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Decoration of plasmonic Cu nanoparticles on WO
3
/
Bi
2
S
3
QDs heterojunction for enhanced
photoelectrochemical water splitting
Palyam Subramanyam
1
, Bhagatram Meena
1
, Gudipati Neeraja Sinha,
Melepurath Deepa, Challapalli Subrahmanyam
*
Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, 502285, Sangareddy, Telangana, India
article info
Article history:
Received 16 March 2019
Received in revised form
9 May 2019
Accepted 20 May 2019
Available online xxx
Keywords:
Photoelectrochemical cell
Copper nanoparticles
Bismuth sulfide
Surface plasmon resonance
Water splitting
Charge transportation
abstract
We report a WO
3
/Cu/Bi
2
S
3
wherein incorporation of Cu nanoparticles (Cu NPs) to enhance
the photoelectrochemical activity over WO
3
/Bi
2
S
3
. Cu NPs effectively harvest the light
energy upon plasmon excitation and transfer the energy to contacted WO
3
, thereby
improving the photoelectrochemical (PEC) performance. The WO
3
/Cu/Bi
2
S
3
composite was
characterized by scanning electron microscopy (SEM), Transmission electron microscopy
(TEM) and X-ray diffraction (XRD) to analyze the morphology and interfacial contact be-
tween the semiconductors. The photocurrent density and Solar-to-Hydrogen conversion
efficiency for this composite is 10.6 mA cm
2
at 1.23 V (versus RHE) and 3.21% at 0.81 V
(versus RHE), which are much higher than WO
3
/Bi
2
S
3
with 4.02 mA cm
2
at 1.23 V (versus
RHE) and 2.46% at 0.81 V (versus RHE) respectively. Moreover, the stability and photo-
response of WO
3
/Cu/Bi
2
S
3
were carried out through chronoamperometric studies. The
composite retained its stability over 50 cycles without decay in PEC performance. High
incident photon conversion efficiency (IPCE) value of about 51% is achieved which is
evident from the high photocurrent density. Incorporation of Cu NPs increase the photo-
activity which is evident from the photocurrent value. The increased activity of Cu NPs
sandwiched composite is attributed for the quick electron transfer to semiconductor due to
surface plasmon resonance (SPR) effect.
©2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction
Photoelectrochemical (PEC) water splitting has received great
attention as it is a promising route for solar-to-chemical en-
ergy conversion and can contribute as a solution to deal with
the ever-growing global energy requirements [1e3]. Different
metal oxide materials including TiO
2
, ZnO, WO
3
,a-Fe
2
O
3
and
BiVO
4
have been used for the development of photoanodes in
PEC water splitting [4e9]. Among them, WO
3
is attractive as a
photoanode due to its unique optical and electrical properties
such as its strong optical absorption, high electron mobility,
moderate hole diffusion length, charge-carrier transport
properties and good electrochemical stability [10e15]. How-
ever, the PEC performance of WO
3
-based photoanode is poor
due to weak absorption in the visible region due to its wide
*Corresponding author.
E-mail address: csubbu@iith.ac.in (C. Subrahmanyam).
1
Contributed equally.
Available online at www.sciencedirect.com
ScienceDirect
journal homepage: www.elsevier.com/locate/he
international journal of hydrogen energy xxx (xxxx) xxx
https://doi.org/10.1016/j.ijhydene.2019.05.168
0360-3199/©2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO
3
/Bi
2
S
3
QDs heterojunction for
enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/
j.ijhydene.2019.05.168
bandgap (2.6e3.0 eV), high charge recombination and poor
charge transfer kinetics at the electrode/electrolyte interface.
Many efforts were put forth to enhance the PEC performance
of WO
3
via heterojunction formation, metal/non-metal
doping, co-catalyst loading and morphology control nano-
architecture [16e21]. In the recent past, heterojunction for-
mation has been extensively used to improve the PEC activity
of WO
3
for this approach can enhance its response to visible
light and minimize the recombination of photo-generated
charge carriers. Narrow bandgap semiconductors like Bi
2
S
3
is used to enhance photo-activity [18,22,23]. For example,
Wang et al., showed good activity for WO
3
/Bi
2
S
3
with a current
density of 5.95 mA cm
2
at 0.9 V (vs RHE) which supports the
injection of photogenerated electrons from Bi
2
S
3
to WO
3
[24].
