Integrated Z-Scheme Nanosystem Based on Metal Sulfide Nanorods for Efficient Photocatalytic Pure Water Splitting

Article abstract of DOI:10.1002/cssc.202002171

Developing efficient metal sulfides for pure water splitting is a challenging topic in the field of photocatalysis. Herein, inspired by natural photosynthesis, an all-solid-state Z-scheme photocatalyst was constructed with Cd_0.9Zn_0.1S (CZS) for water reduction, red phosphorus (RP) for water oxidation, and metallic CoP as the electron mediator. RP@CoP core-shell nanostructures were uniformly attached on the CZS nanorods, which gave rise to multiple monodispersed nanojunctions. The integrated Z-scheme nanosystem exhibited an apparent quantum efficiency of 6.4 % at 420 nm for pure water splitting. Theoretical analysis and femtosecond transient absorption results revealed that the impressive performance was mainly due to efficient hole transfer of CZS, resulting from the intimate atomic contacts between CoP mediator and photocatalysts, together with favorable band alignment. Meanwhile, the multiple monodispersed Z-scheme nanojunctions could provide abundant reaction sites, which was also important for the boosted activity.

Full text of DOI:10.1002/cssc.202002171

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Integrated Z-Scheme Nanosystem Based on Metal Sulfide
Nanorods for Efficient Photocatalytic Pure Water Splitting
Authors: Zhixiao Qin, Xiangjiu Guan, Xu Guo, Penghui Guo, Menglong
Wang, Zhenxiong Huang, and Yubin Chen
This manuscript has been accepted after peer review and appears as an
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To be cited as: ChemSusChem 10.1002/cssc.202002171
Link to VoR: https://doi.org/10.1002/cssc.202002171
0
1/2020

ChemSusChem
10.1002/cssc.202002171
COMMUNICATION
Integrated Z-Scheme Nanosystem Based on Metal Sulfide
Nanorods for Efficient Photocatalytic Pure Water Splitting
Zhixiao Qin, Xiangjiu Guan, Xu Guo, Penghui Guo, Menglong Wang, Zhenxiong Huang, and Yubin Chen*
[
a]
Dr. Z. Qin, Dr. X. Guan, Dr. X. Guo, Ms. P. Guo, Mr. M. Wang, Mr. Z. Huang, Prof. Y. Chen
International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering
Xi’an Jiaotong University
Xi’an, Shaanxi 710049, P. R. China.
E-mail: ybchen@mail.xjtu.edu.cn
Supporting information for this article is given via a link at the end of the document.
3
+
2+
- - [8]
Abstract: Developing efficient metal sulfides for pure water splitting
is a challenging topic in the field of photocatalysis. Herein, inspired
by natural photosynthesis, an all-solid-state Z-scheme photocatalyst
is constructed with Cd0.9Zn0.1S (CZS) for water reduction, red
phosphorus (RP) for water oxidation, and metallic CoP as the
electron mediator. RP@CoP core-shell nanostructures were
uniformly attached on the CZS nanorods, which gave rise to multiple
redox couples, such as Fe /Fe and IO /I , are used as
3
mediators to promote water splitting by consuming
untargeted charge carriers, while the unfavorable shielding
effect and side reactions from ionic redox couples remain
obstacles for further improvement. Recently, transition metal
(
eg. Au, Ag, Rh, Cd) and carbon-based materials (eg. reduced
graphene oxide, carbon dot) as the solid-state electron
mediators were investigated.[ Promising results have been
achieved, because the solid-state mediators provide electron
tunnels to facilitate the consumption of untargeted charge
carriers and inhibit side reactions simultaneously.[
monodispersed
nanosystem exhibited an apparent quantum efficiency of 6.4% at
20 nm for pure water splitting. Theoretical analysis and
nanojunctions.
The
integrated
Z-Scheme
9]
4
femtosecond transient absorption result reveal that the impressive
performance is mainly due to efficient hole transfer of CZS, resulting
from the intimate atomic contacts between CoP mediator and
photocatalysts, together with favorable band alignment. Meanwhile,
the multiple monodispersed Z-scheme nanojunctions can provide
abundant reaction sites, which is also important to the boosted
activity.
