Nickel Doping in Atomically Thin Tin Disulfide Nanosheets Enables Highly Efficient CO2 Reduction

Article abstract of DOI:10.1002/anie.201806043

Engineering electronic properties by elemental doping is a direct strategy to design efficient catalysts towards CO₂ electroreduction. Atomically thin SnS₂ nanosheets were modified by Ni doping for efficient electroreduction of CO₂. The introduction of Ni into SnS₂ nanosheets significantly enhanced the current density and Faradaic efficiency for carbonaceous product relative to pristine SnS₂ nanosheets. When the Ni content was 5 atm %, the Ni-doped SnS₂ nanosheets achieved a remarkable Faradaic efficiency of 93 % for carbonaceous product with a current density of 19.6 mA cm^?2 at ?0.9 V vs. RHE. A mechanistic study revealed that the Ni doping gave rise to a defect level and lowered the work function of SnS₂ nanosheets, resulting in the promoted CO₂ activation and thus improved performance in CO₂ electroreduction.

Full text of DOI:10.1002/anie.201806043

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Accepted Article
Title: Nickel Doping in Atomically Thin Tin Disulfide Nanosheets
Enables Highly Efficient CO2 Reduction
Authors: An Zhang, Rong He, Huiping Li, Yijun Chen, Taoyi Kong, Kan
Li, Huanxin Ju, Junfa Zhu, Wenguang Zhu, and Jie Zeng
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201806043
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10.1002/anie.201806043
Angewandte Chemie International Edition
Revised ms #anie.201806043R1
Nickel Doping in Atomically Thin Tin Disulfide Nanosheets Enables
Highly Efficient CO2 Reduction
An Zhang, Rong He, Huiping Li, Yijun Chen, Taoyi Kong, Kan Li, Huanxin Ju, Junfa Zhu,
Wenguang Zhu, and Jie Zeng*
A. Zhang,[†] R. He,[†] H. Li,[†] Y. Chen, T. Kong, K. Li, H. Ju, Prof. J. Zhu, Prof. W. Zhu, and Prof.
J. Zeng
Hefei National Laboratory for Physical Sciences at the Microscale, Key Laboratory of
Strongly-Coupled Quantum Matter Physics of Chinese Academy of Sciences, National
Synchrotron Radiation Laboratory, Department of Chemical Physics, Department of Physics,
University of Science and Technology of China, Hefei, Anhui 230026, P. R. China
E-mail: zengj@ustc.edu.cn
[†] These authors contributed equally to this work.
[**]This work was supported by Collaborative Innovation Center of Suzhou Nano Science and
Technology, MOST of China (2014CB932700), NSFC (21573206), Key Research Program of
Frontier Sciences of the CAS (QYZDB-SSW-SLH017), Anhui Provincial Key Scientific and
Technological Project (1704a0902013), Major Program of Development Foundation of Hefei
Center for Physical Science and Technology (2017FXZY002), and Fundamental Research Funds
for the Central Universities. This work was partially carried out at the USTC Center for Micro
and Nanoscale Research and Fabrication.
This article is protected by copyright. All rights reserved.

10.1002/anie.201806043
Angewandte Chemie International Edition
Abstract
Engineering the electronic properties by elemental doping represents a direct strategy to
design efficient catalysts towards CO2 electroreduction. Herein, we modified atomically thin
SnS2 nanosheets with Ni doping for efficient electroreduction of CO2. The introduction of Ni into
SnS2 nanosheets significantly enhanced the current density and Faradaic efficiency for
carbonaceous product relative to pristine SnS2 nanosheets. When the Ni content was 5% in
atomic percentage, the Ni doped SnS2 nanosheets achieved a remarkable Faradaic efficiency of
93% for carbonaceous product with a current density of 19.6 mA cm-2 at -0.9 V vs RHE.
Mechanistic study revealed that the Ni doping gave rise to a defect level and lowered the work
function of SnS2 nanosheets, resulting in the promoted CO2 activation and thus improved
performance in CO2 electroreduction.