Liu et al., have also reported WO
3
/Bi
2
S
3
as a photoanode with
different synthetic approach showing considerable current
densities [25].Bi
2
S
3
nanobelt/WO
3
nanoplate array composite
reported by Liu et al. showed current density of 8.91 mA cm
2
at 0.1 V vs. Ag/AgCl [26]. Also, photo-stability is another
major issue. This can be addressed by introducing a plas-
monic metal nanostructure [27e32]. These structures help for
better utilization of solar energy to improve the overall effi-
ciency of PEC. One of the main fundamental forms of surface
plasmon resonance (SPR) is localized SPR. Surface plasmons
(SPs) are coherent delocalized electron oscillations that occur
at the interface of two materials. A SP is generated in the
nanoparticle when light is incident on the material. The
wavelength of light must be smaller or comparable to that of
the nanoparticle so as to generate a plasmon. The frequency
of the incident light and that of the surface free electrons in
nanoparticles (NPs) meet the resonance condition thereby
increasing the optical absorption of the nanoparticle. In an
earlier study, plasmonic Cu nanoparticles have been attracted
research interest due to their high conductive properties as
well as excellent photo-response in near-infrared region
(~800e900 nm) and superior photocatalytic performance.
Moreover, it is a low cost effective material compared to Au
and Ag nanoparticles and Cu NPs have been considered for
hydrogen evolution reaction (HER) [33e35]. Recently, Zhang
et al., reported plasmonic Cu NPs decorated with TiO
2
nano-
tube arrays (TNAs) as photocatalyst, which exhibited high PEC
performance as well as excellent chemical stability for HER
under visible light than the pure TNAs [36]. Yang et al., fabri-
cated sandwiched-Ag NPs based nanocomposite like ZnO/Ag/
CdS photo-electrode for PEC water splitting showed a photo-
current density of 4 mA cm
2
. This results suggest that Ag NPs
act as electron transfer mediators which trigger the electron
transfer chain reaction at the interface between the ZnO and
CdS semiconductor nanostructures [37]. Liu et al., reported
plasmonic Ag NPs decorated with WO
3
/CdS NRs as photo-
anode, which exhibited high PEC performance for hydrogen
production than the pure WO
3
and WO
3
/CdS films [38].On
considering the benefits of SPR effect of Cu NPs and Bi
2
S
3
QDs,
the optical absorption properties in the visible region and the
effect of the same on the PEC activity of WO
3
film is studied.
The incorporation of Cu leads to generation of plasmons on its
surface thereby, electron relays between Bi
2
S
3
and WO
3
thus
increasing the photocurrent.
In this work, we present low cost Cu nanostructures based
WO
3
/Cu/Bi
2
S
3
composite as photoanode for PEC water
splitting. Here, the WO
3
and Cu NPs were prepared by hy-
drothermal process and Cu NPs were decorated via electro-
phoresis deposition. Further, Bi
2
S
3
QDs layers deposited by
successive ionic adsorption reaction (SILAR) process. In the
work presented, case study and comparisons were provided
for various heterojunctions of the synthesized composite for
better PEC activity. Among them, the WO
3
/Cu/Bi
2
S
3
showed
maximum PEC activity which is consistent with the plasmonic
behavior of Cu NPs that were incorporated.
Experimental section
Chemicals
Sodium tungsten dihydrate (Na
2
WO
4
.2H
2
O), sodium sulfide
(Na
2
S), oxalic acid (H
2
C
2
O
4
) and bismuth nitrate trihydrate
(Bi(NO
3
)
3
.3H
2
O) were purchased from Merck. Triton-X 100,
hydrochloric acid (HCl), hydrazine hydrate (N
2
H
4
.H
2
O),
ethylene diamine (C
2
N
2
H
8
) and sodium hydroxide (NaOH),
copper nitrate (Cu(NO
3
)
3
) from sigma Aldrich. FTO (13 U/cm
2
)
glass purchased from Aldrich. FTO glass pre-cleaned with 35%
HCl solution, followed by distilled water and acetone.
Synthesis of copper nanoparticles (Cu NPs)
Cu NPs were synthesized by hydrothermal process con-
taining 7 M NaOH solution (25 mL), 0.1 M Cu (NO
3
)
3
.3H
2
O
(0.15 mL), C
2
N
2
H
8
(150 mL) as capping agent and N
2
H
4
.H
2
O
(60 mL) as reducing agent [39]. To this solution, excess of
NaOH solution was added to maintain a pH of 14. The re-
action mixture was transferred to a Teflon lined autoclave,
sealed and maintained at 150
C for 3 h. The product ob-
tained was separated by centrifugation followed by drying
in oven at 60 C to obtain Cu NPs that can be stored in dark
for further use.
