Visible-light-driven CO2 photoreduction over Zn: XCd1- xS solid solution coupling with tetra(4-carboxyphenyl)porphyrin iron(iii) chloride

Article abstract of DOI:10.1039/c8cp02774a

Construction of solid solution semiconductors has attracted much attention in photocatalysis by virtue of their tunable elemental composition and band structure. The integration of semiconductor sensitizers with molecular catalysts provides a promising wa

Full text of DOI:10.1039/c8cp02774a

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Physical Chemistry Chemical Physics
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DOI: 10.1039/C8CP02774A
Physical Chemistry Chemical Physics
ARTICAL
Visible-light driven CO photoreduction over Zn Cd S solid
2
x
1-x
solution coupling with tetra(4-carboxyphenyl)porphyrin iron(III)
chloride
Received 00th January 20xx,
Accepted 00th January 20xx
Pan Li,a,c Xuehua Zhang,* Chunchao Hou, Lin Lin, Yong Chen* and Tao He*a,c
a
b
a
b,c
DOI: 10.1039/x0xx00000x
www.rsc.org/
Construction of solid solution semiconductor has attracted much attention in photocatalysis by virtue of the tunable
elemental composition and band structure. The integration of semiconductor sensitizers with molecular catalysts provides
a promising way to fabricate highly efficient, selective and stable system for CO
solutions with well-defined floccule-like morphology comprised of nanoribbons are synthesized and used as the
photosensitizer to couple with tetra(4-carboxyphenyl)porphyrin iron(III) chloride (FeTCPP) for CO reduction. The effects of
changes in surface atom of the ZCS solid solution on the performance of CO photoreduction are investigated. Regardless
of the presence of FeTCPP, our results show that introduction of Zn into CdS can affect the activity and selectivity of CO
2
photoreduction. Here Zn
x
Cd1-xS (ZCS) solid
2
2
2
photoreduction, as well as stability of the obtained photocatalysts. More important, the presence of Zn can build efficient
electron transfer channels from ZCS to FeTCPP and, thus, greatly facilitating the interfacial charge transfer. Benefitting
from the efficient charge separation and electron transfer, ZCS-1/FeTCPP (Zn0.14Cd0.84S/FeTCPP) exhibits the highest activity
for CO
2
reduction under visible-light irradiation, with CO yield of 1.28 mol and selectivity up to 93% after 4 h.
antenna and molecular catalyst works as the catalytic center,
in which the strong light-harvesting ability and effective
interfacial electrons transfer are essential. Hence, modulation
Introduction
Inspired by natural photosynthesis, photocatalytic reduction of
CO into solar fuels is regarded as a promising solution to
supply renewable energy and mitigate carbon emission.
of the band structure (conductor/valence band positions and
band gap value) of the semiconductor sensitizer and
interaction between the semiconductor sensitizer and
molecular catalyst are very critical for achieving highly efficient
semiconductor/molecular catalyst hybrid photocatalytic
2
1
-6
Molecular catalysts with multiple and accessible redox states
can facilitate multi-electron transfer process, and thus have
been widely used for CO
2
photoreduction due to their
11-14
systems.
Construction of Zn
extensive attention in the field of photocatalysis by virtue of
7
,8
excellent activity and selectivity. However, the stability of
molecular catalysts is a major obstacle to develop highly
efficient photocatalytic systems. Considering inorganic
semiconductors usually show superior stability and strong
light-absorption ability, semiconductor/molecular catalyst
heterogeneous hybrid systems combining the strengths of
molecular catalyst and inorganic semiconductor can provide a
promising solution to construct highly efficient, selective and
x
Cd1-xS (ZCS) solid solution has attracted
15-17
continuously adjustable band structure.
More importantly,
introduction of Zn can tune the structure of surface atoms in
CdS, which will affect the adsorption/desorption of the
reactants, intermediates, products, and even the hybrid
interaction with molecular catalyst and, thereby, have great
impact on the photocatalytic activity. However, the integration
1
,9-14
stable systems to mimic natural photosynthesis.
hybrid system, the semiconductor acts as light-harvesting
In such a
of ZCS solid solution with molecular catalyst in CO
2
photoreduction has rarely been studied. Several key scientific
issues must be considered in the hybrid system, such as the
balance between light absorption and redox ability in ZCS and
the electron transfer channel from ZCS to molecular catalyst.
