A New Approach to Solar Hydrogen Production: a ZnO–ZnS Solid Solution Nanowire Array Photoanode

Abstract

A ZnO–ZnS solid solution nanowire array photoanode is developed based on an alternative sensitization of a ZnO–ZnS solid solution nanowire array for solar hydrogen generation with considerably enhanced photocurrent – more than 195% greater compared to pristine ZnO nanowires. This solid solution structure demonstrates a better photoactivity enhancement effect than traditional quantum dot sensitization, as well as allowing hydrogen generation.

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Due to increasing global demand for energy and environ-
mental concerns, energy resources and conversion pathways to
replace the burning of fossil fuels are urgently being sought.
Hydrogen produced by water splitting using solar energy
clearly represents an attractive direction for the production of
clean energy.
[ 1 ] Numerous attempts have been made to con-
struct suitable architecture for metal- and semiconductor-based
nanostructured devices.
[ 1–5 ] One innovative study used titanium
dioxide in the photoelectrochemical splitting of water.
[ 6 ] Many
investigations have been conducted to develop photocatalytic
materials or photoelectrochemical cells for hydrogen genera-
tion.
[ 7–17 ] With respect to photoelectrochemical cells for water
splitting, metal oxide photoelectrodes with wide bandgaps, such
as ZnO and TiO
2 , are severely limited by their poor absorption
of visible light. Various schemes have been adopted to increase
absorption in the visible region, including doping with impuri-
ties and sensitization with quantum dots (QDs).
[ 12 , 16 , 18 , 19 ] Semi-
conductor quantum dots, such as CdS, CdSe, CdTe, and InP,
have been assembled on metal oxide semiconductor nanos-
tructures as sensitizers of photoelectrodes.
[ 11 , 12 , 14 , 16 , 18 ] However,
little work has been done on solid solution as a photocatalytic
material and sensitizer in photoelectrochemical devices. Zinc
oxide is a multifunctional semiconductor material with a direct
bandgap of approximately 3.2 eV; furthermore, the addition
of impurities frequently causes dramatic changes in its elec-
trical and optical properties. The formation of a solid solution
in ZnO–ZnS (ZnOS) is expected to modify the electrical and
optical properties because of the large differences between the
electronegativities and sizes between S and O atoms.
[ 20–23 ] The
energy bandgap of an S-doped ZnO fi lm, fabricated by pulse
laser deposition, reportedly shifted to a lower level as sulfur
was incorporated,
[ 24 ] leading to the effective harvest of sun-
light in visible region. Herein, we address a novel sensitiza-
tion approach of ZnO–ZnS solid solution on ZnO nanowires
array for hydrogen generation with considerably enhanced
photocurrent.
Figure 1 a shows a schematic of the fabrication steps for ZnOS
nanowire array photoanode. ZnO nanowire arrays were synthe-
sized over the entire surface of an SnO
2 :F (FTO) glass substrate
using the hydrothermal method. ZnS QDs were deposited in a
chemical bath on ZnO nanowire array, and following thermal
treatment made the formation of ZnO–ZnS solid solution.
Scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) were utilized to further characterize specifi c
nanostructures of a series of samples. SEM images revealed the
growth of dense and vertically aligned ZnO nanowires on an
FTO substrate, with a typical diameter of around 150 nm, and
TEM image of ZnO nanowires demonstrated that they have
uniform diameter (Figure 1 b). The corresponding selected-
area electron diffraction pattern is characteristic of ZnO single
crystal wurtzite structures (spots pattern). Figure 1 c shows
ZnO nanowires after deposition of ZnS QDs, which verifi ed
that individual ZnO nanowires were covered with nanoparticle
aggregated with diameters of about 10–30 nm. Interestingly,
the selected area electron diffraction pattern is characteristic of
the two component crystalline natures. The set of spot pattern
can be indexed to the ZnO wurtzite structure along the [
2 1 10 ]
zone axis, which shows a single crystalline nature. The set of
rings revealed a typical face-centered-cubic polycrystalline struc-
ture corresponding to ZnS, and probably associated with the
large amount of ZnS QDs on the surface of the ZnO nanowires.