Keywords: nickel doping; tin disulfide; atomically thin nanosheet; CO2 electroreduction
Table of Contents
2
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10.1002/anie.201806043
Angewandte Chemie International Edition
Electroreduction of CO2 into value-added fuels represents a promising strategy to meet the
ever-increasing energy demands, because the electricity can be powered by abundant renewable
sources such as solar energy and wind energy.[1,2] In CO2 electroreduction, the critical bottleneck
•-
lies in the activation of CO2 into a radical anion (CO2 ) which requires a high reduction potential
of -1.9 V vs NHE.[3,4] Generally, the CO2 activation concerns an electron transfer from catalysts
to a CO2 molecule, which was closely associated with the electronic structure of catalysts.[5,6]
A
powerful method to tailor the electronic structure of catalysts is elemental doping, because the
introduction of heteroatoms leads to the hybridization of energy levels between dopants and
pristine catalysts.[7,8] Typically, the elemental doping has been reported to engineer the band
structure, d-band centre, valence state of active site, and charge redistribution, resulting in the
enhanced electrochemical activity. For example, n-type doping of MoS2 nanosheets with the Zn
dopants shifted the Fermi energy into the conduction band of MoS2, thereby modulating the
hydrogen evolution activity.[9] Besides, the incorporation of P into Ni-based multi-metal oxides
promoted the oxidation of Ni2+ to Ni4+ which was highly active towards oxygen evolution
reaction.[10] Moreover, the incorporated quaternary N atoms in graphene induced charge
redistribution in the p-conjugated system, endowing the adjacent C atoms with enhanced binding
strength of O2 and hence accelerating the oxygen reduction reaction.[11] In addition, the
introduction of isolated Cu into Pd lattice elevated the d-band centre of Cu site for improved CO2
activation, boosting the conversion of CO2 to CH4.[12] Therefore, elemental doping provided a
promising opportunity to facilitate CO2 activation and thereby improve the performance of CO2
electroreduction by modifying the electronic structure of catalysts.
Herein, we modified atomically thin SnS2 nanosheets with Ni doping for enhanced
performance towards CO2 electroreduction. The Ni doped SnS2 nanosheets exhibited higher
current density and Faradaic efficiency (FE) for carbonaceous product than those of the pristine
SnS2 nanosheets. When the Ni content was 5% in atomic percentage, the Ni doped SnS2
nanosheets achieved a remarkable FE of 93% for carbonaceous product with a current density of
19.6 mA cm-2 at -0.9 V vs RHE. During the potentiostatic test, the Ni doped SnS2 nanosheets
remained a steady FE for carbonaceous product without obvious decay of current density.
Mechanistic study revealed that Ni doping generated a defect level at conduction band edge and
decreased the work function of SnS2 nanosheets, which benefited the activation of CO2 and
accordingly improved the catalytic performance in CO2 electroreduction.
3
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10.1002/anie.201806043
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In the standard synthesis of atomically thin Ni doped SnS2 nanosheets, different amounts of
NiCl2 and a certain amount of SnCl4 were added into anisole solution containing oleylamine and
CS2, followed by heating at 190 °C for 24 h. Pristine SnS2 nanosheets were also prepared for
comparison through the similar procedure except for the addition of NiCl2. Determined by the
inductively coupled plasma atomic emission spectroscopy (ICP-AES) results, the atomic ratios
of Ni/(Ni+Sn) were 1%, 3%, 5% and 7%, denoting these samples as 1%Ni-SnS2, 3%Ni-SnS2,
5%Ni-SnS2, and 7%Ni-SnS2 nanosheets, respectively. As shown by the transmission electron
microscopy (TEM) images, the products with the Ni content from 1% to 5% took the nanosheet
morphology, whereas excessive Ni content of 7% led to the formation of inhomogeneous
nanoparticles on the nanosheets (Figs. 1A and S1). Notably, the doped Ni content of 6.3%
corresponded to the nucleation point of NiS supersaturation (Table S1). The high-resolution
transmission electron microscopy (HRTEM) images show that pristine SnS2 and homogeneously
doped Ni-SnS2 nanosheets were highly crystalline with hexagonal structure (Figs. 1B and S2).