What is the predominate factor, or are these factors
synergistic? Unfortunately, such questions have rarely been
a.CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for
Excellence in Nanoscience, National Center for Nanoscience and Technology,
Beijing 100190, China.
Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
b.
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing
1
00190, China.
studied in CO
molecular catalysts have been employed as electro- or
photocatalysts for CO reduction to CO due to its high activity
and selectivity, earth abundant, and environmentally benign.
2
photoreduction hitherto. Iron porphyrins-based
c.
University of Chinese Academy of Sciences, Beijing 100049, China.
E-mail: het@nanoctr.cn; zhangxh@nanoctr.cn; chenyong@mail.ipc.ac.cn
Tel. +86-10-82545655
Electronic Supplementary Information (ESI) available: Chemicals, characterization,
2
2 2
EDX mapping, N adsorption-desorption isotherms, CO chemical adsorption
isotherms, ICP-MS, Mott-Schotty plots, time dependence of CO evolution amount However, it is noted that majority of the iron porphyrins are
upon CO
after CO
2
photoreduction, UV-vis absorption spectra; SEM, XRD and XPS spectra
reduction. See DOI: 10.1039/x0xx00000x
2
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ARTICLE
Journal Name
o
inactive for CO
without efficient photosensitizers.
2
reduction under visible-light irradiation if of 15 C was illuminated by a 300 W Xe lamp (Microsolar 300,
View Article Online
DOI: 10.1039/C8CP02774A
14,18-20
Beijing Perfect light Technology Co., Ltd.) with a 420 nm cut off
Here we have synthesized ZCS solid solutions with well- filter and an IR-cut filter (i.e., 420 nm < λ < 780 nm), the
–
2
defined floccule-like morphology comprised of nanoribbons via intensity was 260 mW cm . The gas products were analyzed
a facile solvothermal method and fabricated a highly efficient by using Agilent 7890A gas chromatography equipped with a
ZCS/tetra(4-carboxyphenyl)porphyrin
ZCS/FeTCPP) heterogeneous hybrid
photoreduction of CO to CO under visible-light irradiation. (without CO
iron(III)
chloride hydrogen flame ionized detector (FID) and a thermal
for conductivity detector (TCD). A series of control experiments
purging, blank solvent without the photocatalysts,
(
catalysts
2
2
The composition of the obtained ZCS, i.e., Zn/Cd molar ratio, without illumination, and only FeTCPP) were performed to
was modulated by changing the molar ratio of the starting ensure that the observed products come solely from
materials. The effects of the changes in local surface photoreduction of CO
environment/atom and band structure of ZCS on the
2
.
photocatalytic activity of ZCS/FeTCPP hybrid photocatalysts for
Results and discussion
Structure, morphology and optical properties
CO
not the ZCS is integrated with FeTCPP, introduction of Zn into
CdS can greatly change the activity and selectivity of CO
2
reduction have been studied systematically. Whether or
2
Fig. 1(a) displays the XRD patterns of the ZCS powder with
different molar ratio of Zn/Cd. It is found that the CdS, ZCS-1
and ZCS-2 have similar diffraction patterns to those of
hexagonal CdS (JCPDS: 41-1049). From CdS, ZCS-1 to ZCS-2, the
diffraction peaks gradually shift to a larger diffraction angle
photoreduction. This is mainly because, compared with the
pure CdS, the formation of ZCS solid solution can build
effective interfacial electron transfer channels from ZCS to
FeTCPP.
2+
2+
due to smaller ionic radius of Zn (0.74 Å) than that of Cd
0.94 Å), which is the characteristics of the ZCS solid solution.
