Enabling Visible-Light-Driven Selective CO2 Reduction by Doping Quantum Dots: Trapping Electrons and Suppressing H2 Evolution

Article abstract of DOI:10.1002/anie.201810550

Quantum dots (QDs), a class of promising candidates for harvesting visible light, generally exhibit low activity and selectivity towards photocatalytic CO₂ reduction. Functionalizing QDs with metal complexes (or metal cations through ligands) i

Full text of DOI:10.1002/anie.201810550

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Accepted Article
Title: Enabling Visible-Light-Driven Selective CO2 Reduction by
Doping Quantum Dots: Trapping Electrons and Suppressing H2
Evolution
Authors: Jin Wang, Tong Xia, Lei Wang, Xusheng Zheng, Zeming Qi,
Chao Gao, Junfa Zhu, Zhengquan Li, Hangxun Xu, and Yujie
Xiong
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201810550
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10.1002/anie.201810550
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Enabling Visible-Light-Driven Selective CO2 Reduction by Doping
Quantum Dots: Trapping Electrons and Suppressing H2 Evolution
Jin Wang,[a,b] Tong Xia,[a] Lei Wang,[a] Xusheng Zheng,[a] Zeming Qi,[a] Chao Gao,[a] Junfa Zhu,[a]
Zhengquan Li,[b] Hangxun Xu,*[a] and Yujie Xiong*[a]
Abstract: Quantum dots (QDs), a class of promising candidates for
harvesting visible light, generally exhibit low activity and selectivity
toward photocatalytic CO2 reduction. Anchoring QDs with metal
complexes (or metal cations through ligands) is a widely used
strategy for improving their catalytic activity; however, it suffers from
low selectivity and stability in CO2 reduction. Herein, we report that
doping CdS QDs with transition metal sites can overcome these
limitations to achieve highly selective photocatalytic reactions of CO2
with H2O (100% to CO and CH4), with excellent durability over 60 h.
Doping Ni sites into CdS lattice can effectively trap photoexcited
electrons at surface catalytic sites and substantially suppress H2
evolution. The method reported here can be extended to various
transition metal sites, and offers new opportunities for exploring
QDs-based earth-abundant photocatalysts.
the use of bridging ligands.
Doping is a very versatile route to implant metal cations into
QDs. The dopants can create electronic states in the midgap
region of QDs, which may in turn promote charge migration to
surface and locally trap photoexcited electrons on surface.[10,11]
As such, bulk recombination
 a process detrimental to
photocatalysis can be markedly suppressed. Although various
doped QDs have been utilized for photocatalytic H2 production
and organic pollutant degradation,[12-15] it remains largely
unexplored for CO2 photoreduction. The reduction of protons in
water to H2 mainly competes with the CO2 reduction in such a
system. Thus it would be fundamentally important to explore
whether the activity and selectivity for CO2 reduction can be
improved by doping QDs with transition metal cations.
Herein, we report a highly selective and durable system for
CO2 photoreduction with Ni2+-doped CdS QDs (namely, Ni:CdS
QDs). Our investigation based on various characterization
techniques reveals that the incorporation of Ni sites into CdS
lattice can effectively trap photoexcited electrons at surface
catalytic sites and substantially suppress H2 evolution. As a
result, the selectivity of CO2 reduction versus H2 evolution
reaches 100%, and the catalyst exhibits excellent durability for
more than 60 h.
Sunlight-driven CO2 conversion is an appealing approach to
mitigate the impact of greenhouse gases on the environment.[1-3]
As compared with the conventional light harvesters of inorganic
oxides, metal chalcogenide quantum dots (QDs) possess many
attractive features such as high extinction coefficients, multiple-
exciton generation effects, tunable bandgaps and rich surface-
binding sites,[4-6] emerging as a class of excellent candidates for
visible-light photocatalysis.[7,8] Due to the lack of intrinsic
catalytic sites, these QDs-based photocatalysts have to anchor
metal complexes to achieve CO2 reduction. However, the
organic ligands are unfavorable for shuffling photo-generated
charges for surface reactions.[6,9] Moreover, the dangling bonds
derived from the detachment of metal complexes from QDs form
surface defect states at the QDs, which consequently reduces
reaction activity and selectivity at catalytic sites.[7] Thus directly
implanting catalytic sites into light-harvesting centers (i.e., QDs)
could be an ideal solution to shorten electron transfer pathway
as well as enhance the durability of photocatalysts by excluding
Ni:CdS QDs are obtained by simply introducing Ni2+ cations
into the synthesis of CdS QDs (see details in SI). Transmission
electron microscopy (TEM) (Figure S1) reveals that the particle
size of CdS QDs is maintained around 6.6 nm after Ni doping.
