CuO/Pd composite photocathodes for photoelectrochemical hydrogen evolution reaction
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CuO/Pd composite photocathodes forphotoelectrochemical hydrogen evolution reaction
Xin Guo a, Peng Diao a,*, Di Xu a, Shan Huang a, Yang Yang a, Tao Jin a,Qingyong Wua, Min Xiang a, Mei Zhang b,*aKey Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and
Engineering, Beihang University, Beijing 100191, PR Chinab State Key Laboratory of Advanced Metallurgy, School of Metallurgical and Ecological Engineering, University of
Science and Technology Beijing, Beijing 100083, PR China
a r t i c l e i n f o
Article history:
Received 9 October 2013
Received in revised form
27 February 2014
Accepted 7 March 2014
Available online 13 April 2014
Keywords:
Photoelectrochemical cell
Solar water splitting
Hydrogen evolution reaction
Copper oxide
Palladium nanoparticles
* Corresponding authors. Tel./fax: þ86 01 823E-mail addresses: pdiao@buaa.edu.cn, pd
http://dx.doi.org/10.1016/j.ijhydene.2014.03.00360-3199/Copyright ª 2014, Hydrogen Energ
a b s t r a c t
CuO has been considered as a promising photocathodic material for photoelectrochemical
(PEC) hydrogen evolution reaction (HER). In this work, CuO films were prepared by a facile
and cost-effective method that involves solution synthesis, spin-coating and thermal
treatment processes. The resulting CuO films had a monoclinic crystal structure with
bandgap energy of 1.56 eV and a conduction band position of 3.73 eV below the vacuum
level in borate buffer solution. The CuO films exhibited good PEC activity toward HER and
the preparation conditions had great effect on the activity. The photoactivity of the CuO
film decayed to approximately 19% of its original value after reaction for 10 h under illu-
mination. The reduction of CuO to Cu2O has been confirmed to be a parallel competitive
reaction against HER. The mismatched band structure of the resulting CuO/Cu2O hetero-
junction was believed to be the main cause of the decay of photoactivity. The photo-
assisted electrodeposition method was developed to prepare CuO/Pd composite photo-
cathode. The presence of Pd on CuO greatly increased the photocurrent especially at low
overpotentials. In addition, the CuO/Pd composite exhibited significantly improved pho-
tostability compared to CuO. This work demonstrates the feasibility of increasing PEC
activity and stability of CuO-based photocathodes by combining CuO with noble metal
nanoparticles.
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
Introduction
The conversion of solar energy into chemical fuels is an
attractive, clean and sustainable solution to the growing en-
ergy problem [1]. Solar-driven water splitting based on semi-
conductor materials is an artificial photosynthesis process
39562.iao@126.com (P. Diao), zh84y Publications, LLC. Publ
that stores the solar energy in the chemical bonds of the two
products H2 and O2. Photoelectrochemical (PEC) cells are
widely used as solar-powered water splitting devices [2e4]
because they combine solar energy collection with water
electrolysis. Moreover, in PEC cells, the cathodic and the
anodic reactions occur at cathode and anode, respectively,
allowing the investigation of only one half-cell reaction. PEC
angmei@ustb.edu.cn (M. Zhang).
ished by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 7 6 8 6e7 6 9 6 7687
hydrogen evolution reaction (HER) is the cathodic half-cell
reaction which has attracted great research interests [5e9]
because H2 is an environmentally benign fuel that produces
only water upon combustion. PEC HER is usually based on a p-
type semiconductor/electrolyte interface, where the photo-
generated electrons are injected from semiconductor into
water by the field of space charge region to generate H2. In
principle, semiconductors with conduction band position
more negative than the potential of H2O/H2 can be used as
photocathode materials. To develop efficient photocathodes,
materials with small bandgap energy are preferred because
only the photons with higher energy than the bandgap of a
semiconductor can be used to generate electronehole pairs.
Moreover, the photocathode materials should be environ-
mentally benign, low-cost, and highly durable under cathodic
potentials and illumination.
Copper (II) oxide (CuO) is a p-type semiconductor with a
direct bandgap ranging from 1.3 eV to 1.8 eV depending on the
synthesis method and condition [10e15]. The small bandgap
energy allows CuO to absorb vast majority of solar spectrum,
and the direct bandgap endows CuO with a much larger ab-
sorption coefficient compared to the indirect bandgap mate-
rials. Therefore, CuO is believed to be a promising candidate
for water splitting photocathodes. Moreover, the low toxicity
and the relatively high abundance in earth crust make CuO
more attractive. Several works have demonstrated that CuO
can serve as a good photocathodic material by itself [13e19] or
as a component to build heterojunction photoelectrodes with
other semiconductors, such as TiO2 and ZnO [20e23].