10]
To
construct efficient all-solid-state Z-scheme photocatalytic
system, the solid-state electron mediators should have good
conductivity and suitable work function to accommodate
photoinduced electrons in OEP and holes in HEP, as well as
have intimate contacts with photocatalysts to ensure
continuous flow of charge carriers.[ A number of transition
metal compounds have shown metallic characters, and can
form close atomic contacts with metal sulfide
photocatalysts. However, there are few reports on using
metallic transition metal compounds as mediators for Z-
scheme photocatalytic systems. In addition, most currently
developed Z-sheme systems have random distribution of HEP,
OEP, and mediator particles, which is not conductive to
building effective Z-scheme junctions. Therefore, the precise
control of the architecture of the Z-scheme photocatalysts is
crucial to improving the performance.
11]
Developing visible light responsive photocatalysts is
indispensable for efficiently utilizing sunlight to generate
hydrogen from water.[1] Metal sulfides have shown promising
activities for hydrogen evolution under visible light
[
12]
[2]
irradiation. However, sacrificial reagents are needed to
consume the photo-induced holes to avoid photocorrosion
and accelerate charge separation, and metal sulfides are
generally considered unsuitable for pure water splitting. Up
to now, there is only a limited number of studies on metal
sulfides for pure water splitting,[3] and the low efficiencies
restrict their further application. It is believed that the key
point to realize efficient and durable photocatalytic pure
water splitting over metal sulfides is the rapid transfer of
photo-induced holes to avoid self-photooxidation and charge
Herein, an efficient Z-scheme nanosystem is constructed
with Cd0.9Zn0.1S (CZS) for water reduction, red phosphorus
RP) for water oxidation, and metallic CoP as the solid-state
(
electron mediator. The integrated Z-scheme nanosystem
with multiple monodispersed nanojunctions was generated.
Benefiting from the high-quality interfacial contacts between
CoP mediator and photocatalysts, together with favorable
energy band aligment, efficient charge transfer via the Z-
scheme pathway could be realized and hydrogen evolution
from pure water splitting was boosted.
Scanning electron microscope (SEM) and Transmission
electron microscope (TEM) images demonstrated the
successful synthesis of CZS nanorods with a diameter of ~80
nm (Figure S1). As illustrated in Figure S2, CoP nanoparticles
with the size of around 10 nm were tightly attached on the
surface of CZS nanorods. X-ray powder diffraction (XRD)
pattern of RP@CoP/CZS closely resembled these of CZS and
[4]
recombination. Inspired by natural photosynthesis, two-
step excitation (Z-scheme) system has been developed,[5] in
which a hydrogen evolution photocatalyst (HEP) and an
oxygen evolution photocatalyst (OEP) are combined through
an appropriate mediator to realize overall water splitting.[6]
Some pioneer works using metal sulfides as attractive HEP
candidates in Z-scheme systems have been carried out,
where the photogenerated holes of metal sulfides can be
transferred to the electron mediator for the recombination
with photogenerated electrons in OEP. However, it is still
challenging to build efficient Z-scheme water splitting
systems with metal sulfides.[7]
The electron mediator plays an important role in
distracting the photo-induced holes in HEP. Usually, ionic
1
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ChemSusChem
10.1002/cssc.202002171
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after the deposition of CoP or RP, which were caused by the
electronic interaction between different components due to
intimate interfacial contacts (Figure S6). The weaker peak of
Co 2p in RP@CoP/CZS could be ascribed to the core-shell
structure with CoP covered by amorphous RP (Figure 1f). The
similar Co 2p peak positions in CoP/CZS and RP@CoP/CZS
indicated that CoP was not reduced to metallic Co during
phosphorization reaction. The P 2p signals of RP@CoP/CZS
were at 129.7 and 130.6 eV, which were in accordance with
those of RP (Figure 1g and S6d).[ As a comparison, there
were not typical signals of RP in the XPS spectra of CoP/CZS
without core-shell structures, which indicated that the shell
in RP@CoP/CZS corresponded to RP.
15]
The optical properties were analyzed by UV-visible (UV-vis)
absorption spectra. As shown in Figure 2a, RP@CoP/CZS
showed absorption characters of both CZS and RP. The band
g
gap values (E ) of CZS and RP were determined to be around
Figure 1. (a) TEM and (b) HRTEM images of RP@CoP/CZS. (c) STEM image
and corresponding EDS elemental mapping of RP@CoP/CZS. (d) Schematic
illustration of forming RP@CoP core-shell structure. (e) Raman spectra of
CZS, CoP/CZS, and RP@CoP/CZS excited at 514 nm. (f) Co 2p and (g) P 2p
XPS spectra of CoP/CZS and RP@CoP/CZS.
