Applied Catalysis B: Environmental 244 (2019) 1013–1020

Contents lists available at ScienceDirect

Applied Catalysis B: Environmental
journal homepage: www.elsevier.com/locate/apcatb

Probing the role of nickel dopant in aqueous colloidal ZnS nanocrystals for
efficient solar-driven CO2 reduction

T

Hong Panga,b, Xianguang Mengb,c, , Hui Songa,b, Wei Zhoud, Gaoliang Yanga,b, Hongwei Zhange,⁎
Yasuo Izumie, Toshiaki Takeib, Wipakorn Jewasuwanb, Naoki Fukatab, Jinhua Yea,b,f,g,⁎a

Graduate School of Chemical Science and Engineering, Hokkaido University, Sapporo, 060-0814, Japan
International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki, 305-0044, Japanc
Hebei Provincial Key Laboratory of Inorganic Nonmetallic Materials, College of Materials Science and Engineering, North China University of Science and Technology,
Tangshan, 063210, PR Chinad
Department of Applied Physics, Tianjin Key Laboratory of Low Dimensional Materials Physics, Preparing Technology Faculty of Science, Tianjin University, Tianjin,
300072, PR Chinae
Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba, 263-8522, Japanf
TJU-NIMS International Collaboration Laboratory, School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, PR Chinag
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin, 300072, PR Chinab

ARTICLE INFOABSTRACT

Keywords:
Photocatalysis
CO2 reduction
ZnS
DopingVacancy

Photocatalytic CO2 reduction reaction (CO2RR) on a heterogeneous catalyst offers the possibility for CO2
abatement and emerges as a promising avenue for renewable carbonaceous fuels production at ambient temperature and pressure using solar light as the sole energy input. Here, we report a newly aqueous colloidal
comprised of monodispersed Ni-doped ZnS (ZnS:Ni) nanocrystals as excellent visible-light-responsive photocatalysts for CO2RR into formate. The wavelength-dependent quantum yield shows a significant contribution of
Ni doping for visible light activity. A high selectivity (> 95%) of HCOOH production and remarkable quantum
efficiency of 59.1% at 340 nm and 5.6% at 420 nm are obtained over ZnS:Ni (0.1%) colloidal nanocrystals
modified by Cd2+. The proper balance between sulfur vacancies and extended visible light absorption of theconstructed colloidal ZnS:Ni nanocrystals contributes to the prominent performance for CO2RR. However, excessive doping of Ni does not guarantee an increase of photocatalytic CO2RR due to a diminish of sulfur vacancies. The regulation of sulfur vacancies by Ni doping and their interplay on photocatalytic CO2RR activity arepresented and discussed. This work provides an in-depth insight of the role of dopant on vacancy modulation inphotocatalyst beyond light absorption and a guidance for design of the potential photocatalyst for CO2RR.

1. Introduction
Over the past few decades, the ever-increasing CO2 atmosphericconcentration and the diminishing reserves of the chemical feedstocks
on earth have instigated scientists to seek for new alternatives for fuels[1–3]. To address the globally concerned problem, carbon sequestration
was proposed as the long-term blueprint to impede the massive manmade CO2 emissions [4–9]. Among the various methods of carbon mitigation, photocatalytic CO2 reduction reaction (CO2RR) has immense
promise with the solar light as the sole input energy and hence serves as
one of the most appealing and sustainable artificial photosynthetic
approaches [10–15]. Despite enormous efforts and considerable
achievements in solar-to-chemical conversion, the efficiency ofphotocatalytic CO2RR over semiconductors is still far from satisfactory.
With the two symmetric C]O bonds in a short line, CO2 molecule ishighly stable and peculiarly difficult to be activated [10,11,16]. Typically, a photocatalytic reaction is initiated by photogenerated chargepairs (e¯ - h+). As for a CO2RR, the photoexcited electrons are able toreach the surface from conduction band (CB), be captured by the adsorbed carbon species and participate in the photoredox reactions [17].
From a thermodynamic point of view, the CO2RR requires a high CB
position which only a handful semiconductors can satisfy. Zinc sulfide(ZnS) stands out with a CB minimum located at –1.04 V vs. NHE[18,19], indicating a thermodynamic feasibility for efficient CO2RR. In
addition, the fast carriers generation and transportation, non-toxicity
and low-cost make it one of the most promising photocatalysts for⁎
Corresponding authors at: International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1,
Tsukuba, Ibaraki, 305-0044, Japan.
E-mail addresses: mengxg_materchem@163.com (X. Meng), jinhua.ye@nims.go.jp (J. Ye).

https://doi.org/10.1016/j.apcatb.2018.12.010
Received 6 October 2018; Received in revised form 29 November 2018; Accepted 3 December 2018
Available online 04 December 20180926-3373/ © 2018 Elsevier B.V. All rights reserved.

