of 5
Stabilization of Si microwire arrays for solar-driven
H
2
O oxidation to O
2
(g) in 1.0 M KOH(aq) using
conformal coatings of amorphous TiO
2
Matthew R. Shaner,
ab
Shu Hu,
ab
Ke Sun
ab
and Nathan S. Lewis
*
abcd
Conductive, amorphous TiO
2
coatings deposited by atomic-layer
deposition, in combination with a sputter deposited NiCrO
x
oxygen-
evolution catalyst, have been used to protect Si microwire arrays from
passivation or corrosion in contact with aqueous electrolytes. Coated
np
+
-Si/TiO
2
/NiCrO
x
as well as heterojunction n-Si/TiO
2
/NiCrO
x
Si
microwire-array photoanodes exhibited stable photoelectrochemical
operation in aqueous ferri-/ferro-cyanide solutions. The coatings also
allowed for photoanodic water oxidation in 1.0 M KOH(aq) solutions
for >2200 h of continuous operation under simulated 1 Sun conditions
with

100% Faradaic e
ffi
ciency for the evolution of O
2
(g).
Technologically important, small band-gap semiconductors
such as Si are highly attractive materials for use as photoanodes
to oxidize water, but are unstable to corrosion and/or passiv-
ation under anodic conditions in aqueous electrolytes.
1
Single
crystalline n-Si, n-GaAs, n-GaP, n-CdTe, and n-BiVO
4
photo-
anodes have all recently demonstrated enhanced stability (4
100+ hours) under continuous operation for water oxidation to
O
2
(g) in aqueous alkaline electrolytes, with 100% Faradaic
e
ffi
ciency, by use of electrically conductive, optically trans-
parent, 10
100 nm thick protective
lms of amorphous TiO
2
deposited by atomic-layer deposition (ALD).
2
4
Arrays of semi-
conductor microwires or nanowires provide an especially
attractive system architecture for the direct production of fuels
from sunlight, because such a structure provides a minimal
path for ionic conduction, high optical absorption,
5
9
a high
surface-area support for electrocatalyst loading,
10
and other
distinctive, advantageous operational features.
11
The
application of conformal protective
lms to such highly aniso-
tropic structures by use of sputtering or evaporation is expected
to be di
ffi
cult, whereas the self-limiting ALD surface conden-
sation reaction technique is a conformal coating process.
Accordingly, we describe herein the use of ALD-deposited
amorphous TiO
2
lms to enable the continuous oxidation of
water to O
2
(g) by Si microwire array photoanodes for >2200 h in
1.0 M KOH(aq) under simulated 1 Sun illumination conditions.
To fabricate the structures of interest, arrays of n-type and
np
+
-radial junction Si microwires were coated with ALD-grown
TiO
2
,
4
followed by deposition of a nickel
chromium oxide
oxygen-evolution catalyst using magnetron sputtering (see the
ESI
for full Experimental details). Fig. 1a shows a schematic of
the process, and Fig. 1b and c show scanning-electron micro-
graphs (SEM) of the Si microwire arrays before and a
er depo-
sition of the TiO
2
protective coating (2000 ALD cycles,