Synthesis of WO
3
platelets
WO
3
platelets were synthesized via hydrothermal process.
Briefly, Na
2
WO
4
.2H
2
O (0.25 g) was dissolved in 4 mL of
deionized water, followed by addition of 0.7 mL conc. HCl
under stirring. Further, 62 mg of H
2
C
2
O
4
was added and
continued stirring for 1 h. Then, a mixture of ethanol and
water (30 mL in 16:14 ratio) was added to the reaction mixture.
The contents are then transferred to a Teflon lined autoclave
and maintained at 100 C for 16 h. The product was filtered
and dried in oven at 60 C to obtain WO
3
.
Preparation of photoanodes
Preparation of FTO/WO
3
/Bi
2
S
3
photoanode
The grinded powder of WO
3
was coated on pre-cleaned FTO
using acetyl acetone: water (1.5: 8.5 v/v) solution and 0.2 g of
Triton X-100 as binder via doctor blade technique. The FTO/
WO
3
films were annealed at 70 C for 0.5 h followed by calci-
nation at 450 C for 0.5 h. Further, Bi
2
S
3
QDs were deposited on
FTO/WO
3
using SILAR method with 0.1 M Bi (NO
3
)
3
.3H
2
Oin
acetone and 0.1 M Na
2
S in methanol as precursor solutions
international journal of hydrogen energy xxx (xxxx) xxx2
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO
3
/Bi
2
S
3
QDs heterojunction for
enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/
j.ijhydene.2019.05.168
[40]. Totally, eight SILAR cycles were performed resulting in
dark brown FTO/WO
3
/Bi
2
S
3
films.
Preparation of Cu-based photoanodes
Cu-based films such as WO
3
/Cu, WO
3
/Cu/Bi
2
S
3
and WO
3
/Bi
2
S
3
/
Cu were prepared using electrophoretic deposition method. In
the electrophoretic deposition, two electrode system is used
with WO
3
or WO
3
/Bi
2
S
3
films and Pt rod as the working and
counter electrode respectively while the solution of Cu NPs
(10 ml THF) served as the electrolyte for Cu deposition [39].
Further, these plates are decorated with Bi
2
S
3
QDs via SILAR
process to form FTO/WO
3
/Cu/Bi
2
S
3
. The deposition was carried
out at 30 V applying a dc voltage for 5 min. It could be explained
that the Cu NPs were directed by the electric field through the
pores of the WO
3
or WO
3
/Bi
2
S
3
assembly, resulting in brownish
colored WO
3
/Cu and WO
3
/Bi
2
S
3
/Cu. This makes the strong
contact of Cu NPs with the semiconductors. Further, these
electrodes were washed in THF, dried in oven at 45 C.
Characterization
UVeVis spectra for synthesized photoanodes recorded on
Shimadzu UV-3600 instrument. Powder X-ray diffraction
(XRD) patterns of samples were measured by PANalytical,
XpertPRO instrument. Surface morphologies of the samples
were measured using field emission scanning electron mi-
croscope (FESEM-Zeiss supra 40). High resolution trans-
mission electron microscopy (HR-TEM) recorded for
photoanodes using a TECNAI G-2 FEI (300 kV).
Photoelectrochemical measurement
Linear sweep voltammetric (IeV) studies were performed on
Autolab (LOT-Oriel) with a 150 W Xe arc lamp light source
(100 mW cm
2
). Chronoamperometric (I-t) studies, electro-
chemical impedance spectroscopic (EIS) and Mott-Schottky
studies were performed on an Autolab PGSTAT 302 N using
NOVA 2.1 software. The samples were measured in aqueous
electrolyte containing 1:1 mixture of 0.1 M Na
2
SO
3
and Na
2
SO
4
.
Results and discussion
The absorption spectra of the synthesized samples such as
WO
3
,WO
3
/Cu, WO
3
/Bi
2
S
3
and WO
3
/Cu/Bi
2
S
3
are shown in
Fig. 1. Cu NPs show a strong surface plasmon resonance peak
centered at 580 nm as shown in Fig. 1a[41e44].WO
3
has a
narrow absorption with an absorption edge at 450 nm and
bandgap of 2.75 eV (Fig. 1b). While for the WO
3
/Cu sample, the
absorption band edge was found to be at 510 nm, this signifies
a red shift in the absorption range (Fig. 1b). A peak in the
spectrum of WO
3
/Cu around 580 nm is attributed to the SPR
mode of Cu NPs, thus indicating the symbiotic effect in
increasing the photo-activity through coating of Cu NPs on
WO
3
. Additionally, there is a decrease in the absorption for
WO
3
/Cu as compared to WO
3
alone which clearly states the
presence of SPR effect of the Cu NPs in combination with WO
3
.