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CO2RR. Through constructing an all-inorganic reaction system comprised of the colloidal ZnS, we have achieved a breakthrough of photocatalytic CO2 conversion into formate with 76% apparent quantum
efficiency (280 nm) and 95% HCOOH selectivity [20]. Without any
organic surface stabilization, the ZnS particles are quantized with anaverage diameter of ca. 3 nm. The shortened diffusion length within theultra-small colloidal nanocrystals can slow down the recombination
rates of photogenerated electrons and holes [21]. However, its intrinsic
wide bandgap of about 3.7 eV determines it exclusively responsive toultraviolet (UV) light which merely accounts for 4% solar spectrum[22]. It is necessitated to extend its light absorption to visible lightregion or longer wavelengths. Transition-metal dopants such as Bi, Fe,
Pb, Cu, Ni, were reported capable of introducing an additional energy
level within the band gap and narrowing the bandgap of ZnS [23,24].
Thus later, Cu was doped into the colloidal ZnS nanocrystals to narrow
the bandgap and realize the visible light response [25].
Introducing anion vacancies has already been demonstrated as aneffective avenue to boost the performance of oxide photocatalysts as thevacancies can activate the adsorbates and trap the photoexcited electrons to suppress the charge recombination, favorable for the catalytic
reactions [26–39]. On account of the preparation method, it is envisaged that sulfur vacancies (Sv) intrinsically exist within the synthesized colloidal ZnS samples because S atoms are apt to escape in theform of H-S in such aqueous system, which account for the active sites
and exceptional performance. The role of elemental dopants is actuallyquite complicated beyond the light absorption. Previous work pointed
out that Cu dopant changed to Cu+ and formed one sulfur vacancy
adjacently [40]. But our theoretical calculation found the Ni dopant is
not beneficial for the sulfur vacancy formation due to an increase of theformation energy.
Herein this work, Ni, as a noble-metal-free and non-toxic dopant, isselected to modify the electronic structure of ZnS. We constructed acolloidal system of Ni-doped ZnS (ZnS:Ni) nanocrystals in aqueous solution with loosely aggregated architecture furnishing with sufficientsurface area and active sites. The synergy between doping and vacancy
also attract us to look into the effect of Ni doping on Sv where rarereports set foot in, except the modulation of electronic structure. It is
investigated and elucidated the correlation between the amount offoreign dopants and the CO2RR performance. Some new perspectives
into the CO2RR photocatalyst underpinning the relationship between
doping and vacancy are given.