94 nm)
and the NiCrO
x
catalyst layer (20 min sputtering,

40 nm planar
equivalent), respectively. Fig. 1d shows a cross-section near the
base of a single fully processed (np
+
-Si/TiO
2
/NiCrO
x
) microwire
within an array, demonstrating that the fabrication produced
the desired structure as well as a conformal layer of TiO
2
having
a
Division of Chemistry and Chemical Engineering, California Institute of Technology,
Pasadena, CA 91125, USA. E-mail: nslewis@caltech.edu
b
Joint Center for Arti
cial Photosynthesis, California Institute of Technology,
Pasadena, CA 91125, USA
c
Beckman Institute Molecular Materials Research Center, California Institute of
Technology, Pasadena, CA 91125, USA
d
Kavli Nanoscience Institute, California Institute of Technology, Pasadena, CA 91125,
USA
Electronic supplementary information (ESI) available: Experimental methods
and supplementary data. See DOI: 10.1039/c4ee03012e
Cite this:
Energy Environ. Sci.
,2015,
8
,
203
Received 22nd September 2014
Accepted 29th October 2014
DOI: 10.1039/c4ee03012e
www.rsc.org/ees
Broader context
Photoelectrochemical conversion of sunlight into hydrogen has the ability
to provide continuous, carbon-free energy, but in order to be an
economically competitive energy source the system must be both e
ffi
cient
and stable for years of operation. This combination proves challenging as
the most e
ffi
cient semiconductors for solar-energy conversion are
unstable a
er only seconds to minutes of operation, especially under
oxidizing conditions. Accordingly, improving the stability while main-
taining e
ffi
cient operation is an important goal in furthering this tech-
nology and is the subject herein as demonstrated by a two-step approach.
First, increasing the electrochemically active
versus
geometrical surface
area with a periodically structured Si microwire array decreases the
e
ff
ective current density at the solid/solution interface and thus the rate of
corrosion. Secondly, a conformal, transparent, electrically conductive
protection layer serves as a corrosion-resistant barrier between the semi-
conductor and solution while maintaining e
ffi
cient charge transfer to the
reaction site.
This journal is © The Royal Society of Chemistry 2015
Energy Environ. Sci.
,2015,
8
,203
207 |
203
Energy &
Environmental
Science
COMMUNICATION
Published on 05 November 2014. Downloaded on 15/01/2015 15:34:51.
View Article Online
View Journal
| View Issue
a relatively uniform thickness along the height of the wire. A
detailed inspection of an individual Si microwire indicated that
the NiCrO
x
deposited at the top and base of each Si microwire,
due to the relatively line-of-sight deposition pro
le of the
magnetron sputtering process. Only the regions of the NiCrO
x
catalyst on the surfaces of the wires are expected to be electro-
catalytically active, due to the electrically insulating SiO
2
on the
sample substrate.
Fig. 2 shows the current density
vs.
potential (
J
E
) behavior of
an np
+
-Si/TiO
2
/NiCrO
x
microwire-array photoelectrode in
contact with (a) 1.0 M KOH(aq) (pH
¼
13.6) and (b) 0.50 M
K
2
SO
4
0.050 M K
3
Fe(CN)
6
0.35 M K
4
Fe(CN)
6
(aq) in the pres-
ence and absence, respectively, of 100 mW cm

2
of simulated
Air Mass (AM) 1.5G illumination. Fig. 2a also depicts the
J
E
behavior of a p
+
-Si/TiO
2
/NiCrO
x
microwire-array electrode in the
absence of illumination, to allow for a comparison of the onset
potentials for the oxygen-evolution reaction between the illu-
minated photoanode and a degenerately doped unilluminated
p
+
-Si anode. The one-electron, outer-sphere, reversible
Fe(CN)
6
3

/4

redox couple was used to measure the intrinsic
energy-conversion properties of the microwire-array photo-
anodes. Under 100 mW cm

2
of simulated Air Mass (AM) 1.5G
illumination, the np
+
-Si/TiO
2
/NiCrO
x
microwire array produced
an open-circuit potential (
E
oc
)of

0.62 V
vs.
the formal potential
for water oxidation,
E
0
0
(O
2
/OH

), and a light-limited photocur-
rent density (
J
ph
) of 7.1 mA cm

2
in 1.0 M KOH(aq), and
produced
E
oc
¼
0.44 V
vs.
the Nernstian potential of the
solution (
E
(Fe(CN)
6
3

/4

)) and
J
ph
¼
7.3 mA cm

2
in contact
with Fe(CN)
3

/4

(aq). A diode quality factor of 1.9
2.2 was
measured in contact with Fe(CN)
3

/4

(aq) from the
J
E
data
obtained as a function of illumination intensity.
5,12
The intrinsic
photoelectrode behavior observed for the np
+
-Si/TiO
2
/NiCrO
x
photoanode in contact with the one-electron, reversible,
Fe(CN)
6
3

/4

redox system (Fig. 2b) demonstrated an energy-
conversion e
ffi
ciency of 1.8% with a
ll factor of 0.54.
For comparison, Fig. 3 shows the
J
E
behavior in the pres-
ence and absence of illumination, respectively, of n-Si/TiO
2
/
NiCrO
x
microwire-array photoelectrodes in (a) 1.0 M KOH and
(b) an aqueous Fe(CN)
6
3

/4

solution. In contact with 1.0 M
KOH(aq), these samples exhibited
E
oc
¼
0.49 V
vs. E
0
0
(O
2
/OH