Such a low concentration of Ni dopants does not alter the cubic
structure and crystallinity of CdS QDs (Figure S2). As
determined by UV-vis diffuse reflectance spectroscopy (Figure
S3), the bandgap of pristine CdS QDs is 2.43 eV (Figure 1a),
which has not been much altered after Ni doping. We further
estimate the positions of conduction band edge (ECB) through a
linear sweep voltammetry (LSV) technique according to the
peaks for lattice Cd2+ reduction potentials (Figure 1b).[16,17]
Pristine CdS QDs only exhibit one peak assigned to reduction
potential of Cd2+/Cd0 at -0.95 V versus NHE, while two reduction
peaks appear at -1.03 and -0.63 V for Ni:CdS QDs. As a result,
the ECB for CdS and Ni:CdS QDs should be located around -
0.95 V and -1.03 V, respectively, in good agreement with
previous assignments.[16,17] As such, the valence band edge
(EVB) can be determined as illustrated in Figure 1c. Notably, the
additional reduction peak at -0.63 V versus NHE can be
assigned to reduction potential of Ni2+/Ni0 on the surface of
QDs.[18] This designates that photoexcited electrons can be
transferred to surface Ni sites for CO2 reduction.
[a]
Dr. J. Wang,[+] T. Xia,[+] L. Wang, Dr. X. Zheng, Dr. Z. Qi, Dr. C.
Gao, Prof. J. Zhu, Prof. H. Xu, Prof. Y. Xiong
Hefei National Laboratory for Physical Sciences at the Microscale,
iChEM (Collaborative Innovation Center of Chemistry for Energy
Materials), School of Chemistry and Materials Science, and National
Synchrotron Radiation Laboratory
University of Science and Technology of China
Hefei, Anhui 230026, P. R. China
E-mail: yjxiong@ustc.edu.cn; hxu@ustc.edu.cn
Dr. J. Wang,[+] Prof. Z. Li
Key Laboratory of the Ministry of Education for Advanced Catalysis
Materials, Zhejiang Normal University
Jinhua, Zhejiang 321004, P. R. China
These authors contributed equally to this work.
[b]
[+]
In addition to the doping approach, we prepare two
reference samples (Figure 1d): the CdS QDs anchored with Ni2+
through bridging ligands (namely, CdS-b-Ni),[8] and the CdS
Supporting information for this article is given via a link at the end of
the document.
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QDs mixed with Ni2+ in solution that reaches QD surface through
random collision (namely, CdS-Ni).[19] The ligands in CdS-b-Ni
bridge Ni2+ cations with CdS QDs so that the photoexcited
electrons in QDs can be transferred to the anchored metal
cations (Figure S4-S6).[20,21] In the case of CdS-Ni, a small
amount of Ni2+ can be adsorbed on the surface of CdS QDs and
serve as catalytic sites.[22] Figure 2a shows that the selectivity for
CO2 reduction to CO/CH4 versus H2O reduction to H2
impressively approaches 100% by Ni:CdS QDs among various
samples. The CO2 reduction rate by Ni:CdS QDs is about 4.9
and 3.9 times higher than those by CdS-Ni and CdS-b-Ni
systems, respectively. This indicates that doping Ni into CdS
QDs can dramatically suppress H2 evolution. Note that a trace
amount of CO and H2 is generated by pristine CdS QDs, so the
doped Ni2+ cations may serve as catalytic sites for CO2 reduction.
confirms that the carbon source of CO and CH4 indeed
originates from photocatalytic CO2 reduction by Ni:CdS QDs,
while no CO or CH4 can be detected under Ar atmosphere.
Figure 2. a) Average production rates of CH4, CO and H2 using various
photocatalysts based on CdS QDs. b) Average production rates of CH4, CO
and H2 by CdS QDs at various Ni doping amounts. c) Cycling production of
CH4 and CO using Ni(0.26%):CdS QDs. d) Mass spectra of 13CH4 (m/z = 17)
and 13CO (m/z = 29) produced over Ni(0.26%):CdS QDs in the photocatalytic
reduction of 13CO2.
To elucidate the roles of Ni dopants in enhancing
photocatalytic CO2 reduction, we first examine the charge
behaviors in QDs that are often regulated through metal doping.