However, as a photocathodic material, CuO has two main
shortcomings. (1) The overpotential of HER at CuO surface is
high, which is a common problem for most of the semi-
conductor oxides. (2) CuO is unstable under cathodic poten-
tials and illumination [10,15,20], and the HER photocurrent at
CuO photocathodes decays quickly with time [10,15]. It has
been proposed that CuO can be reduced during photocathodic
HER [10,20]. As we know, the reduction of CuO is a photo-
electron consuming side reaction that competes with HER,
and this reaction will inevitably lower the photo-to-hydrogen
efficiency. To solve the first issue, noble metals, such as Pt, Pd
and Rh, were deposited on the surface of semiconductor ox-
ides as a cocatalyst. The presence of noble metals can greatly
improve the HER efficiency of the semiconductors by lowering
the overpotential for HER [24]. Though this strategy has been
proved to be effective in several “semiconductor oxide/noble
metal” systems [24e29], no work has been reported on the
preparation and properties of CuO/noblemetal composites for
PEC HER. As for the second issue, we believe the strategy of
depositing noble metals on CuO surface may also be a good
solution to improve the stability of CuO because of the
following reasons. The reduction of water and the reduction of
CuO by photo-electrons are two competitive photocathodic
reactions, and the presence of cocatalyst on CuO can inhibit
the latter reaction by accelerating the former. However, no
work has been reported to confirm the feasibility of this
strategy. Most of the previous work concerning semi-
conductor/noble metal photocathodes was performed on
stable oxides, such as TiO2 and ZnO [24e29]. As a result, the
effect of noble metal cocatalysts on the stability of photo-
cathode materials has not been well addressed.
In this work, we reported the preparation of the CuO/Pd
composite photocathode by deposition of Pd nanoparticles on
the surface of CuO using a photo-assisted electrochemical
approach.Wedemonstrated that the composite photocathodes
exhibited a substantially enhanced HER photocurrent in low
overpotential region.We showed that the reduction of CuOwas
greatly restrained and the stability of photoactivity of CuO was
significantly improved after deposition of Pd nanoparticles.
Experimental section
Chemicals and materials
CuSO4$5H2O was purchased from Sinopharm Chemical Re-
agent Co., Ltd. L-ascorbic acid was purchased from the Tianjin
Jinke Fine Chemical Institute. Ethylene glycol was purchased
from Beijing Chemical Reagents Company. All chemicals were
of analytical reagent grade and used without further purifi-
cation. The fluorine-doped tin oxide (FTO) glass (8 U sq�1,
transparency 80%, Asahi Glass, Japan) was used as the
conductive substrate. All aqueous solutions were prepared
with deionized water (resistance > 18 MU cm).
Preparation of CuO and CuO/Pd photocathodes
The CuO films were obtained by thermal oxidation of Cu
nanoparticles in air. Cu nanoparticles were synthesized by
mixing the same volume of 100mMethylene glycol solution of
CuSO4 and 100 mM ethylene glycol solution of L-ascorbic acid
in room temperature, and mixture was sonicated for 5 min.
Then, the resulting Cu nanoparticles (NPs) were separated by
centrifugation at 12,000 rpm for 30 min. After being washed
with copious ethanol, the Cu NPs were dispersed in ethanol
and the final concentration was 30 mg/mL. The Cu NPs layers
were prepared on FTO substrates by spin-coating. In detail,
10 mL Cu NPs solution was dropped on FTO substrates and
then spin-coated at 1000 rpm for 15 s. The thickness of Cu NPs
film was controlled by the cycles of spin-coating, and the
sample obtained by one spin-coating cycle was referred as
one-layered Cu NPs. After spin-coating, the Cu NP modified
FTO substrates were thermal treated in a furnace in air at
required temperature (350, 450, and 550 �C) for required time.
During thermal treatment, the Cu NPs were oxidized to CuO.
The Pd NPs were deposited on the surface of CuO using a
photo-assisted electrodeposition method. In detail, the CuO
modified FTO substrates were immersed in 0.1 M KCl con-
taining 0.5 mM Na2PdCl4, and then a deposition potential of
0.1 V was applied to the CuO electrode for required time under
illumination (100 mW cm�2). The resulting CuO/Pd composite
electrodes were rinsed with copious water before they were
used in electrochemical and PEC measurements.
Electrochemical and photoelectrochemical (PEC)measurements
A three-electrode cell was used in PEC measurements. The
CuO modified FTO was employed as working electrode, a
saturated calomel electrode (SCE) and a platinum foil were
used as reference and counter electrodes, respectively. The
Fig. 1 e The SEM images of one-layered (a) and 5-layered (b) Cu NPs prepared on FTO substrates with one and five spin-
coating cycles, respectively. The SEM images of one-layered (c) and 5-layered (d) CuO films after thermal oxidation in air at
550 �C. Typical TEM images and SAED patterns of Cu NPs (e) and CuO film (f).
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area of CuO electrode exposed to solution and light was
0.25 cm2. All electrochemical and PEC measurements were
carried out in 0.2 M H2BO3�/H3BO3 buffer solution (pH ¼ 9.2) on
a CHI660C electrochemical workstation (CH Instruments Co.).
All potentials were reported with respect to SCE or reversible
hydrogen electrode (RHE). Electrochemical impedance mea-
surements were performed at a frequency of 1 KHz and an
amplitude of 5 mV to determine the flat-band potential of the
CuO electrode by using MotteSchottky plot [30]. For PEC
measurements, a 500 W Xenon lamp was used as light source
and the intensity of light was 100 mW cm�2.