)
and
J
ph
¼
3.7 mA cm

2
(Fig. 3a), whereas in contact with
Fe(CN)
6
3

/4

(aq) the n-Si/TiO
2
/NiCrO
x
microwire arrays exhibi-
ted
E
oc
¼
0.18 V
vs. E
(Fe(CN)
6
3

/4

) and
J
ph
¼
3.1 mA cm

2
.
The intrinsic photoelectrode behavior observed for the n-Si/
TiO
2
/NiCrO
x
photoanode in contact with the Fe(CN)
6
3

/4

redox
system (Fig. 2b) demonstrated an energy-conversion e
ffi
ciency
of 0.3% with a
ll factor of 0.51. When measured under nomi-
nally identical conditions, all (6 electrodes) of the microwire-
array photoelectrodes studied herein showed mutually similar
photovoltages, but exhibited variable photocurrent densities
with a spread of

5mAcm

2
, consistent with a variation in wire
height at various locations on the wafer from which the
Fig. 1
(a) Schematic of a structure that consists of an np
+
-Si micro-
wire-array conformally coated with a protective, transparent and hole-
conducting TiO
2
layer, with the TiO
2
layer subsequently coated with a
NiCrO
x
oxygen-evolution catalyst. (b) Scanning-electron micrograph
(SEM) images of an np
+
-Si microwire-array prior to further processing.
(c) SEM image of a fully processed microwire array. (d) SEM cross-
section near the base of a single microwire, showing the conformality
of the TiO
2
coating with a thickness of 94 nm.
Fig. 2
Current density
versus
potential performance of np
+
-Si/TiO
2
/NiCrO
x
microwire-array photoelectrodes in contact with (a) 1.0 M KOH(aq)
and (b) Fe(CN)
6
3

/4

(aq). (a) The solid blue curve is under 1-Sun simulated illumination, the solid black curve is under no illumination, dark, and
the dashed black curve is the performance of a p
+
-Si/TiO
2
/NiCrO
x
microwire-array electrode. The potential is plotted versus the potential of the
reversible hydrogen electrode (RHE). The illumination intensity in (b) was adjusted using a series of 0.3 optical density neutral-density
fi
lters, with
the illumination intensity labeled on the plot.
204
|
Energy Environ. Sci.
,2015,
8
,203
207
This journal is © The Royal Society of Chemistry 2015
Energy & Environmental Science
Communication
Published on 05 November 2014. Downloaded on 15/01/2015 15:34:51.
View Article Online
electrodes were fabricated.
13,14
The addition of scattering parti-
cles can signi
cantly increase
J
ph
by increasing the illumination
path length through the photoactive Si microwire array.
15
Fig. 4a shows the external quantum yield (
F
ext
) for np
+
-Si/
TiO
2
/NiCrO
x
and n-Si/TiO
2
/NiCrO
x
microwire-array photo-
electrodes in contact with 1.0 M KOH(aq). The observed
behavior was similar to that obtained previously for microwire-
array photocathodes and photovoltaics illuminated at normal
incidence.
15
The
F
ext
exhibited a similar dependence on wave-
length for both the np
+
-Si and the n-Si microwire arrays,
consistent with behavior dominated by absorption and charge-
carrier collection in a microwire array. Integration of the
wavelength-dependent spectral response data of protected np
+
-
Si and n-Si microwire-arrays with respect to the AM 1.5G solar
spectrum yielded calculated photocurrent densities of 7.9 mA
cm

2
and 2.9 mA cm

2
, respectively, in excellent agreement
with the
J
ph
values measured from the
J
E
behavior.
Fig. 5a shows the time dependence of the photocurrent
density of an np
+
-Si microwire array under potentiostatic
control at 0.36 V
vs. E
0
0
(O
2
/OH

) (0.70 V
vs.
a Hg/HgO reference
electrode). Fig. 5b shows the
J
E
data at 10 h intervals
throughout the stability test, showing no signi
cant change in
J
ph
,
E
oc
or
ll factor during the stability evaluation. As shown in
Fig. 5c, a comparison between the amount of O
2
(g) expected
based on Coulomb's law and the amount of O
2
(g) detected using
a calibrated
uorescent O
2
(g) probe indicated