The high-resolution TEM shows the lattice spacing between
the (111) planes of 0.311 nm is in agreement with that of ZnS
bulk cubic crystal (insert in Figure 1 c). The spatial distribution
of the compositional elements within the ZnO–ZnS solid solu-
tion nanowire is obtained from scanning transmission electron
microscope (STEM)-EDX line scans in the radial direction of the
nanowires (indicated by the red arrow in Figure 1 d). The spec-
trum verifi ed that the nanowire comprised Zn, O, and S. Most
of the O signals are confi ned in the cores of the nanowires,
while the shell region yields a more intense S signal. Because
of the random orientation of ZnO nanowire and random depo-
sition of ZnO QDs, the compositional profi les had an asym-
metric nature. Since ZnS QDs randomly attached to the surface
of ZnO nanowire during the deposition process, this indicates
Dr. H. M. Chen , C. K. Chen , Prof. Dr. R.-S. Liu
Department of Chemistry
National Taiwan University
Taipei, Taiwan
E-mail: rsliu@ntu.edu.tw
Dr. C.-C. Wu , Dr. W.-S. Chang
Green Energy & Environment Research Laboratories
Industrial Technology Research Institute
Hsinchu, Taiwan
Prof. Dr. K.-H. Chen
Institute of Atomic & Molecular Sciences Academia Sinica
Taipei, Taiwan
Dr. T.-S. Chan , Dr. J.-F. Lee
National Synchrotron Radiation Research Center
Hsinchu, Taiwan
Prof. Dr. D. P. Tsai
Department of Physics
National Taiwan University
Taipei, Taiwan
DOI: 10.1002/aenm.201100246
Hao Ming Chen , Chih Kai Chen , Ru-Shi Liu , * Ching-Chen Wu , Wen-Sheng Chang ,
Kuei-Hsien Chen , Ting-Shan Chan , Jyh-Fu Lee , and Din Ping Tsai
A New Approach to Solar Hydrogen Production: a
ZnO–ZnS Solid Solution Nanowire Array Photoanode

743
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those from other planes because of the [00L] oriented growth
of nanowires. In Figure 2 a, no observable change in the ZnO
nanowires was observed after ZnS QD sensitization, which
meant that sensitization of ZnS QDs did not affect the struc-
ture of ZnO (ZnO@ZnS QDs). In the case of ZnO–ZnS solid
solution, a clear shift forward to low diffraction angle position
could be found, indicating the occurrence of lattice modifi ca-
tion. Since the oxygen in the ZnO nanowire were substituted
by sulfur, which lead to a lattice expansion owing to the larger
radius of sulfur atoms. Furthermore, no diffraction peak from
ZnS QDs was obtained, implaying most of ZnS QDs formed
solid solutions with ZnO nanowires. The UV-vis absorption
spectra powerfully revealed the optical properties of ZnO–ZnS
solid solution. The Kubelka-Munk remission function F ( R )
relates the refl ectivity R of a sample to an absorption coeffi cient
α
and a scattering coeffi cient S, as follows:
F(R)=
(1 −R)2
2R
=
"
S
Based on the assumption of a constant
scattering coeffi cient S , F ( R ) is proportional
to
α
and represents the absorbance of the
sample. The absorption spectra of the sam-
ples were calculated using the Kubelka-Munk
function. The bandgap of the samples was
calculated by fi tting the absorption band edge
of the spectra as:
h<"(<)=B(h<−bg)1/2
Since F ( R ) is proportional to R , the linear
portion of a plot [ F ( R ) h
ν
] 2 (for ZnO) as a
function of h
ν
intercepts the abscissa at h
ν
=
bg (bandgap), and such a plot is commonly
that ZnS QDs exhibited a non-uniform covering on the ZnO
wires. Thus, the S signals present an asymmetric profi le due
to the contribution of ZnS QDs to the sulfur signals. Inter-
estingly, the compositional profi le of O exhibited a signifi cant
signal outside the edge position, indicating that inter-diffusion
between S and O occurred upon the surface of the nanowires.