and room temperature, respectively, over BELsorp II mini (BEL Japan
Inc.). The fluorescence (PL) spectra were measured by a fluorescence
spectrometer (JASCO FP-6500) with the excitation wavelength of
280 nm over the colloidal ZnS:Ni suspensions of 1.0 g L−1. X-ray photoelectron spectroscopy (XPS) (PHI Quantera SXM, ULVAC-PHI Inc.,
Japan) was used to analyse the compositions and valence states of thesamples. Electron spin resonance (ESR) measurements were performed
on JEOL JES-FA-200 at room temperature.
X-ray absorption fine structure (XAFS) spectra were measured atroom temperature for Ni metal foil, ZnS:Ni (2.0%) samples in thetransmission mode in the Photon Factory at KEK (Tsukuba) on beamline
9C. A Si (1 1 1) double-crystal monochromator and a pair of bent
conical/cylinder mirrors and/or double flat mirrors were inserted into
the X-ray beam path. The X-ray intensity was maintained at 67% of the
maximum flux using a piezo-translator applied to the crystal to suppress
higher harmonics. The slit in front of the I0 ionization chamber had an
opening size of 1 mm (vertical) × 2 mm (horizontal). Ni K-edge spectra
for NiS was measured in the transmission mode in the Photon Factory
Advanced Ring at the High Energy Accelerator Research Organization
on beamline NW10 A in which the storage ring energy was 6.5 GeV, andthe ring current was 50 mA. A Si (3 1 1) double-crystal monochromatorand a Pt-coated focusing cylindrical mirror were inserted into the X-ray
beam path.
Spectra for the NiS (5.6 mg) sample diluted by boron nitride were
measured at the beamline. The Ni K-edge absorption energy was calibrated to 8331.65 eV using the spectrum of a Ni metal foil (thickness
5 μm). The obtained Ni K-edge XAFS data were analyzed using XDAP
software package using modified Victoreen function, spline function,
Fourier transform, and multiple-shell curve fitting.
2.3. Photocatalytic CO2 reduction reaction
The photocatalytic CO2 reductions over the aqueous colloidal
ZnS:Ni nanocrystal suspensions were carried out in a Pyrex glass reaction cell with an upside quartz window linked to a gas-closed circulation system connected to a gas chromatography (GC) (GC-8 A,
Shimadzu Co., Japan, Carrier gas: Ar) with a thermal conductivity detector (TCD) for online analysis of the H2 production. 0.001 mol photocatalyst was suspended in 100 ml aqueous solution containing 0.5 M
KHCO3 and 0.5 M K2SO3 sacrificial reagents. For the formate production, 2.0 wt% CdSO4 were added into the reaction solutions. Prior to thephoto-reaction, the solution was evacuated until no O2 and N2 can bedetected. The high-purity CO2 (99.999%) was purged into the system
for several times until the CO2 adsorption was saturated. An AM 1.5 G
solar simulator (WXS-80C-3 AM 1.5 G) and 300 W Xe lamp were used as
the light source. The light intensity was measured on a spectroradiometer (Ushio Spectroradiometer, USR-40) for the measurement ofapparent quantum efficiency.

2. Experimental
2.1. Catalyst preparation
Colloidal ZnS:Ni photocatalysts were fabricated by pouring the Na2S
solution (0.5 M) into equivalent volume of the mixture solution of
ZnSO4 (0.5 M) and a certain amount (0.1%, 0.2%, 0.5%, 1.0%, 2.0%,4.0%) of NiSO4 under vigorous stirring. After stirring overnight, theresulted suspensions were rinsed with water and centrifuged. The fresh
precipitates were then dispersed in 100 ml ultrapure water (Direct-Q
3UV) for further photocatalytic activity evaluation.

2.4. Carbon-species products analysis
The CO and CH4 evolved were sampled and analysed by GC (GC14B, Shimadzu Co., Japan) equipped with a flame ionization detector(FID). The formate ions concentrations were determined by proton (1H)
spectra on a 400 MHz nuclear magnetic resonance (NMR) Spectrometor(ESC-400, JEOL, Japan) using 0.5 ml post-reaction solution supernatantmixed with 0.1 ml deuterated water (D2O) and 0.05 μL dimethyl sulfoxide (DMSO, Sigma, 99.99%). For isotope tracer measurement, 13CO2
was purged in the system. To avoid the confusion of carbon source, theaqueous suspensions only contain 0.5 M K2SO3 sacrificial reagents apartfrom 0.001 mol photocatalyst and a certain amount of Cd2+ cocatalyst.
The identification of 13CO was conducted on a gas chromatographymass spectrometry (GC–MS, JMS-K9, JEOL Co., Japan). H13COOH was
identified on the carbon (13C) spectrum on the NMR.