The interplanar spacings of two distinct lattice fringes were both 0.32 nm, which can be assigned
to (100) and (010) planes. As such, all the nanosheets were orientated along <001> axis. The
atomic force microscopy images showed the thicknesses of all the nanosheets were around 1.2
nm (Fig. S3), around two S-Sn-S layers[13]. As indicated by the high-angle annular dark field
scanning transmission electron microscopy (HAADF-STEM) image and corresponding energy
dispersive X-ray (EDX) elemental mapping of a freestanding 5%Ni-SnS2 nanosheet, Ni, S, and
Sn elements were homogeneously distributed in the whole nanosheet (Fig. 1C). Such
homogeneous distribution was further confirmed by the line-scan profiles and EDX spectrum
(Figs. 1D and S4). In addition, the HAADF-STEM image and elemental mapping spectra of an
individual 7%Ni-SnS2 nanosheets indicate the existence of NiSx nanoparticles (Fig. S5).
To further investigate the structure of atomically thin Ni doped SnS2 nanosheets, we carried
out a series of characterizations including X-ray diffraction (XRD) and X-ray photoelectron
spectroscopy (XPS) measurements. As evidenced by XRD patterns, Ni doped SnS2 nanosheets
with Ni content lower than 5% exhibited similar diffraction peaks which corresponded to
hexagonal SnS2 (Fig. 1E). As the Ni content increased to 7%, extra diffraction peaks assigned to
NiS appeared, indicating 7%Ni-SnS2 nanosheets were inhomogeneous with NiS phase. We
further performed XPS measurements to elucidate the chemical states of Ni. As shown in the
XPS survey spectra, the weak signals of Ni were recorded in Ni doped SnS2 nanosheets (Fig. S6).
4
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10.1002/anie.201806043
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Figure 1F shows the Ni 2p XPS spectra of 5%Ni-SnS2 nanosheets and 7%Ni-SnS2 nanosheets.
The peaks at 871.0 eV and 853.7 eV were attributed to Ni 2p1/2 and Ni 2p3/2 of 5%Ni-SnS2
nanosheets, respectively. The binding energies of Ni were higher than that of reported NiS and
Ni2S3, suggesting that Ni was most likely to occupy the substitution site of Sn in SnS2.[14] In the
case of 7%Ni-SnS2 nanosheets, Ni 2p1/2 and Ni 2p3/2 are dominated by the peaks at the same
position with that of 5%Ni-SnS2 nanosheets. Another set of peaks are located at 870.1 eV and
852.9 eV, which are ascribed to the binding energy of Ni 2p1/2 and Ni 2p3/2 of NiS.[15] In Sn 3d
and S 2p XPS spectra of pristine SnS2 and Ni doped SnS2 nanosheets, the XPS peaks of Sn and S
were slightly shifted to lower binding energies with the increase of Ni content, which was
attributed to the electron transfer from the doped Ni to Sn and S (Figs. S7 and S8).[16]
The catalytic performance of pristine SnS2 and Ni doped SnS2 nanosheets for CO2
electroreduction was explored in an H-cell containing 60 mL of 0.1 M KHCO3 (Fig. S9). As
shown in Figure 2A, the geometrical current densities of SnS2 nanosheets enhanced with the
increase of Ni contents, which was confirmed by the IR-corrected linear sweep voltammetry
measurements (Fig. S10). At overpotentials of -0.9 V and -1.0 V vs RHE, 5%Ni-SnS2 nanosheets
exhibited considerable current densities of 19.6 mA cm-2 and 29.5 mA cm-2, respectively. The
promotion of current density by the Ni doping was further confirmed by the cyclic voltammetry
curves in N2 and CO2 saturated KHCO3 electrolyte (Fig. S11).Quantified by gas chromatography
1
and H NMR analysis, the product in CO2 electroreduction included formate, CO, and H2 over
pristine SnS2 and Ni doped SnS2 nanosheets. The molar quantity of formate was measured by
1
internal standard method of H NMR (Fig. S12). Figure 2B shows the FE for carbonaceous
product (CO and formate), which reflects the efficiency of CO2 reduction. At -0.9 V vs RHE, the
5%Ni-SnS2 nanosheets exhibited the highest FE of 93% for carbonaceous product. By
comparison, the FE for carbonaceous product over the 7%Ni-SnS2 nanosheets obviously reduced
to 71%. The decreased selectivity was attributed to that NiS in 7%Ni-SnS2 nanosheets preferred
hydrogen evolution to CO2 reduction during the electrochemical reaction.[17] These results
demonstrate that the homogeneous incorporation of Ni into SnS2 nanosheets promoted the
activity and selectivity for CO2 electroreduction.