Characterization
The phase analysis of the Cu NPs and the CuO films were
performed by X-ray diffraction (XRD) (Rigaku, Rint 2000
advance theta-2theta powder diffractometer) with CuKa ra-
diation. The morphology and size of the Cu NPs and the CuO
films were characterized by field-emission scanning electron
microscopy (FE-SEM) (Hitachi, S-4800, Japan). High-resolution
transmission electron microscope (HRTEM) measurements
were performed on a field emission JEM-2010F microscope
(JEOL Ltd., Japan) with an accelerating voltage of 200 kV.
Selected area electron diffraction (SAED) patterns were also
recorded on the field emission JEM-2010F microscope. The
diffuse reflectance spectrumof the CuO filmwas recorded by a
double beam UVeVis spectrophotometer (Purkinje General,
China) with a bare FTO substrate as reference.
Results and discussion
Morphology, crystal structure, and bandgap of CuOelectrodes
Fig. 1(a) and (b) shows the SEM images of (one-layered and
five-layered) Cu NPs dispersed on FTO substrates by one and
five repeated spin-coating cycles, respectively. The Cu NPs
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synthesized by reduction of Cu2þ with L-ascorbic acid have a
quasi-spherical shape with an average diameter of about
420 nm. The FTO substrate was not completed coated with Cu
NPs after one cycle of spin-coating, as shown in Fig. 1(a).While
after 5 cycles of spin-coating, an uniform Cu NPs film was
obtained on the FTO substrates and no exposed FTO substrate
areas can be observed in SEM image (Fig. 1(b)). In fact, three
cycles of spin-coating resulted in a complete Cu NPs film that
fully coated the FTO surface. Fig. 1(b) also shows that the Cu
NPs film is porous due to the random stacking of Cu particles.
XRD measurements were carried out to determine the crystal
structure of the Cu NPs, and the result is shown in Fig. 2,
which indicates that the Cu NPs have a cubic crystal structure
(JCPDS NO. 85-1326). To obtain a CuO film, the Cu NP modified
FTO substrates were thermal treated in air at 550 �C for 4 h.
After thermal treatment, the morphology of the sample
changes from a porous film to a more continuous one, with a
significant decrease of the porosity (Fig. 1(c) and (d)). It should
be pointed out here that, though the Cu NPs layer prepared
with one cycle of spin-coating does not fully coat the FTO
surface, a continuous and complete thin layer of CuO was
formed on the FTO surface after thermal oxidation, as can be
seen by comparing Fig. 1(a) and (c). XRD spectra (Fig. 2) confirm
that the Cu NPs were oxidized to CuO during thermal treat-
ment and the resulting CuO film has a monoclinic structure
(JCPDS NO. 5-661). TEM images of Cu NPs and CuO film are
shown in Fig. 1(e) and (f), respectively. The HRTEM image and
the SAED pattern of an individual Cu NP (Fig. 1(e)) further
confirm that the Cu NPs have a single crystalline structure.
The lattice spacing shown in Fig. 1(e) is 0.204 nm, which is in
accordance with that of (111) plane of cubic Cu. This result
agrees well with the XRD spectrum of Cu NPs shown in Fig. 2.
The HRTEM image and the SAED pattern in Fig. 1(f) indicate
that the CuO film is composed of single crystalline CuO small
particles. The lattice spacing shown in Fig. 1(f) is 0.231 nm,
which matches well with that of the (111) planes of the
monoclinic CuO. It should be pointed out that lattice spacing
of 0.158 nm, which corresponds to the lattice distance of (202)
planes of monoclinic CuO, can also be observed in some
HRTEM images of CuO film (data not shown). These TEM
Fig. 2 e XRD pattern of the Cu NPs dispersed on FTO
substrate with 5 cycles of spin-coating and the
corresponding CuO film after thermal oxidation at 550 �C.
results are in good agreement with the XRD pattern of CuO
shown in Fig. 2.
TheUVevis diffuse reflection spectroscopicmeasurements
were performed to investigate the light absorption property of
the CuO films. Fig. 3(a) shows the Tauc plot, which was ob-
tained from UVevis spectra based on Tauc equation [31] and
can be used to determine the bandgap energy (Eg) of the CuO
films [32]. The bandgap energy obtained from Fig. 3(a) is
1.56 eV, suggesting that all the UV and visible light can, in
principle, be employed to generate electronehole pairs in CuO
film. Because one of the key processes for PEC HER is the in-
jection of photo-generated electrons from the conduction
band of CuO to water, the position of the conduction band (Ec)
with respect to the potential of H2O/H2 is very important for
PEC HER. According to the previous work [10], the position of
the valence band (Ev) below the vacuum level is given by the
following equation:
Ev ¼ Vfb þ 4:74þ DEF þ VH (1)
where Vfb is the flat-band potential, 4.74 eV is the position of
SCE below the vacuum level, DEF is the energy difference
Fig. 3 e (a) Tauc plot obtained from the UVevis diffuse
reflectance spectrum of the CuO film. R is the reflectance of
the CuO film and F(R) [ (1 L R)2/2R. (b) The MotteSchottky
plots of CuO film in 0.2 M H2BO3L/H3BO3 buffer solution
(pH [ 9.2). (For interpretation of the references to color in
this figure legend, the reader is referred to the web version
of this article.)