However, several small individual peaks appearing at around
(
Experimental
Synthesis of ZCS solid solution and FeTCPP
o
o
o
3
0.5 , 39.8 and 56.5 that can be attributed to the hexagonal
All the reagents are of analytical grade and used without Wurtzite phase ZnS (JCPDS: 36-1450) are present in the sample
further purification. ZCS solid solution was synthesized via a ZCS-3, implying the occurrence of phase separation with the
solvothermal method using ethylenediamine as solvent and increased Zn content. The structural homogeneity and the
structure-directing agent. In a typical synthesis, 0.5 mmol chemical compatibility of the compounds are the prerequisites
3 2 2 3 2 2 2 2
Zn(NO ) ∙6H O, 0.5 mmol Cd(NO ) ∙4H O and 3 mmol CS(NH )
20
30
40
50
60
70
80
CdS-PDF#41-1049
were dissolved into 30 mL ethylenediamine under constant
stirring at room tempreture for 0.5 h. Subsequently, the
mixture was transferred into an autoclave and heated at 110
CdS
CdS
ZCS-1
ZCS-2
ZCS-3
o
C for 24 h in a furnace. After cooling to room temperature,
ZnS
the obtained precipitate was separated by centrifugation and
washed with ethanol and deionized water alternatively, and
ZnS-PDF#36-1450
2
0
30
40
50
60
70
80
2
(degree)
o
dried overnight in vacuum at 65 C. Finally, the obtained
ZCS-1
ZCS-2
o
powder was annealed at 400 C for 2 h under Ar to remove the
adsorbed ethylenediamine. A series of ZCS solid solutions with
different molar ratio of Zn/Cd were synthesized by changing
3 2 2
the molar ratio of the starting material Zn(NO ) ∙6H O and
Cd(NO ∙4H O (i.e., 0:1, 0.25:0.75, 0.5:0.5, 0.75:0.25 and 1:0),
)
3 2
2
which were denoted as CdS (ZCS-0), ZCS-1, ZCS-2, ZCS-3 and
ZnS (ZCS-4). FeTCPP was synthesized according to the protocol
reported previously.
Cd
Zn
ZCS-1
14,21
Photocatalytic reduction of CO
The photocatalytic reduction of CO
photoreaction system (Labsolar-IIIAG, Beijing Perfect light
Technology Co., Ltd.) saturated with 29 KPa CO using
2
2
was carried out in a
2
S
14
previously reported method. Briefly, 50 mg ZCS solid solution
powder with or without 0.3 mg FeTCPP molecular catalyst was
suspended under stirring in 100 mL solution of
acetonitrile/water/TEOA (3:1:1, v:v:v). The mixed solution was
vacuumed and purged with Ar for several times and then was
continuously purged high purity CO
2
gas for at least 1.5 h Fig. 1 XRD patterns, SEM images, and EDX mapping (only ZCS-1) of the synthesized ZCS
before illumination. The reactor with a constant temperature
samples.
2
| J. Name., 2012, 00, 1-3
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so as to avoid phase separation. As reported previously, it is nanoribbons. In addition, the EDX results from TEVMiew(ATraticbleleOnSli1ne)
difficult to directly prepare the ZCS with a large Zn content due are in good agreement with SEM results. DOI: 10.1039/C8CP02774A
2+
to the discrepant physicochemical property between Cd and
The typical N
Zn ions [17]. In addition, incorporation of Zn into CdS lattice adsorption isotherms of CdS, ZCS-1 and ZCS-2 are displayed in
is much more favourable than the reverse process since Zn Fig. S2. BET surface area and CO chemical adsorption
2 2
adsorption-desorption and CO chemical
2+
2
have a higher mobility than Cd owing to its smaller radius and capability are listed in Table S1. The BET specific surface area is
2
lighter mass [22]. These may account for the phase separation determined to be 57.99, 72.95 and 95.51 m /g for CdS, ZCS-1
occurring at high Zn content. Therefore, only CdS, ZCS-1 and and ZCS-2, respectively, which exhibits a trend of gradual
ZCS-2 will be discussed in the following sections.