Through the introduction of Ni into SnS2 nanosheets, the formation of formate as the major
product significantly increased at the expense of hydrogen evolution (Fig. 2C and Fig. S13). At
-0.9 V vs RHE, the maximum FE for formate production over 5%Ni-SnS2 nanosheets reached
5
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10.1002/anie.201806043
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80%, which was 1.6-fold higher than that of pristine SnS2 nanosheets. The FE for H2 production
significantly decreased from 38% over pristine SnS2 nanosheets to 7% over 5%Ni-SnS2
nanosheets. Figure S14 shows the yield rates for formate and CO formation over pristine SnS2
and Ni doped SnS2 nanosheets at -0.9 V vs RHE. Specifically, the formate yield rates over
5%Ni-SnS2 nanosheets were 587.5 mmol h-1 g-1, 293.7 mmol h-1 cm-2, and 4.16 μmol C-1 based
on the catalyst loadings, geometric areas and charges through the system, respectively. Figure
S15 shows the energy conversion efficiency for formate production (Φformate), which was used to
evaluate the utilization of electricity. At -0.7 V vs RHE, the 5%Ni-SnS2 nanosheets achieved the
highest Φformate value of 57% among the Ni doped SnS2 nanosheets. Considering that the partial
current density for formate production (jformate) represents the production rate of formate, we
further compared the jformate values in this work with that of the recently reported Sn-based
catalysts (Fig. S16). Due to the low overpotential and high partial current density, the 5%Ni-SnS2
nanosheets represented one of the best catalysts for formate production. During an 8-h
potentiostatic test at -0.9 V vs RHE, the 5%Ni-SnS2 nanosheets exhibited less than 3% decay in
geometrical current density, with a steady FE of more than 90% for carbonaceous product (Fig.
2D). Moreover, the Ni 2p XPS spectrum after the durability test and ICP results demonstrated
that the chemical states of Ni in 5%Ni-SnS2 nanosheets exhibited negligible changes during the
electrochemical reaction (Fig. S17).