Fig. 4 e (a) Current density response of 10-layered CuO
photocathode as a function of potential in the dark and
under illumination. (b) Net photocurrent density vs
potential plot obtained on different layered CuO samples.
The different layered CuO samples were prepared by spin-
coating Cu NPs for different cycles on FTO followed by
thermal treatment at 550 �C. The current-potential curves
were obtained in 0.2 M H2BO3L/H3BO3 buffer solution
(pH [ 9.2) with a potential sweep rate of 50 mV sL1.
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between Fermi level (EF) and Ev, which can be calculated by
DEF ¼ EF � Ev, and VH is the Helmholtz potential between the
CuO electrode and the solution. On the basis of previous work
[10,13], DEF is 0.13 eV for CuO. The Helmholtz potential, VH, is
given by [10]:
VH ¼ 0:059ðpHz � pHaÞ (2)
where pHz is the pH of zero charge, which is defined as the pH
value of the solution in which the surface of semiconductor is
zero charged, and for CuO$pHz is 9.5 � 0.4 [33]. pHa is the pH
value of any actual solution in which CuO is immersed. Ac-
cording to Eq. (2), when the pH value of a solution is ca. 9.5, the
Helmholtz potentialVH for CuO in this solution can be neglect.
To meet the requirement for neglecting VH, 0.2 M H2BO3�/
H3BO3 buffer solution with a pH value of 9.2 was employed as
electrolyte in this work. Then, to calculate the value of Ev from
Eq. (1), we also need to know the flat-band potential, Vfb.
In order to obtain Vfb, the space charge capacitance (C) of
CuO at CuO/solution interface was measured as a function of
applied potential (V) at a frequency of 1 kHz in 0.2 M H2BO3�/
H3BO3 buffer solution (pH ¼ 9.2), and the result is shown in
Fig. 3(b), which is also known as MotteSchottky plot. Ac-
cording to theMotteSchottky equation [30], C�2 varies linearly
with V:
1C2
¼ 2eεε0A2ND
��V � Vfb
�� kBTe
�(3)
where ε is dielectric constant of CuO, ε0 is the permittivity of
vacuum, A is the area, ND is the carrier density, e is the elec-
tron charge, kB is Boltzmann’s constant, and T is the absolute
temperature. At room temperature, kBT/e is 0.0257. Therefore,
according to MotteSchottky equation, the value of Vfb can be
obtained from the intercept of the red extrapolation line in
Fig. 3(b) [30]. The MotteShottky plot of CuO in 0.2 M H2BO3�/
H3BO3 solution exhibits a negative slope, indicating that CuO
is a p-type semiconductor, and the value of Vfb obtained from
the intercept is 0.42 V vs SCE. Therefore, according to Eq. (1), Evis calculated to be 5.29 eV, whichmeans that the valence band
of CuO lies at 5.29 eV below the vacuum level. Then, on the
basis of the bandgap energy Eg ¼ Ec � Ev ¼ 1.56 eV, the con-
duction band lies at Ec ¼ 3.73 eV below the vacuum level,
which corresponds to �1.01 V vs SCE. As the equilibrium po-
tential of the H2O/H2 redox couple lies at �0.79 V vs SCE in
0.2 M H2BO3�/H3BO3 buffer solution (pH ¼ 9.2), the conduction
band edge of CuO is more negative than the redox potential of
H2O/H2. As a result, photo-generated electrons in conduction
band can be injected into the solution to reduce water. The
above discussion confirms that the CuO prepared in this work
can be employed as photocathodematerial for water splitting.
However, it should be pointed out that the photo-generated
electrons can also reduce CuO to form Cu2O because the
conduction band edge of CuO ismuchmore negative than that
of the CuO/Cu2O redox couple (�0.06 V vs SCE at pH¼ 9.2) [10].
Therefore, the reduction of water and the reduction of CuO
are two competitive reactions with the latter reaction ther-
modynamically more favorable, though the former may be
dynamically more preferred. The reduction of CuO may show
its side effect on the HER after long reaction time. Although
the quick decay of photocurrent for HER has been reported in
the previous work on CuO photocathodes [10,15], no work
reported what happens to CuO and its effect on HER after
long-term reaction.
PEC properties of the CuO photocathodes for HER
The photoelectrochemical properties of CuO films for HER
were examined in 0.2 M H2BO3�/H3BO3 buffer solution
(pH¼ 9.2). Fig. 4(a) shows a typical current density response of
a 10-layered CuOfilm in linear potential sweepmeasurements
in the dark and under illumination (100 mW cm�2). The onset
potential for HER in the dark is ca. �0.30 V vs SCE and there is
nearly no reduction current can be observed when the po-
tential is higher than �0.30 V. However, under illumination,
the onset potential for HER is positively shifted to 0.1 V vs SCE,
and a large reduction photocurrent is obtained even when the
potential is higher than �0.30 V. These results indicate that
Fig. 5 e (a) Current density-potential responses of the 10-
layered CuO films under chopped illumination
(100 mW cmL2). The 10-layered CuO samples were
prepared under different thermal oxidation temperatures.
