Hydrogen generation by solar water splitting using p-InGaN
photoelectrochemical cells
K. Aryal, B. N. Pantha, J. Li, J. Y. Lin, and H. X. Jiang
a兲
Department of Electrical and Computer Engineering, Texas Tech University, Lubbock, Texas 79409, USA
共Received 18 August 2009; accepted 11 January 2010; published online 2 February 2010兲
Photoelectrochemical effects in p-In
x
Ga
1−x
N 共0ⱕxⱕ0.22兲 alloys have been investigated. Hydrogen
generation was observed in p-InGaN semiconducting electrodes under white light illumination with
additional bias. It was found that p-InGaN alloys possess much higher conversion efficiencies than
p-GaN. Time dependent photocurrent density characteristics showed that the stability of p-InGaN in
aqueous HBr is excellent. The photocurrent density was found to increase almost linearly with hole
mobility and excitation light intensity. © 2010 American Institute of Physics.
关doi:10.1063/1.3304786兴
The generation of hydrogen gas by splitting water has
attracted tremendous research work in recent years, with
the hope of fulfilling demands for environmental friendly
energy.
1
Among the various potential technologies for effi-
cient and nontoxic hydrogen production, photoelectrochemi-
cal 共PEC兲 technology is recognized as one of the most
promising. The currently known photocatalytic materials,
however, are either too inefficient in sunlight due to large
band gaps, or too unstable in aqueous solutions for practical
implementation.
2–6
For example GaInP and GaAsPN, have
shown promising efficiency but suffer from poor stability.
3
Currently, the favored material for the photoanode in a PEC
is TiO
2
due to its high corrosion resistance.
6
However, TiO
2
has an energy band gap of about 3.2 eV and can only be
activated by light energy equal to or greater than 3.2 eV.
Such an energy range is present in less than 3% of the solar
spectrum. TiO
2
is thus intrinsically inefficient. Maximum so-
lar absorption can be attained by minimizing the semicon-
ductor band gap.
7
However, if the band gap becomes too
small, the cell will not generate enough potential to drive the
water splitting reaction. In order to split water in a PEC cell,
the conduction band-edge potential of a semiconductor elec-
trode must be lower than that of the hydrogen-evolving half-
reaction and its valence band-edge potential must be higher
than that of the oxygen-evolving half reaction.
8,9
In
x
Ga
1−x
N is a very promising candidate for solar water
splitting because of its direct band gap, which can be tuned
to cover the entire solar spectrum through band gap engi-
neering. This system not only has the appropriate band gap
energy for water splitting, but also has high corrosion resis-
tance in aqueous solutions.
10–14
Between n-and p-type semi-
conductors, if the band-edge potentials are the same, they
should have similar capabilities for water splitting. However,
they could have different stabilities in an electrolyte solution.
In an n-type semiconductor, photogenerated holes acting
as strong oxidizing agents can oxidize the semiconductor
itself. In p-type semiconductors, the surface exhibits electron
accumulation under irradiation, so using p-type semiconduc-
tor materials as a working electrode offers self-protection
against photocorrosion caused by semiconductor oxidization.
In addition, a p-type semiconductor material has characteris-
tics of hydrogen production at the surface, in contrast to
oxygen production in n-type semiconductors.
15
When
p-In
x
Ga
1−x
N electrode is excited by light irradiation, photo-
excited electrons move toward the p-In
x
Ga
1−x
N/electrolyte
interface and react with H
+
to generate H
2
by a reduction
reaction; 2H
+
+2e
−
→ H
2
.
16
Semiconductors are more resis-
tant to reduction than oxidation reactions, making p-type ma-
terials more stable than n-type materials. Thus, p-type semi-
conductors are preferred over n-type semiconductors as
photocatalytic materials.
17,18
However, there has not been
any work done regarding the use of p-InGaN as the working
electrode in PEC cells. This is because p-type InGaN is no-
toriously hard to make. Recently, our group has succeeded in
producing p-In
x
Ga
1−x
N by metal organic chemical vapor
deposition 共MOCVD兲 for x up to 0.35.
19
In this letter, we report on the studies of PEC effects in
p-In
x
Ga
1−x
N alloys and the observation of hydrogen genera-
tion by solar water splitting using p-In
x
Ga
1−x
N alloys as
working electrodes in a PEC cell. p-In
x
Ga
1−x
N epilayers of
about 0.25
m in thickness were epitaxially deposited
on semi-insulating c-GaN/AlN/sapphire templates using
MOCVD. Trimethylgallium, trimethylindium, and bicyclo-
pentadienyal were used as the precursors for Ga, In, and Mg,
respectively. For an active nitrogen source, high purity am-
monia gas was used. The In content in p-In
x
Ga
1−x
N alloys
was increased by reducing the growth temperature. Ohmic
contacts on p-In
x
Ga
1−x
N working electrodes were prepared
by e-beam evaporation of Ni 共30 nm兲/Au 共120 nm兲 with
subsequent rapid thermal annealing at 550 ° C for 90 s in air.
Hall effect measurement results for the set of samples used in
this study are summarized in Table I. These results showed
that the Mg-doped In
x
Ga
1−x
N epilayers are p-type. The metal
contact was protected by using clear epoxy resin to avoid
direct contact with the electrolyte solution.
a兲
Electronic mail: hx.jiang@ttu.edu.
TABLE I. Electrical properties of p-In
x
Ga
1−x
N alloys employed in this
study.