To further increase the photo-response of WO
3
/Cu, it was
decorated with Bi
2
S
3
QDs. Bi
2
S
3
QDs show absorption in the
300e800 nm with a bandgap of 1.55 eV (Fig. 1c). As compared
to WO
3
/Bi
2
S
3
, Cu NPs embedded composite WO
3
/Cu/Bi
2
S
3
Fig. 1 eAbsorption spectra of the (a) Cu NPs (b) WO
3
and WO
3
/Cu (c) Bi
2
S
3
QDs, WO
3
/Bi
2
S
3
and WO
3
/Cu/Bi
2
S
3
samples (d) PL
spectra of WO
3
,WO
3
/Cu, WO
3
/Bi
2
S
3
,WO
3
/Bi
2
S
3
/Cu and WO
3
/Cu/Bi
2
S
3
at an excitation wavelength of 350 nm.
international journal of hydrogen energy xxx (xxxx) xxx 3
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO
3
/Bi
2
S
3
QDs heterojunction for
enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/
j.ijhydene.2019.05.168
showed enhanced light absorption. This indicates that Cu NPs
form good interfacial contact with semiconductors to have
efficient light harvesting [41,42,44].
The photoluminescence (PL) study for as-prepared samples
such as WO
3
,WO
3
/Cu, WO
3
/Bi
2
S
3
and WO
3
/Cu/Bi
2
S
3
at an
excitation wavelength of 350 nm and corresponding plots are
shown in Fig. 1d. Typically, PL spectra is used to evaluate the
separation ability of the photogenerated charge carriers
where low PL emission intensity indicates less recombination
of photogenerated electrons. The WO
3,
WO
3
/Cu, WO
3
/Bi
2
S
3,
WO
3
/Cu/Bi
2
S
3
and WO
3
/Bi
2
S
3
/Cu samples also have a broad
emission peaks at 426 nm. However, in Cu NPs and Bi
2
S
3
QDs
introduced electrodes namely, WO
3
/Cu/Bi
2
S
3
and WO
3
/Bi
2
S
3
/
Cu heterojunctions, the PL intensity is quenched. Impres-
sively, the WO
3
/Cu/Bi
2
S
3
heterostructure exhibited lowest
emission, which would suggest that it has the most efficient
charge carrier separation among all electrodes and signifi-
cantly inhibited the charge recombination in the hetero-
junction. This quenching in PL emission reveals excited state
interaction between WO
3
and Bi
2
S
3
QDs where photo-
generated electrons can be transferred from Bi
2
S
3
QDs to WO
3
via Cu NPs and this led to the increase in PEC performance of
the ternary heterostructure.
Fig. 2 shows the XRD patterns of WO
3
, Cu NPs Bi
2
S
3
and
WO
3
/Cu/Bi
2
S
3
.WO
3
has a monoclinic crystal lattice (JCPDS-
830950) with XRD peaks of (002), (200), (112), (202), (122), (400)
and (420) which are attributed to d ¼4.05, 3.67, 3.09, 2.62, 2.53,
1.83 and 1.65
A respectively. Cu NPs shows planes at (111),
(200) and (220) of the face centered cubic lattice (JCPDS-892838)
which corresponding to d ¼2.09, 1.82 and 1.28 Å.Bi
2
S
3
Fig. 2 eXRD patterns of the WO
3
, Cu NPs, Bi
2
S
3
QDs and
WO
3
/Cu/Bi
2
S
3
samples.
Fig. 3 eFE-SEM images of the (a) Cu NNs, (b) WO
3,
(c) WO
3
/Cu and (d) WO
3
/Cu/Bi
2
S
3
samples.
international journal of hydrogen energy xxx (xxxx) xxx4
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO
3
/Bi
2
S
3
QDs heterojunction for
enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/
j.ijhydene.2019.05.168
oriented along (120), (220), (101), (310), (211), (221), (311), (240),
(141), (421), (431), (251), (312), (242), (152), (721), (651) and (811)
planes with an orthorhombic primitive crystal lattice (JCPDS-
170320) which attributed to d ¼5.03, 3.96, 3.56, 3.11, 2.81, 2.71,
2.52, 2.23, 2.11, 1.99, 1.95, 1.83, 1.73, 1.56, 1.48, 1.44, 1.39 and
1.28
A respectively. The XRD pattern of WO
3
/Cu/Bi
2
S
3
shows
the characteristic peaks of Cu NPs and Bi
2
S
3
in blue and red
respectively. This indicates the formation of the WO
3
/Cu/Bi
2
S
3
composite.