2.2. Characterization
X-ray diffraction patterns (XRD) were obtained on an X-ray diffractometer (X'pert powder, PANalytical B.V., Netherlands) with Cu-Kα
radiation. The morphologies were observed on a Field-emission
Scanning electron microscopy (FE-SEM, S-4800, Hitachi) and a
Transmission electron microscopy (TEM, FEI Tecnai G2 F30) with a
300 kV accelerating voltage. The diffuse reflectance spectra (DRS) were
measured on a Shimadzu UV-2500 spectrophotometer with an integrating sphere. The absorption spectra were transformed from the
DRS spectra via Kubelka-Munk method. The Brunauer–Emmett–Teller(BET) surface area and CO2 adsorption isotherms were obtained at 77 K
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2.5. Electrochemical measurement

precipitation process. The morphologies are observed by transmission
electron microscopy (TEM) (Fig. 1a) and scanning electron microscopy(SEM) (Fig. S1). Figs. 1a and S1 displays that the majority of particles incolloidal ZnS:Ni are easily agglomerated due to their extremely small
dimensions and high surface energy. Despite the severe agglomeration
under dehydrated conditions for TEM observation, several isolated
nanocrystals are still distinguished as seen in Fig. 1b. The high-angle
annular dark-field (HAADF) image and the energy-dispersive X-ray
spectroscopy (EDX) elemental mappings under scanning transmission
electron microscopy (STEM) mode exhibit a highly uniform distributionof Zn, S, Ni elements as shown in Fig. 1c–f, indicating Ni elements arehomogeneously doped into the colloidal ZnS:Ni nanocrystals. Observation under the high-resolution TEM (HRTEM) found that the colloidal ZnS:Ni nanocrystals gradually crystallized under the electron
beams and showed distinct lattice fringes. The interplanar spacing ofthe ZnS:Ni is 3.21 Å as marked in Fig. 1g, in accordance with theanalytic result of the selected area electron diffraction (SAED) in Fig. 1hwhich shows the radius of the most distinguished diffraction ring is
0.310 nm, corresponding to (111) crystal plane of the cubic ZnS. Closer
examination of the second and third diffraction rings are identified withinterplanar spacing of 0.185 nm and 0.158 nm, well assigned to (220)and (311) crystal planes. This result is in well agreement with the threeintensive peaks located at 28.6°, 47.8°, 56.6° indexed to 111, 220, 311
diffraction, respectively, in X-ray diffraction (XRD) pattern as present in
Fig. 1i. The XRD pattern clearly showed the as-synthesized ZnS:Nicolloidal nanocrystals are indexed to a typical cubic sphalerite structure(JCPDS No. 77-2100) though the diffractogram is broad. There is no
obvious deviation of the peak positions of the XRD patterns with thedoping amount increasing from 0% to 4.0%, which might be attributed
to the reason that the ionic radius of the four-coordinated Ni2+ (0.69 Å)
is close to that of the Zn2+ (0.74 Å) [41]. No other discernable impurity
peaks are detected, indicating Ni doping well retains the crystal structural phase of cubic ZnS nanocrystals in colloidal system.
According to the size distribution in high-resolution TEM (HR-TEM)
images (Inset of Fig. 1b), the estimated average size of the colloidal
ZnS:Ni nanocrystals is 3.5 ± 0.7 nm, in accord with the derived result
from Scherrer’s equation based on the X-ray powder diffraction (XRD)
spectra, which give an average particle size of ca. 3 nm. When dispersing the samples in the liquid phase, the small dimensions can furnish
with more surfaces and active sites for the photocatalytic CO2RR whichfeatures as a surface reaction. This was testified through BET and volumetric CO2 uptake measurement, which clarify the Ni doping effectPhotoelectrochemical measurements were conducted on a CHI
electrochemical workstation ALS/CH model 650 A using a three-electrode system with 0.5 M Na2SO4 (pH = 7) electrolyte. For the preparation of working electrode, 1.0 ml of 2.5 mg mL−1 colloidal ZnS
suspensions were mixed with 30 μL Nafion perfluorinated resin solution(Sigma-Aldrich) and sonicated. For the photocurrent measurement, theas-prepared suspensions were deposited on the indium tin oxide (ITO)
substrates by spin coating. Platinum and Ag/AgCl (RE-1C; BAS Inc.)
were employed as the counter electrode and reference electrode, respectively. An AM 1.5 solar simulator lamp was set as the light sourcewith a light on/off cycle of 10 s. The flatband potential s(Efb) of theundoped and doped ZnS were obtained from Mott–Schottky plots at
different frequencies (500, 1000, and 2000 Hz).
2.6. Theoretical calculations
The calculations of electronic structure were performed based on
the plane-waved density functional theory calculations as implemented
in a Vienna ab initio simulation package (VASP) code. The generalized
gradient approximation (GGA) in a Perdew-Burke-Ernzerhof (PBE)
functional were used for the exchange-correlation potential with acutoff energy of 450 eV for the valence electrons. For the optical absorption calculations, the HSE06 hybrid functional was used to present
the accurate band gap. Substitution of Zn atoms with Ni atom wasconducted with a doping percentage of about 3.125%. The ZnS:Ni (110)
surface is simulated using a 2 × 2 slab model consisting of five atomic
layers of ZnS and a 16 Å vacuum region to avoid the interaction between two adjacent slabs. For the calculation of the density of states,
2 × 2×1 Monkhorst k-point meshes were chosen to sample for the
Brillouin-zone integrations of the slab model. All atoms are relaxed withthe convergence tolerance set to 1 × 10−5 eV per atom and the residual
force less than 0.02 eV/Å.
3. Results and discussion
3.1. Morphology and structure characterization
The colloidal ZnS:Ni photocatalysts with 0%, 0.1%, 0.2%, 0.5%,
1.0%, 2.0% and 4.0% doping amounts were directly synthesized bycontrolling the NiSO4 precursor amount in an aqueous-phase co-Fig. 1. Morphological and structural characterization. (a) Bright-field TEM image, (b)HRTEM image (Inset: size distribution), (c)HAADF-STEM image, (d–f) elemental mappingof Ni, Zn, S, (g) HRTEM image, (h) SAED pattern of the ZnS:Ni (2.0%) colloidal nanocrystals, (i) XRD pattern of the colloidal ZnS:Ni
nanocrystals with doping amount from 0 to 4.0
%.