increase with increasing Zn content. As for CO
2
chemical
Fig. 1 also shows the FESEM images of CdS, ZCS-1 and ZCS-2. adsorption capability, ZCS-1 shows the highest CO
All the synthesized samples exhibit very beautiful floccule-like capability (18.06 m /g) among the three samples. This is
2
adsorption
3
2+
morphology. Interestingly, when Zn ions are incorporated reasonable as the CO
into the lattice of CdS, the floccule-like morphology can be well surface active sites, not just the BET value measured from the
maintained. The chemical compositions are characterized by adsorption of N experiment.
using energy-dispersive X-ray spectroscopy (EDX), which are The chemical composition and electronic structures of CdS,
listed in Table S1. However, the Zn/Cd ratios of ZCS-1 and ZCS- ZCS-1 and ZCS-2 were analyzed by XPS (Fig. 3a). Two peaks
2
adsorption is highly dependent on the
2
2
(
are not well consistent with that used in the raw materials located at 1045.1 and 1021.1 eV with a spin orbit separation of
Zn2+ and Cd ). The elemental contents for CdS, ZCS-1, and 24.0 eV in Fig. 3(b) are attributed to Zn 2p1/2 and Zn 2p3/2 of
2+
2+
ZCS-2 obtained from EDX analysis are measured to be around Zn , respectively. The peaks at 411.5 and 404.8 eV in Fig. 3(c)
Cd0.97S, Zn0.14Cd0.84S, and Zn0.23Cd0.83S, respectively. correspond to Cd 3d3/2 and Cd 3d5/2, respectively. The splitting
Additionally, inductively coupled plasma-mass spectrometry energy of 6.7 eV between Cd 3d3/2 and Cd 3d5/2 is a typical
2+
23,24
(ICP-MS) was further carried out to determine the Zn content. value for Cd in sulfides.
The peaks at around 162.2 and
The atomic ratios of Zn to Cd (Zn/Cd) in ZCS-1 and ZCS-2 161.3 eV in Fig. 3(d) are ascribed to S 2p1/2 and S 2p3/2
,
2−
calculated from ICP-MS are listed in Table S2, which are very respectively, which are typical values for S . The Zn 2p spectra
similar to those obtained from EDX results. EDX-mapping is exhibit almost the same binding energy for ZCS-1 and ZCS-2;
then utilized to get further insights into the element spatial while the binding energy for both Cd 3p and S 2p shifts slightly
distribution. The corresponding elemental mapping images of to a higher value from CdS, ZCS-1 to ZCS-2, due to the
Zn, Cd and S in ZCS-1 are shown in Fig. 1, while the elemental formation of ZCS solid solutions.
mapping images of CdS and ZCS-2 are shown in Fig. S1. It
UV-vis absorption spectra of CdS, ZCS-1 and ZCS-2 are
clearly reveals the homogeneous distribution of S, Cd and/or shown in Fig. 4(a). The optical absorption edge shows a
Zn in CdS, ZCS-1, and ZCS-2. Moreover, there is no phase gradual blue shift with the increased Zn content. The band gap
separation in ZCS-1 or ZCS-2, confirming the formation of ZCS (E
solid solution.