2.49 and 2.01 eV from Tauc plots (Figure S7). Ultraviolet
photoelectron spectroscopy (UPS) of CZS and RP were shown
in Figure 2b. According to the linear intersection method, the
valence band (VB) values of CZS and RP were estimated to be
1
.52 and 1.81 V vs. NHE, respectively (Figure S8).
Correspondingly, the conduction band (CB) values of CZS and
RP were -0.97 and -0.20 V vs. NHE. Meanwhile, the
theoretical calculation demonstrated the metallic behavior of
CoP (Figure S9). The schematic band diagram of CZS, CoP,
and RP before contact was presented in Figure 2c based on
CoP/CZS (Figure S3). When coupling CoP and/or RP with CZS,
no characteristic diffraction peaks for CoP or RP could be
observed for the composite samples, probably due to the low
amount (2 wt%) and high dispersion of CoP nanoparticles, as
well as the amorphous phase of RP. SEM and TEM images
f
the afore-obtained band structure. The Fermi level (E ) of CoP
(
Figure 1a and S4) showed that larger particles with a
was between those of CZS and RP (Figure S9). After contact,
the charge transfer occurred in the system until their Fermi
levels reached an equilibrium.[ The electron transfer from
CZS to CoP could explain the positive shift of the Cd 3d, Zn 2p
diameter of ca. 25 nm grew on the surface of CZS nanorods in
RP@CoP/CZS. The close examination revealed that these
nanoparticles displayed a core-shell structure (Figure 1b and
S4) in which the crystalline CoP particle was surrounded by
amorphous RP. Meanwhile, CoP nanoparticles have intimate
atomic contacts with CZS nanorods and RP layers. Scanning
TEM (STEM) and corresponding elemental mapping images
clearly proved the existence of Cd, Zn, S, Co, and P (Figure 1c),
with much stronger P signal in the region of larger core-shell
nanoparticles. Energy dispersive X-ray (EDX) line scan
mapping also suggested that P signal was varied across the
nanorods, and the P signal arose before the signal of Co in
the core-shell region (Figure S5). These results indicated the
formed shell was RP. The formation mechanism of
16]
amorphous RP can be illustrated in Figure 1d, NaH
2 2
PO
decomposed to release PH , which would fill the reaction
3
space. Owing to the nucleation effect of CoP, RP layers were
in-situ formed around CoP nanoparticles, leading to intimate
interfacial contacts. Therefore, RP@CoP core-shell
nanostructures were uniformly attached on the surface of
CZS nanorods, which gave rise to multiple monodispersed
nanojunctions.
Raman spectra and X-ray photoelectron spectroscopy (XPS)
were further examined to identify RP. As depicted in Figure
Figure 2. (a) UV-vis absorption spectra of CZS, RP, and RP@CoP/CZS. (b)
UPS spectra of CZS and RP. The insets show the enlarged view of the
rectangle areas. Schematic illustration of (c) band structures before contact
and (d) Z-scheme charge transfer pathway after contact.
-
1
1
e, the Raman peaks at ~300 and 600 cm corresponded to
the first- and second-order longitudinal optical (LO) modes of
CZS.[ For RP@CoP/CZS, the additional peak at 480 cm was
13]
-1
assigned to the characteristic peak of RP.[14] The XPS binding
energies of Cd, Zn, and S in CZS slightly shifted to high values
2
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ChemSusChem
10.1002/cssc.202002171
COMMUNICATION
be excited from the GS to the excited state (ES) of CZS.
Subsequently, fast process of charge trapping from ES to TS
and charge recombination from ES to GS occurred (Figure 3e).