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on the surface area and CO2 adsorption capacity. To diminish thetemperature effect on the structure of the low-crystalline nanocrystals,
the colloidal ZnS and ZnS:Ni were dried by freeze-drying instead of heat
treatment. As seen from Fig. S3a, the surface area of the ZnS:Ni (0.1%)
is 192.19 m2 g−1, slightly larger than that of the ZnS:Ni (0.2%)(145.83 m2 g−1) and the undoped ZnS (133.30 m2 g−1) but nearly threetimes higher than that of the ZnS:Ni (2.0%) samples (60.40 m2 g−1).
Although the dried structure can not fully reflect the liquid phase adsorption, it is supposed the smaller Ni doping will cause a disorder ofthe surface and slightly enlarge the surface area but decreased the adsorption sites with the doping amount increase. CO2 uptake amountcomparison in Fig. S3b also indicates the smaller amount of Ni (0.1%)
and Ni (0.2%) doping maintained the CO2 volumetric adsorption at
higher level but the heavy doping amount like 2.0% will cause a slightdecrease.

Fig. 3. UV–vis absorption spectra of ZnS:Ni colloidal nanocrystals.

the occupied Ni 3d orbitals to the hybridization of Zn 4s-S 3p orbitals.
The accordance of UV − vis absorption spectra with the simulated
optical absorption configuration derived from the electron structure,also testifies Ni is successfully doped into ZnS crystal lattice. We canalso observe from the optical photographs in Fig. S6, the colour of theas-synthesized Ni-ZnS colloidal varied from bright yellow to deep
yellow as with the doping amount increment from 0.1% to 4.0%.
Herein we have conducted Mott-Schottky measurement and XPS valence band (VB) to roughly estimate the conduction band and valenceband edges as seen in Fig. S7 (The calculation method can be found in
Supporting Information). It is clear to see the Ni dopant created a newlevel at a position of 0.8 eV above the VBM of ZnS, well correlated withwhat the theoretical simulation predicts. It is worth mentioning that thelonger wavelength absorption tails in the visible region is ascribed to Sv[42], attributed to the transition from a newly created midgap state by

3.2. Electronic structure and photophysical properties
To gain insight into the electronic structure, we constructed the
ZnS:Ni and perfect ZnS supercell slabs as schemed in Fig. S4 to implement the Density Functional Theory (DFT) calculations. The theoretically calculated band structure and projected density of states (PDOS)
are displayed in Fig. 2. Ni dopant introduced an occupied state above
the valence band of the pristine ZnS with an additional contribution
from S orbitals. The created new energy level within the band gap extend the absorption to visible light range as the simulated optical absorbance spectrum predicts in Fig. S5. The UV − vis absorption spectra(Fig. 3) of the ZnS:Ni colloidal crystallites transformed from the diffuse
reflectance spectra (DRS) via K-M method possess an intrinsic absorption feature edged at 340 nm and enhanced photo-absorption in thevisible light ranging from 340∼470 nm, related to the transition from