(b) Current density-potential responses of the 10-layered
CuO samples in the dark and under illumination. The CuO
samples were thermal treated at 550 �C for different time.
All linear potential sweep measurements were performed
in 0.2 M H2BO3L/H3BO3 buffer solution (pH [ 9.2) with a
potential sweep rate of 50 mV sL1.
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the photo-generated electrons in CuO are injected into solu-
tion and contribute greatly to HER. As can be seen in Fig. 4(a),
there is a large reduction current in the dark when the po-
tential is lower than �0.30 V. To extract the pure effect of
illumination on the reduction current, the net photocurrent
(Jph) was calculated out by deducting the current in the dark
from that under illumination. Fig. 4(b) shows the variation of
Jph as a function of the applied potential and CuO layers. For all
the CuO photocathodes, Jph first increases with decreasing the
applied potentials, and then reaches its maximum at ca.
�0.50 V, and still then decreases as the potential is further
negatively shifted. Similar phenomena were observed previ-
ously on the CuO film prepared by flame spray pyrolysis [13]. It
is understandable for Jph to increasewith decreasing potential,
because lowering the applied potential will inevitably in-
creases the driving force for photo-generated electrons to
transfer from CuO to water. As for the decrease of Jph when
potential is more negative than �0.50 V, we believe it is due to
the partial reduction of CuO at very low potentials. Fig. 4(b)
also demonstrates that the number of CuO layers has signifi-
cant effect on Jph, and the maximum value of Jph is obtained
from the 10-layered CuO film. The increase of CuO thickness
has two opposite effects on the photocurrent. On the one
hand, it results in the increase of light absorption, which will
benefit Jph. While on the other hand, it also leads to the in-
crease of resistance, which will surely lower Jph.
The effects of the thermal oxidation temperature and
duration on the photoactivity of CuO toward HER were also
investigated. Three CuO photocathodes prepared under
different thermal oxidation temperature were used to record
the current-potential curve under chopped illumination. The
results are shown in Fig. 5(a), fromwhich it is seen that, raising
the oxidation temperature enhances the photocurrent, and the
highest photocurrent is obtained with a thermal treatment
temperature of 550 �C. We believe this is due to the improve-
ment of crystallinity with increasing temperature. Low tem-
perature thermal treatment results in poor crystalline quality
and then leads to a high density of defects, whichwill act as the
recombination centers for the photo-generated electronehole
pairs [31,34]. It should be pointed out here that raising the
temperature over 550 �Cwill deteriorate the conductivity of the
FTO substrate, and then significantly lower the photocurrent.
Fig. 5(b) shows the influence of thermal treatment duration on
the photoactivity of CuO films. The photocurrent density first
increases and then decreases with increasing oxidation time.
As we know, increasing thermal treatment time at 550 �Ccauses two opposite effects on the photocurrent. First, it im-
proves the crystallinity of CuO and reduces the possibility of
electronehole recombination, and this effect favors the
enhancement of photocurrent. Second, long-time thermal
treatment at 550 �C worsens the electrical properties of FTO
substrate, and then decreases the photocurrent. Accordingly,
there exists an optimal thermal oxidation time for CuO pho-
tocathodes. In consistence with the above discussion, the CuO
sample prepared by thermal oxidation at 550 �C for 4 h exhibits
the highest photoactivity, as shown in Fig. 5(b).
According to the above results, 10-layered CuO samples,
which were thermal treated in air at 550 �C for 4 h, were
employed to study the stability of CuO photocathodes. When
the CuO photocathode was repeatedly scanned within the
potential region from 0.10 V to �0.60 V under continuous or
chopped illumination, a clear decrease of photocurrent with
increasing potential sweep cycles was observed, indicating a
significant decay of the photoactivity. Moreover, the lower the
negative potential sweep end was, the faster the decay of
photoactivity. XRD spectra demonstrate that part of the sur-
face CuO was reduced to Cu2O, providing solid evidence that
the reduction of CuO is not negligible for long-term PEC HER.
As the reduction of CuO by photo-induced electrons is a par-
allel competitive reaction with HER, lowering the over-
potential for HER not only enhances the photoactivity for HER
but also inhibits the reduction of CuO. As Pd has a very low
overpotential for HER [24], Pd nanoparticles (Pd NPs) were
deposited on CuO to prepare CuO/Pd composite, and then
to improve the efficiency and stability of CuO-based
photocathodes.
Fig. 6 e Typical SEM (a) and HRTEM (b) images of Pd/CuO composite prepared by photo-assisted electrodeposition at 0.10 V
for 15 min in 0.1 M KCl containing 0.5 mM Na2PdCl4. The inset in (a) is a high-resolution SEM image.