Inspired by the boosted performance of CO2 electroreduction via the Ni doping, we
investigated the relevant reaction kinetics of pristine SnS2 and 5%Ni-SnS2 nanosheets. The
double-layer capacitance (Cdl) values slightly increased from 6.2 mF cm-2 of pristine SnS2
nanosheets to 6.7 mF cm-2 of 5%Ni-SnS2 nanosheets (Fig. 3A and Fig. S18). Given that the Cdl
value is positively correlated with the electrochemical active surface area (ECSA), the
geometrical current density was normalized by Cdl values to evaluate the intrinsic activity of
catalysts. The Cdl-normalized current density demonstrated that the slightly increased ECSA was
not the main reason for the enhanced current density after Ni doping (Fig. S19). Figure 3B shows
the plot of geometrical current density at -0.9 V vs RHE against Cdl values for pristine SnS2 and
5%Ni-SnS2 nanosheets at different loadings. The slopes of linear fits increased from 2.3 mA
mF-1 of pristine SnS2 nanosheets to 3.4 mA mF-1 of 5%Ni-SnS2 nanosheets, demonstrating that
Ni doping promoted the intrinsic activity of SnS2 nanosheets in CO2 electroreduction. Figure 3C
shows the tafel plots for formate production of the two nanosheets. The tafel slopes of the two
6
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10.1002/anie.201806043
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nanosheets were both close to 118 mV dec-1, indicating that the activation of CO2 serves as the
rate-limiting step in the system of SnS2 nanosheets.[18] Figure 3D shows the Nyquist plots and the
corresponding equivalent circuit of the two nanosheets. Based on the diameter of semicircular
Nyquist plots, the 5%Ni-SnS2 nanosheets exhibited the charge-transfer resistance (RCT) of 3.0 Ω,
which was lower than that (5.0 Ω) of pristine SnS2 nanosheets. Therefore, the charge-transfer
process of SnS2 nanosheets was accelerated after the Ni doping during CO2 electroreduction.
To rationalize the dramatic difference in the charge-transfer process of CO2 electroreduction,
we explored the electronic structures of pristine SnS2 and Ni doped SnS2 nanosheets. Figure 4A
shows the calculated density of states (DOS) of a single-layer SnS2 slab and Ni doped SnS2 slab
by density functional theory methods. Compared with the pristine SnS2, Ni doped SnS2 exhibited
a defect level below the conduction band, which effectively narrowed the band gap from 2.3 eV
of pristine SnS2 to 1.9 eV of Ni doped SnS2. Figure 4B shows the charge density distribution of
Ni doped SnS2 slab near the defect level. The abundant electrons in the vicinity of Ni atom
demonstrate that the defect level was attributed to the Ni doping. We further verified the defect
level through the UV-vis spectra of pristine SnS2 and 5%Ni-SnS2 nanosheets (Fig. 4C). Due to
the low-energy photoexcitation of valance band to defect level, 5%Ni-SnS2 nanosheets exhibited
enhanced absorption at long wavelength (600-900 nm).[19] As the electrons in defect level are
easier to be transferred to molecules compared with that in conduction band, the Ni doping is
expected to facilitate the Faradaic process of SnS2 nanosheets. Figure 4D shows the secondary
electron cutoff of the ultraviolet photoelectron spectroscopy (UPS) spectra. The work function
decreased from 4.1 eV of pristine SnS2 nanosheets to 3.7 eV of 5%Ni-SnS2 nanosheets.[20] It has
been reported that the decreased work function improves the electron transfer process of
electrocatalysts, and thus enhances the performance of CO2 electroreduction.[21] Therefore, the
Ni doping benefited the Faradaic process of SnS2 nanosheets in CO2 electroreduction because of
the defect level and lowered work function.
To verify the promoted Faradaic process through Ni doping, we analyzed the CO2 activation
process of pristine SnS2 and 5%Ni-SnS2 nanosheets. In CO2 adsorption isotherms, 5%Ni-SnS2
nanosheets attained a CO2 adsorption capacity of 186 μmol g-1 under 1 atm at 25 oC, which was
1.9 times larger than that of pristine SnS2 nanosheets (Fig. 4E). This result suggests that Ni
doping favored the CO2 adsorption of SnS2 nanosheets, which was the pre-requisite step for
triggering further CO2 reduction process. We further evaluated the chemisorption abilities of CO2
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on pristine SnS2 and 5%Ni-SnS2 nanosheets by CO2 temperature programmed desorption
o
measurements (Fig. 4F). The temperature of CO2 desorption peak increased from 166 C for
o
pristine SnS2 nanosheets to 225 C for 5%Ni-SnS2 nanosheets. Accordingly, the chemical
adsorption of CO2 on SnS2 nanosheets enhanced after Ni doping. To further understand the
reaction mechanism for CO2 conversion, we analyzed the corresponding proton coupling process
on 5%Ni-SnS2 nanosheets by adjusting the pH values of electrolyte (Fig. S20). The jformate kept
almost unchanged with the variation of pH values, demonstrating the proton coupling process
was not the rate-determining step. Therefore, the Ni doping strengthened the chemical adsorption
of CO2, facilitating the activation process and thus boosting the CO2 conversion to formate.