g
) has been estimated from these spectra using the Kubelka-
Munk method (inset of Fig. 4(a)). The E value of CdS, ZCS-1
g
TEM images further identify the floccule-like morphology of and ZCS-2 is calculated to be 2.42, 2.46, and 2.51 eV,
CdS, ZCS-1 and ZCS-2, which is comprised of nanoribbons (Fig. respectively. So the introduction of Zn into CdS increases the
2
). The selected area electron diffraction (SAED) patterns
g g
E slightly, indicating that the E of ZCS solid solutions can be
clearly show the polycrystalline nature of the whole flower, of tuned via tailoring the Zn/Cd ratio. XPS valence spectroscopy
which the diffraction circles are assigned to (100) and (002) (Fig. 4b) was measured to estimate the valence band (EVB),
crystal facet. The corresponding fast Fourier transform (FFT) which is determined to be about 1.18, 1.20 and 1.24 V (vs NHE)
patterns indicate the crystalline nature of the individual
Cd3d
CdS
ZCS-1
ZCS-2
(
a)
3/2
(b)
ZCS-1
ZCS-2
Zn2p
3
/2 Cd3d5
/2
Zn2p
1/2
Cd3p
(
a)
b)
3/2
Cd3p
5/2
S2p
(
100)
Cd4d
(
002)
Cd(MNN)
140012001000 800 600 400 200
0
1020
1030
1040
1050
(
Binding energy (eV)
Binding energy (eV)
(
100)
(
002)
(c)
CdS
ZCS-1
ZCS-2
(d)
CdS
ZCS-1
ZCS-2
(c)
(
100)
(
002)
400
405
410
415
420
158
160
162
164
166
Binding energy (eV)
Binding energy (eV)
Fig. 3 (a) XPS survey spectra of CdS, ZCS-1 and ZCS-2, high-resolution XPS spectra of (b)
Fig. 2 TEM image, SAED pattern, and live FFT of (a) CdS, (b) ZCS-1, and (c) ZCS-2.
Zn 2p, (c) Cd 3d, and (d) S 2p for CdS, ZCS-1 and ZCS-2.
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CdS
ZCS-1
ZCS-2
DOI: 10.1039/C8CP02774A
(a)
(b)
CdS
ZCS-1
ZCS-2
slightly stronger PL signal than CdS, indicating that apropriate
amount of Zn can reduce the electron-hole combination, i.e.
increase the charge seperation. The broad asymmetric tail in
all of the luminescence peaks can be attributed to the large
amount of the defect/surface states formed during synthesis.25
2
.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0
Photon energy (eV)
2+
2+
Since the ionic radius of Zn and Cd has approximately 10%
difference, compared with the PL spectrum of CdS, the slighly
red-shifted PL spectra of ZCS-1 and ZCS-2 may be caused by
higher amount of the defect/surface states due to the release
of lattice strain in the crystals upon Zn incorporation.22
4
00 450 500 550 600 650 700 750 800
5
4
3
2
1
0
-1 -2 -3 -4 -5
Wavelength (nm)
Binding energy (eV)
Fig.4 (a) UV-vis absorption (inset, Tauc plot) and (b) XPS valence band of CdS, ZCS-1 and
ZCS-2.
Fig. 5(b) shows the time-resolved PL decay curves of the as-
synthesized ZCS samples, the corresponding fitted
fluorescence lifetime and calculated average lifetime are listed
in Table 1. All of the samples show the tri-exponential decay
characteristics, corresponding to the three different processes
for CdS, ZCS-1 and ZCS-2, respectively. Combining with E
g
obtained from Fig. 4(a), the conduction band (ECB) value for
CdS, ZCS-1 and ZCS-2 is calculated to be −1.24, −1.26 and −1.27
V (vs NHE), respectively. The band structure of the obtained
ZCS solid solution changes obviously with the introduction of
Zn. The CB value for all the CdS, ZCS-1 and ZCS-2 is negative
than the redox potential of FeTCPP (−1.02 V, vs NHE),
indicating that the electrons transfer from the CB of CdS and
that contribute to the lifetime, non-radiative process (τ
radiative process (τ ) related to the recombination of
photogenerated electrons and holes, and energy transfer
1
),
2
26,27
process (τ
transfermechanisms, fluorescence resonance energy transfer
FRET) and radiation-reabsorption. FRET is a distance-
3
).
There are two different energy
1
4
ZCS to FeTCPP is thermodynamicly feasible.
(
The Mott-Schottky plots of CdS, ZCS-1, and ZCS-2 (Fig. S3)
are further used to determine the flatband potential (EFB),
which is the key factor that determines the charge transfer
direction, which is ‒1.09 V, ‒1.10 V, and ‒1.16 V (vs. NHE) for
CdS, ZCS-1 and ZCS-2, respectively. So all the EFB values are
dependent interaction between the electronic excited states
of two molecules, in which excitation is transferred from the
donor molecule to acceptor molecule without photon emission.