This observation verifi es the formation of ZnO–ZnS solid solu-
tion on the surface of ZnO nanowires. To examine the struc-
tural properties of these ZnO–ZnS solid solution nanowires,
X-ray diffraction (XRD) studies were conducted ( Figure 2 a).
All of the samples yielded similar diffraction patterns. The
indexing of the patterns reveals that all of the diffraction peaks
are consistent with the wurtzite ZnO structure with lattice
constants of a = 3.250 Å and c = 5.207 Å. The high intensity
of the (002) diffraction peaks in Figure 2 a indicates that refl ec-
tions from the (002) plane in ZnO nanowires are stronger than
Figure 1 . a) Schematic of the fabrication of ZnOS nanowire photoanode. b) SEM and TEM images, and corresponding SAED of pristine ZnO nanowires.
c) TEM image of ZnO nanowires with deposition of ZnS QDs. d) Elemental profi le extract from STEM in direction of nanowires (indicated by arrow
in TEM).
Figure 2 . a) X-ray diffraction patterns of ZnO, ZnO nanowires sensitized with ZnS QDs, and
ZnO-ZnS solid solution nanowires. b) bandgap plots of ( F ( R ) h
ν
) 2 for ZnO and ZnO–ZnS solid
solution nanowires.
a. u.)
ZnO-ZnS solid solution
ZnO
a. u.)
(a) (b)
(002)
(102)
(101)
ZnO-ZnS solid solution
30 35 40 45 50
Intensity (a
2.8 3.0 3.2 3.4
Energy (eV)
ZnO
(F(R)xh
ν
)
2
(
2θ(degree)
ZnO@ZnS QDs
ZnO rod

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adopted in estimating the bandgap energy of semiconductors.
A model for direct bandgap transitions was used, and good
linear fi ts were obtained, as displayed in Figure 2 b. It should
be noted that there was no signifi cant changes in bandgap
value (approximately 3.19 eV) in pristine ZnO nanowires. The
bandgap of the case of ZnO–ZnS solid solution sample shifted
to a lower energy (approximately 3.10 eV) due to a bowing
effect in the bandgap.
[ 24 ] When a S atom is incorporated into
the ZnO lattice, the potential energy of the top of the valence
band that is formed by O
2p atomic orbitals (i.e., O
2p –S 3p repul-
sion), narrowing the bandgap for ZnO–ZnS solid solution.
[ 24–26 ]
This reveals that ZnO–ZnS solid solution sensitization can sig-
nifi cantly reduce the bandgap and absorb visible region from
sunlight.
To obtain the photocurrent for the photoactivity under light
illumination, photoelectrochemical (PEC) studies were car-
ried out in a 0.5
M Na 2 S and K
2 SO 3 solution (pH = 11) as the
supporting electrolyte medium, which operated as sacrifi cial
reagents.
[ 9 ] The photocatalytic reaction is carried out in an
aqueous solution including sacrifi cial reagents, hole scaven-
gers (sulfi de ions) – photogenerating holes that react with the
sacrifi cial reagents instead of water. Figure 3 a displays a set of
linear-sweep voltammograms that were recorded on pristine
ZnO nanowires and ZnO–ZnS solid solution nanowire arrays.
ZnO–ZnS solid solution nanowire arrays showed a pronounced
photocurrent starting at around –0.7 V and continuing to
increase to 1.5 mA cm
− 2 at + 1.0 V under illumination. In
comparison to pristine ZnO nanowires, the photocurrent den-
sity of ZnS–ZnO solid solution nanowire arrays was approxi-
mately two times greater (around 0.88 mA cm
−
2 ) than the
value for pristine ZnO nanowires (around 0.45 mA cm
−
2 )
at 0 V, which suggests that the formed ZnS–ZnO solid solu-
tion harvested solar light more effectively than the pristine
nanowires. The two most important metrics associated with a
photocurrent are the plateau current and the onset potential.