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PEC properties of the CuO/Pd composite photocathodes forHER
The CuO/Pd composites have been employed as catalysts in
several reactions, including Suzuki reaction [35], cyanation
reaction [35], hydrogen generation reaction by methanol par-
tial oxidation [36], and photodegradation [37]. In this work, the
Pd is used as an electrocatalyst, and the modification of CuO
photocathode with Pd is aimed facilitating HER by lowering
the HER overpotential. The CuO/Pd composite photocathodes
were prepared by photo-assisted electrodeposition of Pd on
the surface of CuO film. Under illumination, the photo-
generated electrons can reduce the high-valence palladium
precursor (PdCl42�) to zero-valence metal Pd NPs. As a result,
the produced Pd NPs can be loaded on the surface of CuO film.
The applied potential is much lower than the redox potential
of PdCl42�/Pd couple, and then can facilitate the reduction of
PdCl42� by photo-induced electrons. The advantage of this
deposition method is that PdCl42� is directly reduced to Pd at
the photoactive sites on CuO, which ensures a faster electron
transfer fromCuO to Pd. Themorphology of the resulting CuO/
Pd composite is shown in Fig. 6(a), fromwhich a uniform layer
of small nanoparticles with particle size ranging from 3 nm to
20 nm is observed on the surface of CuO. Energy dispersive
spectroscopy (EDS) was used to identify the elemental com-
positions of the particle modified CuO. Pd peaks appear on
EDS spectra (see Fig. S1), providing the solid evidence that the
small particles in SEM image (Fig. 6) are Pd NPs. Moreover, the
compositional analysis by EDS indicates that, for the CuO/Pd
composite prepared by photo-assisted electrodeposition of Pd
for 15min, the atomic weight percentage of Pd on CuO surface
is 3.57%. A typical HRTEM image of CuO/Pd composite is
shown in Fig. 6(b), from which the lattice fringes of both CuO
and Pd are clearly seen. The Pd particles have a single crys-
talline structure with a lattice spacing of 0.225 nm, in accor-
dance with that of (111) facet of Pd. The CuO film exhibits a
lattice spacing of 0.158 nm, which corresponds to the lattice
distance of (202) facet of monoclinic CuO. All these SEM, EDS
and TEM results clearly demonstrate that small Pd nano-
crystals can be successfully generated on the surface of CuO
by photo-assisted electrodeposition.
As shown in Figs. 4(a) and 5, a relatively large dark reduc-
tion current is observed on CuO photocathodes when the
potential is more negative than 0.35 V. This current may
partially come from the electrochemical reduction of CuO.
Accordingly, to avoid electrochemical reduction of CuO, PEC
measurements on CuO/Pd composite photocathodes were
performed with the applied potential higher than �0.35 V.
Fig. 7(a) demonstrates the current-potential responses of CuO
film in the dark and under illumination before and after Pd
deposition. Because of the low HER overpotential of the Pd
cocatalyst [24], the CuO/Pd composite exhibits a much larger
reduction current under illumination compared to CuO. In
fact, after Pd deposition, the photocurrents increase by ca.
110%, 105%, 59%, 21% at potentials of 0.0, �0.10, �0.20 and
�0.30 V, respectively, indicating an enhancement of photo-
activity especially at low overpotentials. Electrochemical
impedance spectroscopy (EIS) was employed to provide the
information of the electron transfer across CuO/solution
interface. Fig. 7(b) shows the Nyquist plots of the CuO and the
CuO/Pd photocathodes at a constant potential of �0.35 V.
Clearly there are two semicircles apparent in the EIS spectra
on both photocathodes. The semicircle in high frequency re-
gion is attributed to the impedance of the CuO/FTO interface,
which can be represented by the CuO/FTO contact resistance
and the CuO/FTO interfacial capacitance in parallel. This
semicircle keeps almost unchanged regardless of illumination
and the presence of Pd because the contact resistance and the
capacitance of CuO/FTO are independent of illumination and
Pd modification. The low frequency semicircle corresponds to
the impedance of the CuO/solution interface. The diameter of
this semicircle represents the charge transfer resistance (Rct)
across the CuO/solution interface. Fig. 7(b) clearly shows that,
for the CuO/Pd electrode, Rct under illumination is much
smaller than that in the dark, indicating that photo-induced
electrons are involved in the electron transfer across the
CuO/solution interface. Moreover, under continuous illumi-
nation, the value of Rct (108 U cm2) on the CuO/Pd electrode is
much smaller than that on the CuO electrode (258 U cm2),
providing another evidence that the deposition of Pd on CuO
surface facilitates the transfer of photo-induced electrons
from CuO to solution.
The amount of deposited Pd, which can be adjusted by
varying the deposition time, is a key factor controlling the
photoactivity of CuO/Pd composite. Fig. 8 shows the influence
of Pd deposition time on the photocurrent of CuO/Pd
Fig. 7 e (a) Current density-potential curves of the CuO
films in the dark and under illumination before and after
photo-assisted electrodeposition of Pd for 15 min. The
potential sweep rate is 50mV sL1. (b) EIS spectra of the CuO
and the Pd/CuO photocathodes at an applied potential of
L0.35 V. All the electrochemical measurements were
performed in 0.2 M H2BO3L/H3BO3 buffer solution (pH[ 9.2).