Here the τ
3
is mainly associated with the radiation-
reabsorption process, as no photon emission and reabsorption
is involved in FRET. The average lifetime (τ) is calculated by the
I/0
negative than the reduction potential of FeTCPP (EFe , −1.02 V
vs NHE), implying that the electron transfer from both CdS and
ZCS to FeTCPP is thermodynamicly feasible. Considering the
equation of τ=f
1
τ
1
+f
contribution. ZCS-2 exhibits a very similar τ to CdS, while ZCS-
has the slowest decay kinetics and shows the longest τ
2 2 3 3
τ +f τ , where f is the relative fractional
2
8
ECB of n-type semiconductors is more negative than EFB, the
1
electron can indeed transfer from both CdS and ZCS to FeTCPP,
consistent with the above results.
among CdS, ZCS-1 and ZCS-2, implying the highest charge
separation efficiency in ZCS-1 (i.e., efficient charge transfer
from ZCS-1 to FeTCPP, as discussed later).
PL and ESR characteristics
The amount of photogenerated electrons are not only related Table 1. Fluorescence lifetime of CdS, ZCS-1 and ZCS-2, derived from time-resolved PL
decay curves.
with the light absorption ability of semiconductor materials,
but also highly dependent on the separation of charge carriers,
Sample
CdS
ZCS-1
ZCS-2
τ
1
/ ns (rel. %)
τ
2
/ ns (rel. %)
τ
3
/ ns (rel. %)
τ / ns
which is usually studied by PL and time-resolved PL
measurements. Fig. 5(a) shows the PL spectra of CdS, ZCS-1
and ZCS-2. CdS exhibits a strong PL signal at around 540 nm
when excited by 350 nm. It is found that ZCS-1 (Zn0.14Cd0.84S)
has weaker PL signal than CdS, while ZCS-2 (Zn0.23Cd0.83S) has a
1.04 (38.07%) 6.05 (31.76%) 100.90 (30.17%) 32.77
0.90 (33.21%) 5.82 (28.88%) 126.48 (37.92%) 49.94
0.99 (38.37%) 6.33 (32.98%) 107.52 (28.65%) 33.27
CdS
ZCS-1
ZCS-2
CdS
ZCS-1
ZCS-2
CdS
ZCS-1
ZCS-2
(b)
(c)
(a)
g=2.003
4
00
500
600
700
800
0
100
200
300
400
500
3400
3450
3500
3550
3600
Wavelength (nm)
Lifetime(ns)
Magnetic field (G)
Fig. 5 (a) PL spectra excited by 350 nm, (b) time-resolved PL decay curves at 540 nm, and (c) ESR spectra of CdS, ZCS-1 and ZCS-2.
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DOI: 10.1039/C8CP02774A
E and ECB; the former can affect the light absorption and the
g
1
1
1
0
0
0
0
0
.4
.2
.0
.8
.6
.4
.2
.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
latter can have an impact on the reduction power) is not
considered, as the difference among the three samples is not
large enough to cause an obvious change.
Compared with the case using pure CdS, the amount of both
CH4 and CO using CdS/FeTCPP hybrid catalysts decreases
(
a)
CH
(b)
CH
4
4
CO
CO
slightly (0.024 and 0.39 mol, respectively), indicating that the
CdS
ZCS-1
ZCS-2
CdS/FeTCPP ZCS-1/FeTCPP ZCS-2/FeTCPP
electrons in the CB of CdS cannot efficiently transfer to FeTCPP.
This maybe because the interaction between CdS and FeTCPP
is not strong enough and/or the FeTCPP may hinder light
harvesting of CdS since FeTCPP has a strong Soret-band
absorption at 428 nm but inactive under visible-light
irradiation without the light-harvesting absorber.3
Fig.6 CO
FeTCPP (0.3 mg) in CH
illumination (420 nm <  < 780 nm), light intensity = 260 mW cm–2
2
photoreduction results over different catalysts (a) without and (b) with
CN:H O:TEOA (3:1:1, 100 mL) mixed solution under visible-light
3
2
.