The value of the plateau current depends mainly on the photo-
generated holes/electrons that reach the semiconductor/liquid
junction and react with water, rather than recombining.
[ 15 ] In
determining the onset potential, the overpotential must be
taken into account. A large overpotential is caused mainly by
the slow kinetics of water oxidation, and causes holes/electrons
to accumulate at the surface; subsequent surface recombina-
tion occurs until suffi ciently positive potentials are applied.
Modifi cation of the electrode surface lowers the kinetic barrier
to interfacial charge transfer, reducing the required overpoten-
tial and shifting the curve to the left.
[ 15 ] The onset potentials of
ZnO nanowires sensitized with ZnS QDs or ZnO–ZnS solid
solution are more negative than that of pristine ZnO, with both
samples exhibiting identical behavior, suggesting that their
surfaces are similar (as shown in Figure S1 in the Supporting
Information). Notably, the plateau current of the ZnO sensitized
with ZnS QDs is close to that of pristine ZnO nanowires, which
means that ZnS QD sensitization cannot markedly improve the
plateau photocurrent. This phenomenon further reveals that
solid solution structure can lead to a better enhancement effect
in photoactivity than traditional quantum dot sensitization. The
improvement in the plateau current is caused by the formation
of ZnS–ZnO solid solution, which means the solid solution can
harvest sunlight to generate more photoelectrons. To evaluate
Figure 3 . a) Linear-sweep voltammograms from ZnO nanowires, ZnO
nanowires with sensitization of ZnS QDs, and ZnO–ZnS solid solution
nanowires. b) IPCE spectra of ZnO, ZnO nanowires with sensitization of
ZnS QDs, and ZnO–ZnS solid solution nanowires at a potential of 0.5 V
versus Ag/AgCl. c) H
2 production upon illumination of each samples.
300 350 400 450 500 550 600
0
5
10
15
20
25
IPCE / %
Wavelength / nm
ZnO-ZnS solid solution
ZnO@ZnS QDs
ZnO
0.5
1.0
1.5
j / mAcm-2
ZnO-ZnS solid solution
ZnO@ZnS QDs
ZnO
Dark scan
(a)
(b)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
0
1
2
3
4
5ZnO-ZnS solid solution
ZnO@ZnS QDs
ZnO
-0.5 0.0 0.5
0.0
Potential / V vs Ag/AgCl
(c)
Time (h)
H
2
evolved (mmol)

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solid solution nanowire arrays. A strong peak at around 1.85 Å
was observed in the Fourier transform (FT) of the Zn K-edge
EXAFS spectrum of the ZnO nanowires with phase correction,
suggesting that central Zn atoms were surrounded by O atoms
in fi rst shell scattering. Another strong peak at around 3.2 Å
was obtained with phase correction, indicating that the second
shell around the Zn atoms included neighboring atoms of Zn
atoms (as shown in the insert of Figure 4 ). This result reveals
that the ZnO nanowires are consistent with the wurtzite ZnO
structure. In the case of ZnO–ZnS solid solution, the peak scat-
tered from fi rst shell atoms slightly shifted to longer distance,
which indicates that the fi rst shell around the Zn atoms may
be affected by incorporation of S atoms and including neigh-
boring atoms of two elements (O and S). To better reveal this
structural transformation, the strong fi rst peak possesses two
deconvoluted peaks occurring at 1.85 and 2.0 Å, corresponding
to Zn–O and Zn–S, respectively. The peak at 1.85 Å is assigned
to single scattering by O atoms within ZnO wurtzite structure
and other peak at 2.0 Å corresponds to single scattering by
the S atoms. It was worth noting that second shell scattering
(around 3.2 Å) around central Zn atoms was identical in both
of ZnO and ZnO–ZnS solid solution nanowire arrays, this
could be attributed to the existence only of Zn atoms in the
second scattering shell. This phenomenon demonstrated that
ZnO–ZnS solid solution nanowire arrays exhibited a primary
wurtzite crystal structure while ZnO and ZnS formed a solid
solution phase. This crystalline nanostructure is able to provide
the additional potential advantage of improved charge transport
over a traditional doping approach, which results in a dramatic
increase in the plateau current.