Both pristine CdS QDs and Ni:CdS QDs exhibit
a
photoluminescence (PL) around 600 nm (Figure 3a). Such a PL
should originate from the radiative electron-hole recombination
on their surface rather than in bulk. This argument is supported
by the PL spectra collected in the presence of methyl viologen
(MV2+) – an agent to sacrifice surface electrons of QDs, which
displays a weak emission peak for the radiative recombination in
bulk at 500-550 nm (Figure 3b). Typically, enhanced surface
photoemission indicates that more photoexcited electrons can
reach QD surface for radiative recombination with the holes in
valence band, rather than being trapped inside the QDs.[10,11,14]
Here, Ni doping can enhance the PL around 600 nm (Figure S8),
as more photoexcited electrons are transferred to surface and
trapped at surface metal sites for surface-states emission.[10,11,14]
In a practical photocatalytic CO2 reduction, the photoexcited
electrons trapped at surface Ni sites would participate in the
reactions with Ni atoms as catalytic centers. However, the PL
intensity of Ni:CdS QDs around 600 nm is reduced when further
increasing the doping concentration of Ni, as photoexcited
electrons are more trapped at bulk Ni sites and participate in
non-radiative recombination (also see PL decay in Figure S9
and Table S1). It is worth pointing out that the formation of NiS is
another cause for the deteriorative photocatalytic performance at
a very high doping concentration (Figure S10-S13).
Figure 1. a) The bandgaps of CdS QDs and Ni:CdS QDs. b) LSV curves of
CdS QDs, Ni:CdS QDs and free Ni2+ cations in water under identical conditions.
c) Energy band diagrams for CdS QDs and Ni:CdS QDs with respect to the
CO2 reduction potentials (pH ~7). d) Schematic illustration for three different
configurations of combining QDs with metal cations.
We further look into the effect of dopant concentration on
photocatalytic performance as the properties of QDs are often
tuned by dopant concentrations.[13] Figure 2b shows the
photocatalytic performance of Ni:CdS QDs at the Ni contents
from 0.12% to 1.03%, in which a volcano tendency is observed
for the photocatalytic activity with the maximum at 0.26% by
continuously increasing the amount of Ni dopants. Notably
Ni(0.26%):CdS QDs achieve a turnover number (TON) of ~35
and ~300 in terms of Ni atoms and QDs, respectively, which
turns out to be stable in five 12-hour cycles (i.e., 60 h in total,
Figure 2c). In comparison, the catalytic activity for CdS-b-Ni
begins to significantly decay in the second cycle (Figure S7).
The strong metal-sulfur binding in lattice can avoid the leaching
of Ni2+ cations from Ni:CdS QDs,[23] which appears as an
advantage over metal complex catalysts (Figure S7). The
isotopic-label experiment using 13CO2 as substrate (Figure 2d)
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The PL measurements have highlighted the process for
transferring photoexcited electrons from CdS to Ni sites.
Electron spin resonance (ESR) spectroscopy is further
employed to examine the local electronic state of Ni2+ (Figure
3c). Ni(0.26%):CdS QDs give a typical ESR signal in dark that
can be ascribed to the high-spin state of Ni2+ (g = 2.1).[24] After
irradiated for 2 min, the signal for Ni2+ in Ni:CdS QDs is
substantially reduced, which can be assigned to the lowered
spin state by the reduction of Ni2+. In contrast, no distinct ESR
signal can be detected for pristine CdS QDs. This observation
confirms that photogenerated electrons are transferred from
QDs to Ni2+ centers upon photoexcitation. Moreover, near-edge
X-ray absorption fine structure (NEXAFS) spectroscopy (Figure
3d) reveals that the binding energies for Ni:CdS QDs are 0.5 eV
higher than those for pristine CdS QDs, indicating a decrease in
electron density for sulfur.[25] This demonstrates that the
photoexcited electrons trapped by Ni2+ are mainly supplied by
sulfur. Accordingly, varying Cd/S stoichiometry is able to control
the number of sulfur vacancies that trap photoexcited electrons
(Figure S14-S16 and Table S2-S4).[26] In photocatalytic CO2
reduction, the surface Ni sites in sulfur vacancies would receive
the photoexcited electrons to trigger the reduction of adsorbed
Ni:CdS QDs will be transferred to the doped Ni atoms in sulfur
vacancies and relax to shallow charge trap states. At a low
dopant concentration (Figure S8), the electrons are mainly
trapped at the surface Ni sites in the vacancies. The addition of
Ni dopants can enhance the electron trapping in sulfur
vacancies (Figure 3a) so as to substantially promote charge
separation, as confirmed by photocurrent and transient open-
circuit voltage decay measurements (Figure S18 and S19).
CO2 molecules so that
a higher Cd/S ratio can benefit
photocatalytic CO2 reduction.
Figure 4. a) In-situ DRIFTS spectra for the reactions of CO2 with H2O on
Ni(0.26%):CdS QDs under irradiation. b) Schematic illustration for the
proposed mechanism of photocatalytic CO2 reduction at Ni:CdS QDs. c)
Electrochemical polarization curves for HER using various samples in PBS
buffer solution (pH = 7). d) Average production rates of CH4, CO and H2 by
CdS QDs doped with different metal cations.