0,31
It is noted that the charge separation in a semiconductor
can be related to the defects. Therefore, ESR characterization
was further performed. As shown in Fig. 5(c), the typical sulfur
vacancy signals at g = 2.003 was observed.29 ZCS-1 shows the
highest ESR signal, followed by ZCS-2 and CdS. Thus, the
introduction of Zn into CdS can facilitate the formation of
sulfur vacancy, which can affect the catalytic efficiency of
ZCS/FeTCPP hybrid catalysts via two approaches, i.e., charge
separation in ZCS and interaction mode between ZCS and
FeTCPP. The sulfur vacancy can serve as the center for electron
trapping and active sites for CO adsorption. In this sense, one
2
can also say the sulfur vacancy can facilitate the separation of
charge carriers. The more sulfur vacancy, the higher charge
separation efficiency is. This also explains why the CO
chemical adsorption follows the order of CdS < ZCS-2 < ZCS-
(Fig. S2(b)). Furthermore, sulfur vacancy can also enhance the
interaction between the ZCS and FeTCPP, which can promote
the interfacial electron transfer between the ZCS and FeTCPP.
Interestingly, compared with pure ZCS-1 (0.078 and 0.11
for CH and CO, respectively), the amount of CO using ZCS-
1/FeTCPP hybrid catalysts (1.28 mol) increases greatly, while
the amount of CH (0.023 mol) decreases obviously. The TON
mol
4

4

value of the ZCS-1/FeTCPP hybrid catalyst is calculated to be
about 9.2 after 12-h reduction based on the FeTCPP amount
(
Fig. S4). If further increasing the Zn amount (such as ZCS-2),
the CO yield for ZCS-2/FeTCPP (0.27 mol) is much less than
that of ZCS-1/FeTCPP, while still noticeably higher than that of
pure ZCS-2 (0.095 mol). However, the CH yield for ZCS-
/FeTCPP (0.017 mol) is less than that of both ZCS-1/FeTCPP
0.023 mol) and pure ZCS-2 (0.039 mol). Nevertheless,
considering the electrons involved in the CO reduction for the
observed products, the integration with FeTCPP (0.844 and
.744 mol for ZCS-1 and ZCS-1/FeTCPP, respectively; 0.502
and 0.676 mol for ZCS-2 and ZCS-2/FeTCPP, respectively) can


4
2
(



2
2
2

1

improve the production yield, not just greatly change the
reduction selectivity. In addition, it is found that the ZCS solid
solution based hybrid catalyst exhibits higher stability than the
CdS based hybrid catalyst (Fig. S5). Moreover, the ZCS-
Photoreduction of CO
2
over ZCS and ZCS/FeTCPP catalysts
Fig. 6(a) and 6(b) show the respective results of CO
photoreduction over ZCS and ZCS/FeTCPP catalysts under
visible-light illumination (420 nm ≤ λ ≤ 780 nm). The major
4 2
products are CH and CO, with very little amount of H due to
the competitive reaction of water splitting. No any other gas or
liquid products were observed within 4-h irradiation. The
accurate amount of the produced H
to the detection limit of the GC instrument. Without the
FeTCPP, the amount of produced CH and CO from CO
reduction over CdS after 4-h illumination is 0.028 and 0.43
mol, respectively. When the ZCS-1 and ZCS-2 were used, the
amount of CH increase obviously (0.078 and 0.039 mol for
ZCS-1 and ZCS-2, respectively), while the amount of CO
decreases greatly (0.11 and 0.095 mol for ZCS-1 and ZCS-2,
2
1
/FeTCPP catalyst shows linear increase of the CO yield over
upon CO reduction under visible-light illumination even for 12
h (Fig. S5). So the ZCS-1/FeTCPP is quite stable, which is well
2
32,33
consistent with the previous work.
Based on the results of
XRD, XPS, and SEM images of the CdS, ZCS-1 and ZCS-2 after
CO2 photoreduction (Fig. S6 and S7), it is concluded that the
deactivation of CdS/FeTCPP may originate from the instability
of CdS, rather than the decomposition of FeTCPP.