In conclusion, we developed atomically thin Ni doped SnS2 nanosheets for efficient
electroreduction of CO2. The current density and FE for carbonaceous product exhibited a
dramatic increase through the introduction of Ni into SnS2 nanosheets. At -0.9 V vs RHE, the
5%Ni-SnS2 nanosheets exhibited a remarkable FE of 93% for carbonaceous product, with a
considerable current density of 19.6 mA cm-2. Moreover, the 5%Ni-SnS2 nanosheets remained a
steady FE for carbonaceous product without obvious decay of current density during
potentiostatic test. Mechanistic study revealed that the improved performance of SnS2 nanosheets
by Ni doping originated from the defect level at conduction band edge and decreased work
function. This work not only developed Ni doped SnS2 nanosheets as promising catalysts for
CO2 electroreduction, but also provided a strategy for the design of electrocatalysts.
8
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Figure 1. (A) TEM image of 5%Ni-SnS2 nanosheets. (B) HRTEM image of 5%Ni-SnS2
nanosheets. (C-F) HAADF-STEM and STEM-EDX elemental mapping images of an individual
5%Ni-SnS2 nanosheet. (G) Line-scanning profiles of Ni, S, and Sn along the yellow line in panel
C. (H) XRD patterns of pristine SnS2 and Ni doped SnS2 nanosheets. (I) Ni 2p XPS spectra of
5%Ni-SnS2 nanosheets and 7%Ni-SnS2 nanosheets.
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10.1002/anie.201806043
Angewandte Chemie International Edition
Figure 2. (A) Geometrical current densities over pristine SnS2 and Ni doped SnS2 nanosheets. (B)
Faradaic efficiencies for carbonaceous product (C-product) over pristine SnS2 and Ni doped SnS2
nanosheets. (C) Faradaic efficiencies for formate production over pristine SnS2 and Ni doped
SnS2 nanosheets. (D) Plot of geometrical current density (j) and Faradaic efficiencies for
carbonaceous product versus time over the 5%Ni-SnS2 nanosheets at a constant potential of -0.9
V vs RHE.
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10.1002/anie.201806043
Angewandte Chemie International Edition
Figure 3. (A) Charging current density differences plotted against scan rates for pristine SnS2
and 5%Ni-SnS2 nanosheets. The fitting slopes are twice that of the Cdl values. (B) Geometrical
current density at -0.9 V vs RHE plotted against Cdl for pristine SnS2 and 5%Ni-SnS2 nanosheets
at different loadings. (C) Tafel plots of pristine SnS2 and 5%Ni-SnS2 nanosheets. (D) Nyquist
plots of pristine SnS2 and 5%Ni-SnS2 nanosheets.
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10.1002/anie.201806043
Angewandte Chemie International Edition
Figure 4. (A) Calculated DOS of SnS2 and Ni doped SnS2 slabs. (B) The distribution of charge
density at the defect level of Ni doped SnS2 slab. (C) UV-vis-NIR diffuse reflection spectra of
pristine SnS2 and 5%Ni-SnS2 nanosheets. (D) Secondary electron cutoff of the UPS spectra for
pristine SnS2 and 5%Ni-SnS2 nanosheets. (E) CO2 adsorption isotherms of pristine SnS2 and
o
5%Ni-SnS2 nanosheets at 25 C. (F) CO2-TPD spectra of pristine SnS2 and 5%Ni-SnS2
nanosheets.
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