For CZS, the short and long lifetimes were respectively 13 and
1
38 ps (τ
lifetimes from excited CZS were increased to 15 and 182 ps
, τ , Table S1). The longer electron lifetimes were owing to
1 2
, τ , Table S1). With regard to CoP/CZS, the electron
(
τ
1
2
the presence of a hole transfer pathway from the VB of CZS
to CoP mediator (Figure 3f). For RP@CoP/CZS (Figure 3g), the
electron lifetimes of 16 and 301 ps (τ , τ , Table S1) became
1 2
much longer compared to those in CoP/CZS, implying that
the hole transfer of CZS was further accelerated due to the
electron injection from RP to CoP mediator for the
recombination with holes from CZS. The efficient charge
transfer in RP@CoP/CZS heterostructure was also proved by
photocurrent responses and electrochemical impedance
spectra (EIS) Nyquist plots (Figure S11 and S12).[ In addition,
linear sweep voltammetry (LSV) results demonstrated that RP
could efficiently promote water oxidation reaction (Figure
S13).
19]
The obtained samples were tested for water splitting
under visible light irradiation without using sacrificial
reagents. CZS, CoP/CZS, or RP did not show apparent
hydrogen evolution, while RP@CoP/CZS delivered
a
significantly boosted hydrogen evolution rate (Figure 4a).
Notably, the apparent quantum efficiency (AQE) for
hydrogen production reached 6.4% at 420 nm without the
assistance of cocatalysts. To our knowledge, this is the best
value for pure water splitting utilizing metal sulfide-based
photocatalysts (Table S2). However, the molar ratio of
Figure 3. Fs-TA spectra of (a) CZS, (b) CoP/CZS, and (c) RP@CoP/CZS at
various delay times with the excitation of 400 nm laser. (d) Normalized fs-TA
decay kinetics (dotted lines) with biexponential fitting curves (solid lines)
probed at 490 nm. Schematic representation of proposed charge transfer and
recombination routes in (e) CZS, (f) CoP/CZS, and (g) RP@CoP/CZS.
generated H
2
and O
2
was not as expected 2:1 after
was
photocatalytic reaction (Figure 4b and S14). When MnO
2
and S 2p peaks of RP@CoP/CZS to more positive binding
energy values as compared to CoP/CZS and CZS (Figure S6).[17]
As shown in Figure 2d, the band bending in RP@CoP/CZS
facilitated the separation and transfer of photoinduced
charges via the Z-scheme pathway, while CoP with metallic
nature served as an electron mediator. In comparision, a
traditional type II charge tranfer was unfavorable (Figure S10).
Following Z-scheme pathway, the holes in VB of CZS and
electrons in CB of RP would transfer to CoP electron mediator
for recombination. As a result, the strong reducibility of
electrons in the CB of CZS and oxidizability of holes in the VB
of RP were retained to drive redox reactions.
To further study the charge transfer behavior in
RP@CoP/CZS, femtosecond transient absorption (fs-TA)
spectroscopy was measured.[18] Upon excitation by 400 nm
light, all samples showed broad negative absorption (Figure
3
a-c). Over the next 350 picosecond (ps), the negative peaks
dramatically reduced in intensity. Kinetic traces of
photoinduced charges were monitored under 490 nm, which
corresponded to the bleach feature of photoexcited
electrons in CZS (Figure 3d). The decay curves could be fitted
with biexponential decay, and the fitting parameters were
Figure 4. (a) Photocatalytic hydrogen production from pure water splitting
under visible light irradiation (λ > 420 nm). (b) Amounts of H and O evolution
by adding MnO into the suspension after one hour of photocatalytic reaction.
(c) RRDE current-time curves (Idisk disk current; ring current). (d)
Detection of superoxide radicals with DMPO in MeOH and hydroxyl radicals
with DMPO in H O by EPR.
2
2
2
summarized in Table S1. Generally, the short lifetime (τ
1
) and
:
ring
I :
long lifetime (τ ) components were assigned to electrons
2
2
trapped at defect-related trap state (TS) and recombined at
ground state (GS).[4b] Upon light irradiation, electrons could
3
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ChemSusChem
10.1002/cssc.202002171
COMMUNICATION
introduced into the suspension after reaction, the H
2
amount industrial product, was the main oxidation product in this
remained unchanged and O
The eventual ratio of evolved H
indicated that H was generated during photocatalytic ideas to design an efficient Z-scheme system for
oxidation of water. To further identify whether it was a water photocatalytic pure water splitting. Meanwhile, the multiple
splitting process, MnO was added into the solution before monodispersed Z-scheme nanojunctions provide abundant
irradiation and the evolved H and O were detected reaction sites, which is also important to the enhanced
simultaneously. As shown in Figure S15, the molar ratio of photocatalytic hydrogen production.
generated H and O was as expected 2:1. Because the In summary, all-solid-state metal
incident light would be absorbed by the MnO catalyst, the photocatalytic Z-scheme system was successfully constructed.