The Pd/CuO composite photocathodes were prepared by
photo-assisted deposition of Pd on CuO at 0.1 V for 15 min
in 0.1 M KCl containing 0.5 mM Na2PdCl4.
Fig. 8 e Current density-potential responses of the Pd/CuO
composite photocathodes with different Pd deposition
time. The Pd/CuO composite photocathodes were prepared
by photo-assisted deposition of Pd on CuO at 0.1 V for
different time in 0.1 M KCl containing 0.5 mM Na2PdCl4.
The linear potential sweep measurements were performed
in 0.2 M H2BO3L/H3BO3 buffer solution (pH [ 9.2) with a
potential sweep rate of 50 mV sL1 in the dark and under
illumination.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 7 6 8 6e7 6 9 6 7693
composites. The photocurrent first increases and then de-
creases with increasing deposition time, and a maximum
photocurrent was obtained with a deposition time of 15 min.
This result is due to the fact that the amount of deposited Pd
influences the photoactivity of CuO/Pd in two opposite di-
rections. A large Pd deposition amount (long deposition time)
can surely benefit the electron transfer from CuO to water.
However, it may also inhibit the light absorption of CuO and
then lowers the photoactivity. We believe that the maximum
photocurrent shown in Fig. 8 is a balance between the two
opposite effects.
The photocatalytic stability of the CuO and the CuO/Pd
photocathodes were examined by using chronoamperometry.
Fig. 9(a) show the variation of photocurrent density of CuO
and CuO/Pd as a function of time under chopped illumination
at fixed potentials of �0.35 V. According to the discussion
in PEC properties of the CuO photocathodes for HER, the
potential of �0.35 V was used in the chronoamperometric
measurements because it is the most negative potential that
can produce the largest HER current while avoiding the elec-
trochemical reduction of CuO. Fig. 9(a) clearly demonstrates
that, the photocurrent of CuO decreases to approximately 19%
of its original value after 10 h reaction, whereas the photo-
current of the CuO/Pd composite decays more slowly
compared to that of CuO. After 10 h reaction, the remaining
photocurrent density of the composite photocathode is ca.
0.30 mA cm�2, which is over two times larger than that of the
CuO electrode. In addition, after reaction for the first 3 h, the
photocurrent on CuO/Pd keeps almost constant, only less than
15% photocurrent decay is observed for the next 7 h. However,
as for the CuO film, the photocurrent decreases by over 60%
for the last 7 h. These results provide solid evidence that the
combination of Pd with CuO can significantly improve the
photocatalytic stability of the photocathode. Fig. 9(a) also re-
veals that, even on the CuO/Pd composite photocathode, the
photocurrent density still decreases with increasing reaction
time. This is due to the fact that the CuO surface was not
completely coated with Pd NPs, as can be seen in the SEM
image in Fig. 6. Therefore, the CuO surface exposed to solution
on the CuO/Pd electrode may be reduced by photo-induced
electrons and then leads to the decrease of photocurrent.
When the exposed active sites on CuO surface were reduced
by photo-induced electrons and lost their activity, the rate of
current decay on the CuO/Pd composite slowed down, as can
be seen from the stable photocurrent for the last seven hours
(Fig. 9(a)).
It should be pointed out here that, even at the same po-
tentials, the photocurrents obtained from chronoampero-
metric measurements under chopped illumination on both
the CuO and the CuO/Pd photocathodes (Fig. 9(a)) are smaller
Fig. 9 e (a) Current density response as a function reaction
time on the CuO and CuO/Pd photocathodes at a constant
applied potential of L0.35 V under chopped illumination.
(b) XRD spectra of the CuO and the Pd/CuO photocathodes
after held at L0.35 V for 10 h under chopped illumination.
Fig. 10 e The band energy diagrams of the CuO/Cu2O (a)
and the Pd/CuO (b) photocathodes in 0.2 M H2BO3L/H3BO3
buffer solution (pH [ 9.2).
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 7 6 8 6e7 6 9 67694
than those in linear potential sweep measurements under
continuous illumination (Fig. 7(a)).We believe the difference is
due to the different methodology used to obtain photocur-
rents, and the detailed reason is not fully understood.
After long-time reaction under illumination, the phase
structure of the CuO and the CuO/Pd electrodeswere analyzed
by XRD. Fig. 9(b) shows the XRD spectra of the CuO and the
CuO/Pd electrodes after they were held at �0.35 V for 10 h
under chopped illumination. For both electrodes, two new
diffraction peaks at 36.4� and 42.4� appear besides the peaks of
monoclinic CuO. These two peaks correspond to the (111) and
(200) diffraction peaks of cubic Cu2O, as can be seen from
standard XRD pattern of cubic Cu2O (JCPDS No. 77-199). No
diffraction peaks from metal Cu can be observed, indicating
that no detectable amount of Cuwas generated as the product
of the two-electron reduction of CuO. These XRD results pro-
vide direct and solid evidence that part of CuO is reduced by
photo-generated electrons at �0.35 V under illumination, and
the main product is Cu2O. The intensity ratio of the (111)
diffraction peak of Cu2O to the ð111Þ diffraction peak of CuO,
which were denoted as g, reflects the ratio of the amount of
Cu2O to that of CuO. In other words, the value of g indicates, to
some extent, the percentage of the surface CuO that was
reduced to Cu2O. As shown in Fig. 9(b), after ten hour’s reac-
tion, the g values for the CuO and the CuO/Pd electrodes are
0.77 and 0.35, respectively, with the latter less than half of the
former. This result reveals the fact that the amount of surface
CuO, which were reduced to Cu2O on the CuO/Pd composite
photocathode during PEC HER, is much smaller than that on
the CuO photocathode. Therefore, it can be safely concluded
that the deposition of Pd cocatalyst not only facilitates the
electron transfer across CuO/solution interface, but also
significantly inhibits the reduction of CuO to Cu2O.