The morphology of the samples were analyzed by FE-SEM
and are displayed in Fig. 3. The synthesized Cu NPs have
needle shaped morphology (Fig. 3a). WO
3
formed appear as
nanoplatelets with average diameter of 150e200 nm as shown
in Fig. 3b. Fig. 3c displays the presence of Cu NPs on WO
3
nanoplatelets. The composite WO
3
/Cu/Bi
2
S
3
shows a combi-
nation of the individual morphologies of WO
3
/Cu and aggre-
gate randomly with no specific shape displaying high
roughness. FE-SEM images of WO
3
/Bi
2
S
3,
WO
3
/Bi
2
S
3
/Cu and
Bi
2
S
3
samples are shown in S1.
Fig. 4 e(aed) HR-TEM images of WO
3
/Cu/Bi
2
S
3
composite.
Fig. 5 e(a) LSV plots and (b) STH efficiency of pristine WO
3
and WO
3
/Cu, WO
3
/Bi
2
S
3
,WO
3
/Bi
2
S
3
/Cu and WO
3
/Cu/Bi
2
S
3
photoanodes under solar radiation (intensity of 100 Wm
-2
).
international journal of hydrogen energy xxx (xxxx) xxx 5
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO
3
/Bi
2
S
3
QDs heterojunction for
enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/
j.ijhydene.2019.05.168
The HR-TEM images of WO
3
/Cu/Bi
2
S
3
composite are shown
in Fig. 4. The distance between the consecutive fringes in WO
3
,
Cu NPs and Bi
2
S
3
respectively are shown in the figure. The d-
spacing of 0.43 nm relates to (002) of monoclinic WO
3
(JCPDS:
830950). The d-spacing of 0.42 nm is due to the (220) of
orthorhombic Bi
2
S
3
(JCPDS: 170320) and the d-spacing of
0.24 nm corresponds to the (200) of Cu (JCPDS: 892838). The
TEM images of Bi
2
S
3
QDs as shown in the Fig. S2a. Further-
more, the spots in the SAED pattern confirms the presence of
WO
3
, Cu NPs and Bi
2
S
3
in the WO
3
/Cu/Bi
2
S
3
composite
(Fig. S2b).
Photoelectrochemical studies
The PEC activity of photo-electrodes were analyzed under
dark and light conditions and the current-potential (IeV)
behavior are presented in the Fig. 5a. The measurements were
performed using a three electrode system where prepared
composites, Pt and Ag/AgCl are working, counter and refer-
ence electrodes respectively. Equi-molar (0.1 M) mixture of
Na
2
SO
3
and Na
2
SO
4
is used as the electrolyte. The following
relation is used to convert the measured potential versus Ag/
AgCl into RHE:
ERHE¼EAg=AgCl þE
Ag=AgClþ0:059pH(1)
where standard electrode potential of Ag/AgCl (E
Ag/AgCl
)-
¼0.197 V at 25 C with electrolyte pH ¼12.7.
Under dark conditions, the photoanodes: pristine WO
3
,
WO
3
/Cu, WO
3
/Bi
2
S
3
,WO
3
/Cu/Bi
2
S
3
and WO
3
/Bi
2
S
3
/Cu exhibit
nearly negligible currents. Under illumination, the measured
photocurrent densities for WO
3
,WO
3
/Cu, WO
3
/Bi
2
S
3
,WO
3
/Cu/
Bi
2
S
3
and WO
3
/Bi
2
S
3
/Cu composite photoanodes are 0.92, 1.85,
4.02, 10.6 and 8.46 mA cm
2
at 1.23 V versus RHE respectively.
The same photoanodes exhibited an onset potentials of 0.32,
0.30, 0.29, 0.21 and 0.25 V versus RHE respectively. Among
these photo-electrodes, sandwiched type WO
3
/Cu/Bi
2
S
3
com-
posite exhibited highest photocurrent density with a low
onset potential as compared to the other photo-electrodes.