Fig. 2. The electronic band structure and the decomposition of the total density of states (DOS) into partial DOS of Ni, Zn and S orbitals of (a) ZnS:Ni (2.7%) and (b)undoped ZnS, respectively.
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Sv just below CB to the CB of ZnS, suggesting the abundant Sv within the
as-prepared samples. In addition, the tails level off with an increase of
the doping amount until the 4.0% Ni-doped sample, indicating the Ni
doping can modulate the amount of sulfur vacancies within the doped
ZnS colloidal nanocrystals.

solar light.
The isotope experiment using 13CO2 under the identical reactant
conditions excluding the utilization of KHCO3 was carried out to validate the source of the produced CO and HCOOH. The GC–MS results in
Fig. S13 clearly demonstrate the production of 13CO (m/z = 29). The
identification of liquid H13COO¯ was performed by 13C nuclear magnetic resonance (NMR) spectra. The peaks at d = 170.5 ppm and
160.5 ppm assigned to H13COO¯ and H13CO3¯ in 13C NMR spectrum(Fig. S14) and the doublet peak at 8.1 and 8.6 ppm in 1H NMR spectrum(Fig. S15) together confirme the liquid product HCOOH all comes fromthe 13CO2.
To test the ZnS:Ni photocatalysts’ durability, long-term photochemical CO2 reduction experiments were performed over 80 h. As
displayed in the Fig. S16, no obvious deactivation was observed in thislong period, underlying the colloidal ZnS:Ni exhibits quite strong durability. Notably, even the CO production rate were slightly higher in thelate stage as shown.

3.3. Photocatalytic CO2RR activity
The CO2RR of the ZnS:Ni colloidal suspension was assessed over theCO2-saturated KHCO3 solvent with K2SO3 as the electron donors in thepyrex cell with a quartz lid. The experimental setup can be found in theFig. S8. A 300 W full arc Xe lamp with and without cutoff filter(> 420 nm) were first adopted as the outer irradiation source. As reported previously, Cd2+ can serve as an efficient cocatalyst for thehighly selective formate production [20,25]. Herein 2.0% Cd2+ wasadded to promote the formate production. When the photocatalytic
reaction was carried out, photogenerated holes were irreversibly consumed by the reducing reagent SO32− and thereby boosting the photogenerated electrons in the conduction band transfer to chemisorbed
CO2 for the reduction. The evolved rates of H2, CO and the liquid
product, HCOOH under irradiance with wavelength > 420 nm are figured in Fig. 4; H2, CO and HCOOH were produced at a rate as high as
23.8 μmol h-1, 0.18 μmol h-1 and 427.5 μmol h-1, respectively, when fullarc Xenon lamp was employed. As contrast, the undoped ZnS has noactivity under the visible light irradiation. Then to simulate the photocatalytic CO2 reduction under solar light, the reaction was carried out
under an AM 1.5 G solar simulator. The time-conversion plot of thegaseous products, H2 and CO over time course and the average production rate of formate are figured in Fig. 5a–c. Even under the weaksunlight, the formate production can reach up to 137.9 μmol h-1 with aselectivity up to 99% over ZnS:Ni (0.1%) (Fig. 5c). The apparent
quantum efficiency (AQY) curve in Fig. 5d clearly showes a wavelength-dependence tracking the absorption of the ZnS:Ni. In this experiment, the AQY is 59.1% @ 339.3 nm and 5.6% @ 417.9 nm,
dominated by the intrinsic ultraviolet absorption and the Ni dopinginduced visible light absorption, respectively. In addition, despite anegligible absorption induced by Ni doping at 480 nm, there is an obvious AQY at 480.4 nm with a value of 1.19%, suggesting the abundant
Sv also contributes to the visible-light-responsive activity over the
ZnS:Ni.
The CO2RR activity over the series of ZnS:Ni were also tested under
different light source as shown in Figs. S10-12 with a different optimal
doping amount of Ni. It can be found the optimal doping amount of Ni
shifts to larger when increasing the visible light component in the irradiation source, demonstrating the Ni doping effect and existence of Sv
successfully extends the CO2RR activity of colloidal ZnS nanocrystals
into visible light region and enhances the formate production under