100% Faradaic
e
ffi
ciency for oxygen evolution. Assuming four electrons per Si
atom dissolved, the total number of coulombs of charged
passed during the stability test, 6000 C, exceeded by a factor of
20 the 300 C of charge that would have been required to dissolve
the entire Si microwire array. Furthermore, assuming that 10
nm of oxide formation would result in complete passivation of
the electrode, the amount of charge passed establishes a lower
limit of 9

10
4
on the branching ratio for water oxidation to
O
2
(g) relative to oxidation of the Si.
Accounting for the

20% capacity factor of sunlight, the
2200 h of continuous operation contained the same amount of
charge as would be passed during >1 year of outdoor opera-
tion. Because lower current densities away from peak illumi-
nation times of day would likely increase the stability, this
projected >1 year stability plausibly represents a lower limit on
the actual stability of the NiO
x
-coated Si photoanodes under
operational conditions. A detai
led failure analysis study and
validated accelerated testing protocols, additionally incorpo-
rating possible e
ff
ects of temperature cycling and extended
periods of no photocurrent cur
rent due to day/night cycling,
would clearly be required to establish the ultimate limit on the
stability of the photoanodes described herein. The high
internal surface area of a highly anisotropic structure such as a
microwire array produces a correspondingly low current
density at the areas exposed to the electrolyte. This low current
densityisexpectedtobene
cially reduce the rate of light-
intensity-dependent photocorrosion or photopassivation
processes, because the photon
ux per projected geometric
area provided by sunlight produces minority-carrier currents
that are distributed over a lar
ge internal surface area of the
solid/liquid contact in the in
ternal volume of a microwire
array. Consistently, the microw
ire arrays exhibited a greater
degree of stability than crystalline Si electrodes protected by
amorphous TiO
2
lms and operated at 30 mA cm

2
,which
exhibited a small but signi
cant decay in photocurrent a
er 24
h of continuous operation in the same electrolyte.
4
Fig. 3
Current density
versus
potential data for n-Si/TiO
2
/NiCrO
x
microwire-array photoelectrodes in contact with (a) 1.0 M KOH(aq) and (b)
Fe(CN)
6
3

/4

(aq). (a) The solid blue curve is under 1 Sun simulated illumination, the solid black curve is in the absence of illumination, dark, and
the dashed black curve is the performance of a p
+
-Si/TiO
2
/NiCrO
x
microwire-array electrode. (b) The illumination intensity was adjusted using a
series of 0.3 optical density neutral-density
fi
lters, with the resulting illumination intensity labeled on the plot.
Fig. 4
External quantum yield (
F
ext
)
versus
wavelength plot of
representative protected n-Si and np
+
-Si microwire-array photo-
electrodes in contact with 1.0 M KOH(aq).
This journal is © The Royal Society of Chemistry 2015
Energy Environ. Sci.
,2015,
8
,203
207 |
205
Communication
Energy & Environmental Science
Published on 05 November 2014. Downloaded on 15/01/2015 15:34:51.
View Article Online
Incorporation of the np
+
-Si/TiO
2
/NiCrO
x
photoanode into a
complete water-splitting device operating at 10% solar-to-
hydrogen e
ffi
ciency would require a photocathode capable of
producing >10 mA cm

2
at 1.5 V and addition of scattering
particles to increase the absorption and current density in the
Si microwires.
15
For e
ffi
cient operation, such a photocathode,
operating near its Shockley
Quiesser limit, would need to have
abandgapof