2
cannot be calculated due
4
2
Scheme
1
shows the schematic diagram of CO
2
photoreduction over the ZCS/FeTCPP hybrid catalyst, which
consists of the following processes, generation of
photoelectrons in ZCS upon visible-light illumination, followed

4

by the electrons transfer from ZCS to FeTCPP, and then CO
2

reduction on FeTCPP. Meanwhile, the holes left in VB of ZCS
are consumed by sacrificial agent TEOA. Obviously, the
respectively) (Fig. 6a). These indicate that the introduction of
Zn into CdS can significantly affect the product selectivity of
CO reduction and facilitate the formation of CH , which may
2 4
construction of an efficient ZCS/FeTCPP hybrid for CO
2
photoreduction strongly depends on the charge
generation/separation in ZCS and the effective electron
transfer from ZCS to FeTCPP. However, it seems that the
photogenerated electrons cannot efficiently transfer from CdS
to FeTCPP even in the case that the CB potential of CdS (‒1.24
V) is more negative than the redox potential of FeTCPP (‒1.02
V), as mentioned above.
be due to the different adsorption ability of the intermediates
and products on ZCS solid solutions. Moreover, it is noted that
the amount of both CH
ZCS-1. Considering ZCS-2 has a larger BET value, this is possibly
because the ZCS-2 exhibits inferior CO adsorption capability
4
and CO for ZCS-2 is less than that for
2
and charge separation efficiency to the ZCS-1, as discussed
above. Here the influence of band structure (mainly including
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ARTICLE
Journal Name
solid solution/molecular catalyst hybrid systems, and provide a
feasible approach to fabricate highly efficient heterogeneous
photocatalysts for CO reduction.
2
View Article Online
DOI: 10.1039/C8CP02774A
FeTCPP
FeTCPP
-
2
e-
-1.27V
e
-1.24V
e
-1.26V
-
-
e-
-
1.02V
e
-
-
1
Conflicts of interest
There are no conflicts to declare.
CO2
2
.51 eV 2.46 eV 0
2
.42 eV
CO
TEOA+
1
+
h
h+
.24V
h
+
Zn
Acknowledgements
h+
1
.18V
CdS
Cd
S
1
.20V
ZCS-1
1
TEOA
This work was supported by the National Natural Science
Foundation of China (21673052), the Ministry of Science and
Technology of China (2015DFG62610), and The Belt and Road
Initiative by Chinese Academy of Sciences.
ZCS-2
NHE(V)
2
Scheme 1. Schematic diagram of CO photoreduction over ZCS/FeTCPP hybrid catalyst.
While for the ZCS, introduction of Zn may affect the interaction
between the FeTCPP and ZCS due to the changes in surface
atoms of CdS and/or the differences in the coordination ability
Notes and references
2+
2+
of carboxyl groups with Zn and Cd , which can build an
effective electron transfer channel from ZCS solid solutions to
FeTCPP. It is noted that the ZCS contains more sulfur vacancy
than CdS, which can facilitate the interaction between the ZCS
and FeTCPP. Moreover, the binding energy for both Cd 3p and
S 2p shifts slightly to a higher value upon the introduction of
Zn into CdS (Fig. 3), implying that the electronic density
decreases upon Zn introduction. This is in favor of the
modification of FeTCPP onto the semiconductor, as the
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Physical Chemistry Chemical Physics
Page 8 of 8
View Article Online
DOI: 10.1039/C8CP02774A
Visible-light driven CO photoreduction over
2
Zn Cd S solid solution coupling with tetra(4-
x
1-x
carboxyphenyl)porphyrin iron(III) chloride
a,c
a,
b
a
b,c,
a,c,
Pan Li, Xuehua Zhang, * Chunchao Hou, Lin Lin, Yong Chen, * Tao He *
a
CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence
in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China.
b
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute
of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
c
University of Chinese Academy of Sciences, Beijing 100049, China
Photocatalytic reduction of CO into solar fuels is a promising approach to supply sustainable
2
energy and efficiently use CO as a resource.
2
1