[ 15 ] Moreover, since O
2p
and Zn
4p
orbitals involved the conduction band of ZnO nanowires, X-ray
absorption near edge structure (XANES) of the Zn K-edge could
be utilized to reveal the conduction band structure of ZnO–ZnS
the PEC performance, incident photon-to-current-conversion
effi ciency (IPCE) measurements were made to determine the
photoresponses of the pristine ZnO nanowires and ZnO–ZnS
solid solution nanowire arrays as functions of incident light
wavelength (Figure 3 b). The IPCE shows a clear enhancement
at around 380 nm, which can be attributed to the presence of
ZnS. The IPCE curve of the ZnO–ZnS solid solution sample
demonstrates substantial photoactivity in the visible light region
from 400 to 480 nm in addition to strong photoresponse in the
near-UV region, which implies that the ZnS–ZnO solid solu-
tion can signifi cantly enhance the IPCE. This phenomenon is
consistent with the observation of the UV-vis spectra (Figure S2
in the Supporting Information), which may be due to the modi-
fi cation of electronic structure and orbital coupling derived
from the formation of ZnO–ZnS solid solution (vide infra).
Figure 3 c shows the amount of H
2 that was produced by ZnO,
ZnO nanowires with sensitization of ZnS QDs, and ZnO–ZnS
solid solution photoelectrodes (3 × 2 cm
2 ) upon solar simulator
illumination (300–900 nm, 100 mW cm
−
2 , spectral peak at
500 nm). The ZnO–ZnS solid solution photoelectrodes
exhibited a signifi cant enhancement in hydrogen generation
compared to both ZnO nanowires and ZnO nanowires with
sensitization of ZnS QDs.
Although XRD has demonstrated the lattice expansion
caused by sulfur incorporation in ZnO–ZnS solid solution,
little structural information with respect to the zinc is revealed.
Extended X-ray absorption fi ne structure (EXAFS) is a short-
range probe of structure and yielded results concerning local
correlations around the absorbing atom, specifi cally the
nearest neighbor interatomic distances and coordination num-
bers.
[ 27–29 ] EXAFS was conducted to obtain better evidence of
the structural parameters of ZnO–ZnS solid solution nanowire
arrays. Figure 4 shows the Zn K-edge of ZnO and ZnO–ZnS
0
1
ZnO-ZnS solid solution
ZnO@ZnS QDs
ZnO
Normalized Absorbance (a. u.)
(a) (c)
9650 9675 9700
Energy (eV)
(b)
Figure 4 . a) EXAFS spectra of Zn K-edge for ZnO and ZnO-ZnS solid solution nanowires. b) Wurtzite structure of ZnO and each scattering shell modes
of central Zn atom. c) XANES spectra of Zn K-edge for ZnO nanowires, ZnO@ ZnS QDs, and ZnO–ZnS solid solution nanowires.

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solid solution (1s to 4p transition). Figure 4 c shows the
XANES spectra of pristine ZnO and ZnO–ZnS solid solution
nanowires. Features A and B refl ect dipole transition from Zn
1s to 4p
π
(along the c axis) states.
[ 30 ] To compare with pristine
ZnO nanowire, the electronic transition to p state along the c
axis increased with the formation of ZnO–ZnS solid solution,
indicating that orbital coupling occurred in c axis direction of
ZnO nanowire. Accompanying the formation of the ZnO–ZnS
solid solution, the conduction band was derived from S
3p
, O 2p
,
and Zn
4p
orbitals, the coupling of S
3p
orbital dominated unoc-
cupied states of c axis direction. As a result, ZnO–ZnS solid
solution nanowires array was characteristic of a “hetero-coor-
dination zinc” nanostructure, which modifi ed the conduction
band derived from O
2p
and Zn
4p
orbitals. Orbital coupling of
S
3p
with O
2p
and Zn
4p
leads to an increase in the number of
conduction band unoccupied state and enhances the transition
probability from the valence band to the conduction band. This
enhancement in transition probability may contribute to the
photoelectrochemical response.