2.0eVwitheitheraburiedjunctionorproper
band positioning relative to the hydrogen-evolution potential.
The observations reported herein therefore illustrate an
additional advantage of the amorphous TiO
2
-based protection
strategy in that the deposition method, ALD, is especially well
suited to be compatible with a wide range of high-e
ffi
ciency
materials while also being compatible with a broad range of
morphologies associated with hi
ghly anisotropic structures of
the light absorber.
Acknowledgements
The authors would like to acknowledge Dr Ragip Pala for
assistance with the spectral response measurement system.
This material is based upon work performed by the Joint
Center for Arti
cial Photosynthesis, a DOE Energy Innovation
Hub, supported through the O
ffi
ce of Science of the U.S.
Department of Energy under Award Number DE-SC0004993.
M. R. S. is supported by a graduate fellowship from the
Resnick Institute for Sustainability. The authors also
acknowledge support from the Gordon and Betty Moore
Foundation.
Notes and references
1 J. R. McKone, N. S. Lewis and H. B. Gray,
Chem. Mater.
, 2014,
26
, 407
414.
2 M. F. Lichterman, A. I. Carim, M. T. McDowell, S. Hu,
H. B. Gray, B. S. Brunschwig and N. S. Lewis,
Energy
Environ. Sci.
, 2014,
7
, 3334
3337.
3 M. T. McDowell, M. F. Lichterman, J. M. Spurgeon, S. Hu,
I. D. Sharp, B. S. Brunschwig and N. S. Lewis,
J. Phys.
Chem. C
, 2014,
118
, 19618
19624.
4 S. Hu, M. R. Shaner, J. A. Beardslee, M. Lichterman,
B. S. Brunschwig and N. S. Lewis,
Science
, 2014,
344
, 1005
1009.
5 S. Hu, C.-Y. Chi, K. T. Fountaine, M. Yao, H. A. Atwater,
P. D. Dapkus, N. S. Lewis and C. Zhou,
Energy Environ. Sci.
,
2013,
6
, 1879.
Fig. 5
(a) Current density
versus
time for an n-p
+
-Si/TiO
2
/NiCrO
x
microwire-array photoelectrode under 1 Sun simulated illumination in 1.0 M
KOH(aq) under potential control at 0.36 V
vs. E
0
0
(OH

/O
2
). (b) Current density
versus
potential behavior of cyclic voltammograms taken at 10 h
intervals throughout the duration of the stability test. (c) Oxygen production as a function of time in 1.0 M KOH(aq) while under potential control
at 0.36 V
vs. E
0
0
(OH

/O
2
). The Faradaic e
ffi
ciency for oxygen evolution,

100%, was determined by comparing the observed oxygen produced
relative to the amount expected based on the total current passed in conjunction with the use of Faraday's law.
206
|
Energy Environ. Sci.
,2015,
8
,203
207
This journal is © The Royal Society of Chemistry 2015
Energy & Environmental Science
Communication
Published on 05 November 2014. Downloaded on 15/01/2015 15:34:51.
View Article Online
6 A. R. Madaria, M. Yao, C. Chi, N. Huang, C. Lin, R. Li,
M. L. Povinelli, P. D. Dapkus and C. Zhou,
Nano Lett.
,
2012,
12
, 2839
2845.
7 J. Zhu, Z. Yu, G. F. Burkhard, C.-M. Hsu, S. T. Connor, Y. Xu,
Q. Wang, M. McGehee, S. Fan and Y. Cui,
Nano Lett.
, 2009,
9
,
279
282.
8 A. Chutinan and S. John,
Phys. Rev. A
, 2008,
78
, 023825.
9 T. Tayagaki, Y. Hoshi, Y. Kishimoto and N. Usami,
Opt.
Express
, 2014,
22
, A225.
10 E. L. Warren, J. R. McKone, H. A. Atwater, H. B. Gray and
N. S. Lewis,
Energy Environ. Sci.
, 2012,
5
, 9653.
11 S. Haussener, C. Xiang, J. M. Spurgeon, S. Ardo, N. S. Lewis
and A. Z. Weber,
Energy Environ. Sci.
, 2012,
5
, 9922
9935.
12 S. M. Sze and K. K. Ng,
Physics of Semiconductor Devices
,
Wiley-Interscience, 3rd edn, 2006.
13 S. M. Eichfeld, H. Shen, C. M. Eichfeld, S. E. Mohney,
E. C. Dickey and J. M. Redwing,
J. Mater. Res.
, 2011,
26
,
2207
2214.
14 E. L. Warren, H. A. Atwater and N. S. Lewis,
J. Phys. Chem. C
,
2014,
118
, 747
759.
15 M. D. Kelzenberg, S. W. Boettcher, J. A. Petykiewicz,
D. B. Turner-Evans, M. C. Putnam, E. L. Warren,
J. M. Spurgeon, R. M. Briggs, N. S. Lewis and
H. A. Atwater,
Nat. Mater.
, 2010,
9
, 239
244.
This journal is © The Royal Society of Chemistry 2015
Energy Environ. Sci.
,2015,
8
,203
207 |
207
Communication
Energy & Environmental Science
Published on 05 November 2014. Downloaded on 15/01/2015 15:34:51.
View Article Online