RP@CoP/CZS exhibited low activity after MnO was added.
as the oxidation product was also detected by
2
amount was gradually increased. system. The favorable two-electron kinetics in oxidation
2
and O was about 2:1, which reaction can promote the performance, which offers new
2
2 2
O
2
2
2
2
2
sulfide-based
2
2
Accelerated hole transfer in metal sulfide photocatalysts via
Z-scheme approach and favorable monodispersed
2 2
H O
electrochemical analysis (Figure S16). The electron transfer nanojunctions lead to efficient hydrogen evolution from pure
number (n) under visible light irradiation was determined to water splitting, achieving an AQE of 6.4% at 420 nm, which is
be 2.19 via the rotating ring disk electrode (RRDE) collection the highest value of metal sulfide-based photocatalysts. This
experiment (Figure 4c, the calculation details are shown in work demonstrates the possibility of utilizing metal sulfide-
the Supporting Information).[20] Therefore, pure water based photocatalysts for efficient pure water splitting and
splitting over RP@CoP/CZS under visible-light irradiation was provides new insights to design integrated Z-scheme systems
dominated by a two-electron process for H
2
O
2
evolution. with precisely controlled configurations.
-
Besides H
2
O
2
, ·OH and ·O
2
radicals were also detected in
photocatalytic systems by electron paramagnetic resonance
EPR) measurements (Fig. 4d). The formation of H could
be associated with a series of complicated chemical reactions
(
2 2
O
-
[21]
with the assistance of ·OH and ·O
generation via four-electron process, H
two-electron pathway is kinetically prone to occur.
2
radicals. Compared to
O
2
2 2
O
evolution via
[
22]
The
stability test indicated that after three cycles in pure water,
the photocatalytic activity of RP@CoP/CZS maintained 70%
of its initial activity (Figure S17). As shown in XRD patterns
Figure 5. Schematic illustration of photocatalytic water splitting in
RP@CoP/CZS via Z-scheme approach under visible light irradiation.
(
Figure S18), there were not apparent changes before and
after photocatalytic reaction, which revealed the good
stability of its crystal structure. P 2p XPS results of
photocatalysts before and after the reaction didn’t change
apparently (Figure S19), suggesting that RP was not
consumed as a sacrificial reagent during reaction. TEM image
after photocatalytic reaction showed kind of photocorrosion
Acknowledgements
The authors thank supports from the National Natural
Science Foundation of China (Nos. 51888103, 52076177),
Shaanxi Technical Innovation Guidance Project (Grant No.
(
Figure S20), probably due to oxidization by photo-induced
2
018HJCG-14), and Fundamental Research Funds for the
holes. Although the holes in VB of CZS and electrons in CB of
RP would transfer to CoP electron mediator for
recombination, the photocorrosion could not be fully avoided.
The particular role of metallic CoP as a solid-state electron
mediator in the integrated Z-scheme photocatalytic water
splitting system can be illustrated in Figure 5. Upon excitation
with visible light, electrons are photoinduced from the VB of
CZS and RP to their CB. The holes in VB of CZS and electrons
in CB of RP would transfer to CoP electron mediator for
recombination. Benefiting from the intimate atomic contacts
between CoP mediator and photocatalysts, as well as
favorable energy band bending, efficient charge transfer via
the Z-scheme pathway could be realized. The electrons in the
Central Universities (No. xzy012019017).
Keywords: Photocatalysis • Z-scheme • Metal sulfide • Water
splitting• Hole transfer
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ChemSusChem
10.1002/cssc.202002171
COMMUNICATION
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An integrated Z-scheme nanosystem is constructed with Cd0.9Zn0.1S (CZS) for water reduction, red phosphorus (RP) for water
oxidation, and metallic CoP as the solid-state electron mediator. The strong reducibility of electrons in CZS and oxidizability of holes
in RP are retained to drive pure water splitting without the assistance of any cocatalysts, achieving an apparent quantum efficiency of
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.4% at 420 nm.
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