As is well known, Cu2O is a p-type semiconductor with a
direct bandgap energy of ca. 2.0 eV, which is also suitable for
solar energy harvesting [7]. The conduction band position of
Cu2O is located at 0.7 V negative of the hydrogen evolution
potential, making Cu2O a suitable photocathode material for
HER [7,38]. Then, an important issue arises: why the presence
of Cu2O on the surface of CuO lowers photocurrentwhen Cu2O
is also a photocathode material? We believe this is due to the
mismatch in the band structure between CuO and Cu2O.
Fig. 10(a) shows the band energy diagrams of the CuO/Cu2O/
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 7 6 8 6e7 6 9 6 7695
solution heterojunction, which are drawn based on the
bandgap energy and the conduction band position of CuO and
Cu2O. Since Cu2O is generated on the surface of CuO, photo-
induced electrons have to pass through the CuO/Cu2O/solu-
tion interface to reducewater. However, as shown in Fig. 10(a),
the conduction band position of Cu2O is higher (more nega-
tive) than that of CuO, making it impossible for the photo-
induced electrons to transfer from CuO to Cu2O [39]. More-
over, some of the photo-generated electrons at Cu2O can be
injected into CuO, and accumulate in the conduction band of
CuO. This process greatly increases the possibility of electro-
nehole recombination inside CuO and decreases the
photocurrent.
Fig. 10(b) shows the band energy diagram of the CuO/Pd
composite in solution. Because the conduction band position
of CuO is 0.22 V negative of hydrogen evolution potential in
0.2 M H2BO3�/H3BO3 buffer solution (pH ¼ 9.2), the photo-
generated electrons have two alternative paths to transfer
from CuO to water on CuO/Pd electrode. (1) Electrons transfer
directly fromCuO towater, and (2) electrons first transfer from
CuO to Pd and then from Pd towater, with Pd as amediator. As
is well known, Pd exhibits a high electrochemical HER activity
[40], which ensures a very low HER overpotential on Pd [24].
Therefore, the latter path via Pd mediator has a much faster
electron transfer rate than the former one. This is the reason
why Pd deposition greatly improves the photocurrent for HER.
As we have demonstrated before, two competitive reactions
are involved during PEC HER: the reduction of water to
generate H2 and the reduction of CuO to form Cu2O. The
presence of Pd on the surface of CuO can greatly accelerate the
electron transfer fromCuO towater, andwill surely inhibit the
reduction of CuO by photo-generated electrons. That’s why
the deposition of Pd significantly improves the PEC stability of
the CuO photocathodes.
It should be pointed out here that the purpose of this work
is to demonstrate that the fabrication of CuO/Pd composite
photocathodes by depositing Pd on the surface of CuO can
greatly enhance the activity and stability of the photocathode
toward PEC HER. We do not take much effort to improve the
photocurrent on CuO film. Therefore, this leaves vast spaces
for further improving the photoactivity of the CuO/Pd com-
posite electrodes by, for example, increasing the specific area
(porosity) of the CuO film.
Conclusion
The monoclinic CuO films with bandgap energy of 1.56 eV
were prepared by a facile and cost-effective method, which
included solution reduction, spin-coating and thermal
oxidation processes. The conduction band edge of the CuO
film was located �1.01 V vs SCE in aqueous solution with
pH ¼ 9.2, making CuO a good candidate for water splitting
photocathode. The highest photoactivity toward HER was
obtained on 10-layered CuO films that were thermal treated at
550 �C for 4 h. We provided the first solid evidence that part of
the surface CuO was reduced to Cu2O by photo-induced
electrons during PEC HER. The reduction of CuO greatly low-
ered the photoactivity and photostability of the CuO film. A
photo-assisted electrodeposition method that ensured the
deposition of Pd on the photoactive sites of CuO surface was
developed to prepare CuO/Pd composite photocathodes. We
demonstrated that the deposition of Pd on CuO not only
enhanced the photocurrent for HER but also significantly
improved the photocatalytic stability of the CuO film. This
work shows the feasibility of increasing PEC activity and sta-
bility of CuO-based photocathodes by developing CuO/noble
metal composites.
Acknowledgments
The authors acknowledge the financial support of thiswork by
National Natural Science Foundation of China (NSFC
21173016, 20973020), Beijing Natural Science Foundation
(2142020), Doctoral Fund of Ministry of Education of China
(20101102110002).
Appendix A. Supplementary data
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.ijhydene.2014.03.084.
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