This illustrates that incorporation of Cu NPs in WO
3
/Cu/Bi
2
S
3
composite leads to high current at a 40 mV less potential than
WO
3
/Bi
2
S
3
/Cu. The high photocurrent exhibited by WO
3
/Cu/
Bi
2
S
3
indicates its ability to mitigate charge recombination.
The sandwiched Cu NPs in WO
3
/Cu/Bi
2
S
3
ternary composite
showed superior PEC performance among the prepared pho-
toanodes. Bi
2
S
3
improves the charge separation and transport
of electrons while Cu NPs due to SPR generated surface plas-
mons increase charge separation in Bi
2
S
3
, improve charge
transfer and reduce the charge recombination rate. Compared
to previous reports on WO
3
and Bi
2
S
3
based photo-electrodes,
our reported composite exhibits higher photocurrent density
for water oxidation reaction.
Fig. 6 eLSV of (a) pristine WO
3
and WO
3
/Cu, (b) WO
3
/Bi
2
S
3
(c) WO
3
/Bi
2
S
3
/Cu, and (d) WO
3
/Cu/Bi
2
S
3
photoanodes under solar
radiation exposure for 50 repeated cycles of operation.
international journal of hydrogen energy xxx (xxxx) xxx6
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO
3
/Bi
2
S
3
QDs heterojunction for
enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/
j.ijhydene.2019.05.168
The efficiency of the photo-electrodes for PEC water-
splitting is calculated using the following equation:
hð%Þ¼Jð1:23 VÞ=Plight (2)
where J is photocurrent density at a measured potential, V is
applied bias (versus RHE) and P
light
is power density of the
source (P
light
¼100 Wm
-2
).
The evaluated STH (solar to hydrogen) conversion effi-
ciencies of pristine WO
3
,WO
3
/Cu, WO
3
/Bi
2
S
3
,WO
3
/Bi
2
S
3
/Cu
and WO
3
/Cu/Bi
2
S
3
composite photo-electrodes are 0.14, 0.57,
1.23, 2.46 and 3.21% at 0.81 V versus RHE respectively as shown
in Fig. 5b. As expected, the WO
3
/Cu/Bi
2
S
3
photoanode ach-
ieved a maximum STH over other photo-electrodes. WO
3
/Cu/
Bi
2
S
3
show enhanced efficiency over WO
3
/Bi
2
S
3
/Cu, thus
indicating Cu NPs coated act as recombination sites for the
carriers.
The stability of prepared pristine WO
3
,WO
3
/Cu, WO
3
/
Bi
2
S
3
,WO
3
/Cu/Bi
2
S
3
and WO
3
/Bi
2
S
3
/Cu nanostructured
photo-electrodes were examined through linear sweep vol-
tammetry (LSV) measurements under light illumination for
50 repeated cycles of operation (Fig. 6). Fig. 6a shows
comparison between WO
3
and WO
3
/Cu, WO
3
shows very
low photo-activity after 50 cycles while WO
3
/Cu showed
great activity even -after 50 cycles. This indicates good
interfacial contact and the ability of Cu NPs to enhance
photo-activity. Fig. 6b shows activity of WO
3
/Bi
2
S
3
for 1st
and 50
th
cycle, indicating slight loss in photo-response due
to corrosion with electrolyte. Fig. 6c and d shows photo-
activities of WO
3
/Bi
2
S
3
/Cu and WO
3
/Cu/Bi
2
S
3
, a significant
difference is observed in the current densities after 50 cy-
cles. Therefore, it is concluded that Cu plasmons increase
the stability of the WO
3
/Bi
2
S
3
.
Chronoamperometric (I-t curves) studies reveal instant
photo-response and stability response of the photoanodes
(Fig. 7). The photocurrent response of the anodes were tested
under repeated on-off cycles at 1.23 V versus RHE. Under
switch on/off, all the photo-electrodes exhibit an excellent
photo-response. The values of photocurrent are consistent
with that obtained from LSV. It reveals that the WO
3
/Cu/Bi
2
S
3
composite electrode exhibited higher transient photocurrent
than the other photo-electrodes. The stability for photo-
electrodes was tested over a period of 5000 s under light illu-
mination as shown in Fig. 7b. For pristine WO
3
photo-
electrode, photocurrent density decreases continuously with
time, and a nearly 60% decrement was observed after 3200 s.