Furthermore, a simple relationship is satisfi ed when
both ZnO and ZnS are randomly distributed in the
nanowires:
[ 27 , 31 ]
NZnO =(XZnO/XZnS )NZnS
N ZnO is the coordination number of the O atoms around
the Zn atoms, and N ZnS is the coordination number of the S
atoms around the Zn atoms. X ZnO and X ZnS denote the frac-
tions of ZnO and ZnS in the sample, respectively. Notably,
the coordination number determined from the EXAFS spectra
of the ZnO–ZnS solid solution nanowires satisfi ed the afore-
mentioned relationship (S/O = 23/77 from elemental analysis,
Figure S3 in the Supporting Information), indicates that the
nanowires formed a solid solution phase. Based on the above
observations, we propose a model of the ZnO–ZnS solid
solution nanowires. Figure 5 presents a model of the ZnO–
ZnS solid solution nanowires array photoelectrode for solar
hydrogen generation. The EXAFS and XRD results revealed
that ZnO and ZnS formed a solid solution in this study. How-
ever, the STEM-EDX line scans spectra exhibited a signifi -
cant asymmetric distribution, implying that S incroporation
dominated on the surface rather than core part of nanowires.
Consequently, ZnS QDs are utilized as a source of sulfur to
form a solid solution which leads to a narrow bandgap and
increases the number of conduction band unoccupied states.
The energy level diagram in Figure 5 indicates that the conduc-
tion bands (CB) of the ZnS–ZnO solid solution are higher than
that of ZnO, allowing the effi cient transfer of photoexcited
electrons from ZnS–ZnO solid solution to ZnO nanowires
(determination of the detail of band position is described in
the Supporting Information) When electrons are transported
to the cathode, generating hydrogen in a PEC device, the holes
are consumed in an oxidation reaction at the anode/electrolyte
interface.
In conclusion, we have demonstrated signifi cant improve-
ment in the ZnO–ZnS solid solution sensitization, which
results in a dramatic increase in the plateau photocurrent, as
well as a substantial shift in the onset potential. This study is
the fi rst demonstration of sensitization using ZnO–ZnS solid
solution. A photocurrent of 0.88 mA cm
−
2 at 0 V was observed
Figure 5 . Sketch of nanostructure of ZnO–ZnS solid solution nanowires. The lower part displays pathways of electrons for generating hydrogen, and
a relative energy diagram.

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and it was more than 195% greater than that of pristine ZnO
nanowires. The STEM results demonstrated the interdiffu-
sion between S and O, and sulfur atoms became incorporated
into the lattice of ZnO nanowires to form the solid solution of
ZnO–ZnS. ZnO–ZnS solid solution nanowires array was char-
acteristic of a “hetero-coordination zinc” nanostructure, which
modifi ed the conduction band derived from O
2p
and Zn
4p
orbital. The enhancement in photoactivity can be attributed to
the increase of O
2p
+ S 3p
and Zn
4p
unoccupied states. This solid
solution structure can deliver a better enhancement effect in
photoactivity than traditional quantum dot sensitization, and
also reveals hydrogen generation. Although this investigation
was concerned with the ZnO–ZnS system, we believe that this
strategy can be extended to other solid solution systems.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors gratefully acknowledge the fi nancial support of the Institute
of Atomic & Molecular Sciences Academia Sinica (Contract No. AS-98-
TP-A05) and the National Science Council of Taiwan (Contracts Nos.
NSC 97-2113-M-002-012-MY3 and NSC 100-2120-M-002-008).
Received: May 10, 2011
Revised: June 22, 2011
Published online: August 16, 2011
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