲
where I f and Isub are the intensities of the Mg 2s and Ti 2p
peaks, respectively, K is a factor that takes into account the
different photoelectron cross-sections or atomic sensitivity
factors 共ASF兲, ␪ is the angle between emitted photoelectrons
and the surface normal, ␭ is the inelastic mean free path
共IMFP兲 of the photoelectrons, and d is the MgO tunnel bar-
97, 063111-1
© 2010 American Institute of Physics
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063111-2
Guo et al.
FIG. 1. 共a兲 Ti 2p XPS spectra and 共b兲 Mg 2s spectra for TiO2,
TiO2 / MgO共1 nm兲, and TiO2 / MgO共2 nm兲 at a takeoff angle of 45°.
The dotted line shows the Mg 2s spectra for TiO2 / MgO共1 nm兲 after 1 h
white light illumination in 1 M KOH. 共c兲 Angle-resolved XPS plots for
TiO2 / MgO共1 nm兲 and TiO2 / MgO共2 nm兲 tunnel barriers according to
Eq. 共1兲.
rier thickness. We take ␭ ⬇ 3.1 nm 共Ref. 15兲 for Ti 2p and
Mg 2s photoelectrons in MgO, and K = 0.19 taking ASF
= 2.001 for Ti 共2p兲 and 0.252 for Mg 共2s兲.16 The XPS data
show excellent agreement with Eq. 共1兲 关Fig. 1共c兲兴 and extrapolate to the origin, indicating the good uniformity of the
MgO layers.14 From the slopes of the fits we obtain thicknesses of 1.2 and 2.4 nm in good agreement with the nominal
thickness of 1 nm and 2 nm, respectively.
Figure 2共a兲 shows I-V curves for TiO2 and TiO2 / MgO
tunnel barrier electrodes under white light illumination in
1 M KOH solution.13 The photocurrent onset for TiO2 occurs
at ⫺0.7 V and the photocurrent rapidly reaches a plateau at
about ⫺0.2 V. The TiO2 / MgO photoelectrodes exhibit similar I-V curves although the photocurrent onset shifts to more
positive potentials. The smaller photocurrent for the 2 nm
MgO layer is consistent with a decrease in the probability of
hole penetration across the tunnel barrier. The inset in Fig.
2共a兲 shows oxygen evolution at a TiO2 / MgO 共2 nm兲 tunnel
barrier photoanode. We note that a pH gradient or an external
bias is required to achieve simultaneous hydrogen evolution
at the platinum counter electrode with the tunnel barrier photoelectrodes or bare TiO2.
Figure 2共b兲 shows photocurrent-time transients for TiO2
and TiO2 / MgO tunnel barrier photoelectrodes in a twoelectrode configuration. The photocurrents are approximately
constant suggesting that the MgO barrier layers remain
stable under typical operating conditions for at least one
hour. Dissolution of the MgO results in a gradual increase in
the photocurrent to a value characteristic of bare TiO2. After
1 h illumination, the TiO2 / MgO共1 nm兲 photoelectrode
showed a well-defined Mg 2s spectrum in XPS 关Fig. 1共b兲兴.
The decrease in intensity of the Mg 2s peak is due to the
Appl. Phys. Lett. 97, 063111 共2010兲
FIG. 2. 共a兲 Current-voltage curves for TiO2 and TiO2 / MgO tunnel barrier
photoelectrodes in 1 M KOH under white light illumination
共300 mW cm−2兲. The dashed line is the dark current on bare TiO2. Experiments were performed in a two electrode configuration with a platinum
counter electrode at a scan rate of 50 mV s−1. 共Inset兲 Photograph of a
TiO2 / MgO共2 nm兲 tunnel barrier photoanode in a two electrode cell configuration in 1 M KOH under white light illumination showing oxygen gas
evolution 共iph ⬇ 15 mA cm−2兲. 共b兲 Photocurrent-time transients for TiO2 and
TiO2 / MgO tunnel barrier photoelectrodes under the same condition as 共a兲.
共3x兲 smaller sample area and the slight peak shift toward
larger binding energy is due to a contribution from hydroxide
resulting from immersion in KOH solution.15
Figure 3 shows incident photon-to-electron conversion
efficiency 共IPCE兲 measurements for TiO2 and TiO2 / MgO
tunnel barrier electrodes. These measurements were reproducible over multiple cycles.13 The IPCE for TiO2 photoelectrodes increases at wavelengths less than about 400 nm. Consistent with Fig. 2, the IPCE decreases with increasing tunnel
barrier thickness. The IPCE for the 2 nm MgO tunnel barrier
is about three times smaller than the IPCE for TiO2 at photon
energies close to the band gap but is about an order of mag-
FIG. 3. 共a兲 IPCE vs wavelength for TiO2 and TiO2 / MgO tunnel barrier
photoelectrodes in 1 M KOH solution under open circuit conditions. The
IPCE for TiO2 is attenuated by the high reflectivity of the polished surface
and the indirect band gap of rutile. 共Inset兲 Tauc Plot showing that the band
gaps are 3.03 eV irrespective of the tunnel barrier thickness.
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063111-3
Appl. Phys. Lett. 97, 063111 共2010兲
Guo et al.
nitude smaller at shorter wavelengths. A Tauc plot of the
IPCE data 共inset兲 for TiO2 and the TiO2 / MgO photoelectrodes shows an indirect band gap of 3.03 eV, very close to
the value of 3.02 eV for rutile TiO2 reported in the
literature.17
The ability to isolate a light absorbing semiconductor
from the strongly acidic or basic conditions used for water
splitting, while allowing reasonable levels of charge transport across the interface, is highly challenging. We have fabricated TiO2 / MgO photoelectrodes and show that the MgO
tunnel barrier is stable in strong base, at least for the time
scale of the experiments. Although the photocurrent is attenuated by the presence of the tunnel barrier, the incorporation of an oxygen evolution catalyst should improve the performance. These proof-of-principle results for a model
system indicate that tunnel barrier photoelectrodes with light
absorbers well matched to the solar spectrum may be a route
to efficient water splitting.
This work was supported by NSF 共Grant No. CHE0905869兲 and the JHU MRSEC 共NSF under Grant No.
DMR05–20491兲. The authors thank Shane Ardo for assistance with optical characterization.
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