The improvement of charge separation and transfer by
doping is critical to promote photocatalytic activity; however, it
remains unclear why the selectivity for CO2 reduction can be
improved. The evolution of CO2 and H2O gas at catalytic sites is
investigated by employing in-situ diffuse reflectance infrared
Fourier-transform spectroscopy (DRIFTS) (Figure 4a and S20).
Along with light irradiation, the peaks located from 1200 to 2200
cm–1 arise with the gradually increased intensity for CdS-Ni,
CdS-b-Ni and Ni:CdS QDs due to the formation of intermediate
carbonate species (Table S5).[27,28] Thus the mechanism for
Ni:CdS QDs in photocatalytic CO2 can be outlined in Figure 4b.
In the first step, Ni2+ adsorbs a CO2 molecule and captures two
electrons to give a Ni-CO2 complex as resolved by the feature at
1682 cm–1.[7] The second step is to form Ni-COOH by accepting
a proton from the surrounding medium, corresponding to the
peaks located at 1305 cm-1, 1412 cm–1 and 1640 cm–1 for C–OH,
C–O and C═O stretching, respectively.[29] Subsequently, the Ni-
COOH accepts another proton and undergoes dehydration to
form a Ni-CO complex which exhibits two additional stretching
peaks at 1903 and 1982 cm–1.[7] The CO molecule in Ni-CO can
be readily released and desorbed from the surface of QDs as
indicated by the peak appearing at 2156 cm–1.[30] Intuitively, the
minor product of CH4 should originate from the hydrogenation at
the Ni-CO complex. During this process, the QDs with a high
Cd/S ratio offer abundant coordinatively unsaturated Ni sites for
Figure 3. a) PL emission spectra of bare CdS and Ni(0.26%):CdS QDs (λex
=
380 nm). b) PL emission spectra of bare CdS and Ni(0.26%):CdS QDs in the
presence of MV2+ (10-5 M, λex= 380 nm). c) ESR spectra of bare CdS
and Ni(0.26%):CdS QDs. d) S L2,3-edge NEXAFS spectra of bare CdS and
Ni:CdS QDs.
Based on the information above, we can depict the process
of electron transfer as Figure S17. When the electrons in CdS
QDs are photoexcited, they will be transferred to surface and
relax to surface defect states. This electron trapping by surface
sulfur vacancies becomes prominent at a high ratio of Cd/S
(Figure S15). In comparison, the photoexcited electrons in
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In summary, we have successfully demonstrated that doping
QDs, the most common strategy applied to tune the
photophysical properties of QDs, can markedly reduce CO2 into
CO and CH4 with a nearly 100% selectivity, a TON of ~35 in
terms of Ni atoms and an excellent durability for more than 60 h.
The doped metal sites play a dual role in the photocatalytic
process – trapping photoexcited electrons at surface catalytic
sites and suppressing H2 evolution. This work is the first
experimental report that doping QDs with metal cations can lead
to efficient photocatalytic systems for reducing CO2 with high
selectivity and durability, which provides insights into
simultaneously maneuvering the two key steps – charge transfer
and separation, and surface reactions through
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component. It offers us a valid approach to earth-abundant
photocatalysts for CO2 photoreduction using visible light.
Experimental Section
See Supporting Information for material synthesis and characterization
methods.
Acknowledgements
This work was financially supported in part by National Key R&D
Program of China (2017YFA0207301), NSFC (21725102,
21471141, U1532135, 21701143, 21875235), CAS Key
Research Program of Frontier Sciences (QYZDB-SSW-SLH018),
and CAS Interdisciplinary Innovation Team.
Keywords: photocatalysis • quantum dots • catalytic sites •
doping • CO2 reduction
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Entry for the Table of Contents
COMMUNICATION
Quantum dots (QDs), a class of
promising candidates for harvesting
visible light, commonly possess low
activity and selectivity toward
Jin Wang, Tong Xia, Lei Wang, Xusheng
Zheng, Zeming Qi, Chao Gao, Junfa
Zhu, Zhengquan Li, Hangxun Xu,* and
Yujie Xiong*
photocatalytic CO2 reduction. Doping
CdS QDs with transition metal cations,
which can trap photoexcited electrons
and suppress H2 evolution, provides
an approach to visible-light-driven
highly selective CO2 reduction with
excellent durability.
Page No. – Page No.
Enabling Visible-Light-Driven
Selective CO2 Reduction by Doping
Quantum Dots: Trapping Electrons
and Suppressing H2 Evolution
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