For the WO
3
/Cu photo-electrode, no significant decrement of
photocurrent density was observed, thus demonstrating su-
perior stability of WO
3
/Cu photo-electrode due to SPR effect
and chemical stability of Cu NPs in an aqueous medium. The
WO
3
/Bi
2
S
3
shows nearly 25% decay over 4500 s since sulfide
based QDs are less stable in aqueous solution. Finally, Cu-
based composite photoanodes, WO
3
/Cu/Bi
2
S
3
and WO
3
/Bi
2
S
3
/
Cu showed significant long-term stability without decay in
performance. This suggests that decoration with Cu NPs can
Fig. 7 e(a) Transient photo-response curves and (b) stability curves of pristine WO
3
,WO
3
/Cu, WO
3
/Bi
2
S
3
,WO
3
/Bi
2
S
3
/Cu and
WO
3
/Cu/Bi
2
S
3
photoanodes under solar radiation exposure at 1.23 V vs RHE (c) H
2
evolution over prepared samples under
solar light irradiation at 1.23 V vs RHE in 0.1 M of Na
2
SO
4
and Na
2
SO
3
solution during period time 2 h and (d) IPCE data of
pristine WO
3
,WO
3
/Cu, WO
3
/Bi
2
S
3
,WO
3
/Bi
2
S
3
/Cu and WO
3
/Cu/Bi
2
S
3
photoanodes under monochromatic solar radiation.
international journal of hydrogen energy xxx (xxxx) xxx 7
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO
3
/Bi
2
S
3
QDs heterojunction for
enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/
j.ijhydene.2019.05.168
enhance stability of the photo-electrode and reduce the onset
potential as well as increase the photocurrent density.
The hydrogen evolution activity of all photoelectrodes
were performed as a function of time at 1.23 V vs RHE in
0.1 M of Na
2
SO
4
and Na
2
SO
3
solution and results are pre-
sented in the Fig. 7c. During the 2 h period, the hydrogen
evolution of the WO
3
,WO
3
/Cu, WO
3
/Bi
2
S
3,
WO
3
/Bi
2
S
3
/Cu and
WO
3
/Cu/Bi
2
S
3
photoanodes were 0.32, 0.61, 1.54, 2.09 and
2.36 mmol respectively. The above results confirms that the
higher hydrogen evolution was achieved for WO
3
/Bi
2
S
3
/Cu
and WO
3
/Cu/Bi
2
S
3
photoanodes than other electrodes. The
higher hydrogen evolution for WO
3
/Cu/Bi
2
S
3
could be
attributed to Cu NPs which enhanced the photocurrent and
the solar to-hydrogen efficiency of the WO
3
-based photo-
anodes. The IPCE response of the fabricated photo-
electrodes as shown in Fig. 7d are evident to state that
WO
3
/Cu/Bi
2
S
3
exhibited high photo conversion. For the WO
3
photoanode, the maximum IPCE of 8.5% observed at 350 nm
andphoto-responseobservedupto455nmduetothewide
bandgap (~2.7 eV). On introduction of Cu NPs on WO
3,
IPCE
maximum improved from 8.5 to 13.1% at 350 nm with photo-
response extended up to 700 nm, which is due to SPR effect
of Cu NPs that could be enhance the visible light absorption.
For the WO
3
/Bi
2
S
3
photo-electrode, the highest IPCE value is
21% in 350e750 nm region, the contribution of Bi
2
S
3
QDs
gives the additional enhancement. The highest IPCE of 51%
is achieved at 350 nm with spectral range of UV-NIR region
for WO
3
/Cu/Bi
2
S
3
photoanode after decoration of Bi
2
S
3
QDs.
The high IPCE for WO
3
/Cu/Bi
2
S
3
can be explained by the dual
effect of Bi
2
S
3
QDs that enhance the interfacial active sites
due to high surface area and Cu NPs improving the light
harvesting in the visible region due to the SPR effect. Usually,
Cu NPs display SPR peak in the wavelength range of
550e650 nm. Upon excitation, the appearance of a plas-
monic peak of Cu NPs at ~580 nm indicates the formation of
plasmon induced photo-excitation which contributed to the
enhanced photocurrent.