Samples
Mobility
共cm
2
/ Vs兲
Hole Concentration
共cm
−3
兲
Resistivity
共⍀ cm兲
GaN 15 2.0⫻10
17
2.1
In
0.05
Ga
0.95
N 13 3.0⫻10
17
1.6
In
0.15
Ga
0.85
N 2 2.4⫻10
18
1.3
In
0.22
Ga
0.78
N 3 5.0⫻10
18
0.4
APPLIED PHYSICS LETTERS 96, 052110 共2010兲
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The PEC cell consists of a working electrode
共p-In
x
Ga
1−x
N兲, a counter electrode and a reference electrode.
The counter and reference electrodes were made of platinum
共Pt兲 and Ag/AgCl/NaCl 共sodium-chloride-saturated silver-
chloride electrode兲. The light source was a standard AM1.5
solar simulator. 1 mol/L of hydrobromic acid 共HBr兲 solution
was used as the electrolyte. A Keithley source meter was
used to apply bias voltage between the working and counter
electrodes 共V
CE
兲. The photocurrent was recorded using an
electrometer. H
2
gas generation was visible when V
CE
ex-
ceeded 0.7 V and bubbles accumulated on the surface of the
p-In
x
Ga
1−x
N electrodes.
Figure 1 shows the photocurrent density 共J
ph
兲 as a func-
tion of V
CE
under white light irradiation using an AM1.5
solar simulator. The measured J
ph
values are much higher in
p-InGaN than in p-GaN. However, an apparent dependence
of J
ph
on the In-content is not seen here. This is most likely
due to the fact that the material quality of p-In
x
Ga
1−x
N domi-
nates over all other factors so that the advantage of the lower
band gap is not noticeable in present p-InGaN materials. This
speculation is corroborated by the results shown in Fig. 2,
where a clear correlation between J
ph
and hole mobility 共
h
兲
is presented. It was found that J
ph
increases almost linearly
with
h
, which is reasonable because higher hole mobility
would help the photogenerated holes to move faster in semi-
conductor electrodes, which would result in higher photocur-
rents.
The stability of p-In
x
Ga
1−x
N working electrodes in an
HBr solution was tested by recording J
ph
for a prolonged
period of time 共24 h兲. Figure 3 shows J
ph
as a function of
light irradiation time 共t兲. The results show that the stability of
p-In
x
Ga
1−x
N in an HBr solution is excellent. It was observed
that J
ph
dropped quickly in the first few seconds and became
completely stable after about 10 min. Further, we did not
observe any etching effects occurring on the surface of the
p-In
x
Ga
1−x
N working electrodes. The reason for the excellent
stability of p-In
x
Ga
1−x
N in an electrolytic solution is that
p-type conductivity provides a reduction reaction and pre-
vents the photocorrosion of p-In
x
Ga
1−x
N electrodes.
The dependence of J
ph
on light intensity was measured.
Figure 4 shows J
ph
共at V
CE
=1.2 V兲 for p-In
0.05
Ga
0.95
Nasa
function of light intensity. Maximum J
ph
was observed at
FIG. 2. 共Color online兲 Photocurrent densities 共J
ph
兲 as a function of hole
mobility of p-In
x
Ga
1−x
N electrodes at V
CE
=1.2 V. The light intensity at the
sample surface was about 132 mW/ cm
2
.
FIG. 3. 共Color online兲 Photocurrent densities 共J
ph
兲 of p-In
x
Ga
1−x
N electrodes
at V
CE
=1.2 V as a function of the measurement time 共t兲. The light intensity
at the sample surface was about 132 mW/ cm
2
.
FIG. 4. 共Color online兲 Photocurrent densities 共J
ph
兲 of p-In
0.05
Ga
0.95
Nasa
function of light intensity at V
CE
=1.2 V.
FIG. 1. 共Color online兲 Photocurrent densities 共J
ph
兲 as a function of V
CE
, the
voltage applied between working and counter electrodes under white light
illumination using a standard AM1.5 solar simulator. The light intensity at
the sample surface was about 132 mW/ cm
2
. Open symbols indicate gen-
eration of H
2
gas, while solid symbols indicate no H
2
gas generation.
052110-2 Aryal et al. Appl. Phys. Lett. 96, 052110 共2010兲
Downloaded 12 Jul 2010 to 129.118.86.45. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
intensity 132 mW/ cm
2
. Further increase in light intensity is
limited by our light source. Higher light intensity would gen-
erate more electron-hole pairs, resulting in a higher photo-
current. The linear dependence implies that it is possible to
adopt concentrator solar cell concept for PEC to further en-
hance conversion efficiency.
In summary, PEC effects in p-In
x
Ga
1−x
N alloys have
been investigated. p-type In
x
Ga
1−x
N alloys exhibit much
higher conversion efficiency compared to p-GaN. Continu-
ous hydrogen bubbles evolved from the surface of p-InGaN
samples when the bias voltage exceeded 0.7 V. The time
dependent photocurrent density measurement showed that no
morphological degradation of the surface of p-In
x
Ga
1−x
N
electrodes was visible and the stability of p-In
x
Ga
1−x
N elec-
trodes in an aqueous solution of HBr is excellent. Further
enhancement of conversion efficiency is anticipated upon
further improvements in the material quality of p-InGaN.
This work is supported by NSF 共under Grant No. DRM-
0906879兲. H.X.J. and J.Y.L. would like to acknowledge the
support of Whitacare endowed chair positions through the
AT&T Foundation.
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