Electrical impedance spectroscopy (EIS) experiments
were performed for the kinetic study of composites at
photoanode/electrolyte interface. The photoanodes were
tested in the electrolyte (0.1 M Na
2
SO
3
and 0.1 Na
2
SO
4
mixed solutions) under light illumination with frequency
rangeof10KHzto1Hz.TheNyquistplotsforWO
3,
WO
3
/
Cu, WO
3
/Bi
2
S
3
,WO
3
/Cu/Bi
2
S
3
,andWO
3
/Bi
2
S
3
/Cu composite
photoanodes are shown in Fig. 8. The diameter of the
semicircle represents charge transfer resistance (R
ct
)in
the Nyquist plot. Lower the R
ct,
better the charge transfer
process at the electrode-electrolyte interface. Lowest R
ct
is
observed for WO
3
/Cu/Bi
2
S
3
among the synthesized photo-
anodes (inset Figure). By incorporation of Cu NPs between
the WO
3
and Bi
2
S
3
,theR
ct
value was further lowered
resulting in the enhancement of PEC performance
compared to that of WO
3
/Bi
2
S
3
photo-electrode. This sug-
geststhattheplasmonsgeneratedonCuNPsfacilitieslow
charge transfer resistance which is also evident from high
current density, leading to a significant enhancement in
the photoelectrochemical performance for HER.
The CV plots of WO
3
and Bi
2
S
3
and Cu NPs films are
displayed in the supporting information (Fig. S3). The
conduction band (CB) and valence band (VB) positions of
WO
3
are 3.69 eV and 6.44 eV at pH 12.7 respectively,
which are calculated from the reduction and oxidation
peak potentials. For Bi
2
S
3,
the CB and VB are 3.62 eV and
5.17 eV respectively. For Cu NPs, the work function or
Fermi level (E
F
)is4.32 eV which is obtained from the
oxidation potential value (Fig. S3c). These values are ob-
tained by the CV and optical studies and this values are
used in the energy band diagram of the photoanode for
PEC water splitting.
Thepossiblereasonfortheelectrontransferprocessin
Cu NPs-Bi
2
S
3
QDs sensitized WO
3
system could be
explained via a schematic representation as shown in
Fig. 9.TheexposureofBi
2
S
3
to solar light generates ex-
citons due to its high absorption in the visible region. The
photogenerated electrons are introduced from the CB of
Bi
2
S
3
and are injected onto Cu, where the SPR effect re-
sults in efficient transfer of carriers to the CB of WO
3
,
finally to the contact (FTO). While, the holes generated in
the VB of Bi
2
S
3
react with electrolyte and drive the water
oxidation to produce oxygen gas and hydrogen ions.
Fig. 8 eNyquist plots of pristine WO
3
,WO
3
/Cu, WO
3
/Bi
2
S
3
,
WO
3
/Bi
2
S
3
/Cu and WO
3
/Cu/Bi
2
S
3
photoanodes at open
circuit potential.
Fig. 9 eSchematic of energy alignment of and charge
transfer in the WO
3
/Cu/Bi
2
S
3
photoanode under solar light
illumination.
international journal of hydrogen energy xxx (xxxx) xxx8
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO
3
/Bi
2
S
3
QDs heterojunction for
enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/
j.ijhydene.2019.05.168
While the electrons transfer to counter electrode (Pt) via
external circuit leads to hydrogen evolution. The Cu NPs
embedded between Bi
2
S
3
and WO
3
facilitate efficient
charge transfer and improves the PEC performance for
hydrogen production.
Conclusions
In summary, a WO
3
/Cu/Bi
2
S
3
composite was synthesized via
hydrothermal process, followed by electrophoresis and SILAR
methods. The prepared composite served as photoanode for
PEC water splitting with photocurrent density of 10.6 mA cm
2
at 1.23 V (versus RHE) and solar to hydrogen conversion effi-
ciency of 3.21% at 0.81 V (versus RHE), which is higher than
WO
3
/Bi
2
S
3
,WO
3
/Bi
2
S
3
/Cu, WO
3
/Cu and pristine WO
3
films.
This improved photocurrent is mainly due to plasmons
generated on the surface of Cu NPs that are in conjugation
with WO
3
and Bi
2
S
3
. The effect of Cu NPs is clearly visible in
terms of photo-stability and low charge transfer resistance.
Overall, this improved PEC performance of the composite is
possibly due to high optical absorption by Bi
2
S
3
while Cu NPs
serving as electron relays from Bi
2
S
3
to WO
3
. This synergic
charge transfer explains the low charge recombination in the
composite that led to the improvement in the number of
photoexcited carriers for water splitting efficiency.
Acknowledgements
PS thanks CSIR for a senior research fellowship. G.N.S thanks
DST-Inspire (IF170949), New Delhi, India for the award of
research fellowship.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.ijhydene.2019.05.168.
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Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO
3
/Bi
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enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/
j.ijhydene.2019.05.168