3.4. The correlation between Ni dopant and sulfur vacancy
It has been already stated the abundant sulfur vacancies in suchaqueous colloidal structure induce the excellent performance for
CO2RR and Ni doping increase the valence band position with regard to
the band structure, improving the visible light utilization and hence theactivity under solar light. But the fact that light doping increased theactivity nearly three times and heavier doping of Ni leading to a decrease of CO2RR production raise a question why the optimal activity is
obtained at 0.1% doping amount. We measured the photocurrents (inFig. S17) of the representative samples to describe the charge separation efficiency and carrier transport under irradiation but found that
only slight discrepancies were presented, not sufficient to account for
the large difference in activity. Then we turned to the adsorption sites
indicated by the BET and CO2 uptake experiment for the reason. Our
theoretical calculation predicts the sulfur vacancy formation energy
increases 0.76 eV after incorporation of Ni, indicating the Ni doping is
not beneficial for the formation of the Sv.
The factors of Ni dopants and Sv are actually complex and interrelated. To get a deeper insight into how the Ni doping affects the Sv
and photocatalytic CO2RR performance, the electron spin resonance(ESR) spectroscopy was further employed to observe the electron performance as shown in Fig. 6a. It was clearly noted that a sharp signal at
g1 = 1.999 ± 0.004 and a broad signal located at g2 = 2.007 ± 0.004
were detected both over the undoped and ZnS:Ni colloidal nanocrystals.
According to the reference, the broad signal is the feature of the dipolarinteraction between the physically absorbed molecular oxygen on theparamagnetic centers as we performed the ESR in air [43]; the sharp
signal can be assigned to the F center, a singly negatively charged Sv inthe cubic ZnS crystals, strongly supporting the existence of Sv [44]. No
Ni-related signatures were observed in this range [45]. As the ESR
signal is correlated with the density of paramagnetic electrons from Sv,
it was found the smaller Ni doping (< 0.2%) results in a higher level ofsurface active sites according to the larger intensity of the chemisorbed
oxygen signals but the doping amount more than 0.2% will dramatically decrease the Sv within the nanocrystals, consistent with the surface area and absorption observations stated above.
Photoluminescence (PL) spectroscopy is another efficient and direct
way to figure out the photogenerated charge carriers dynamics under acertain excitation wavelength. From the PL emission spectra in Fig. 6b,
we can have a clear view of the energy states within the bandgap.
Under 280 nm excitation, there are two major emission peaks. The
320 nm emission peak (green area) is assigned to the intrinsic transitionof photoelectrons and holes from conduction band to valence band.
With the new states above the valence band maximum (VBM) createdby Ni to trap the holes and promote the charge separation, the reduced
intensity at 320 nm with the increasing Ni amount is easy to correlatedwith the suppressed recombination. The broad peak centered at 425 nm

Fig. 4. Evolution rates of H2, CO and HCOOH under 300 W full arc Xenon lampwith a cutoff filter of 420 nm (left panel) and without any cutoff filter (rightpanel) over colloidal ZnS:Ni (0.1%) nanocrystals.
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Fig. 5. Time-conversion plot of the gaseous product under AM 1.5 simulator: (a) CO and (b) H2. (c) HCOOH production rate of colloidal ZnS:Ni with different doping
amount of Ni (d) The wavelength-dependent AQY of photocatalytic HCOOH production over colloidal ZnS:Ni (0.1%) nanocrystals.

peak (yellow area) arising from the deep trap level near conduction
band was reported from Sv in the previous literature [46,47]. From thedecreasing intensities of the blue emission at 425 nm, it is deduced Ni
doping can curtail the excessive Sv in the undoped ZnS, in well accord
with what we have observed in ESR spectra.
Further chemical states analysis of the colloidal ZnS:Ni nanocrystals
are conducted on X-ray photoelectron spectroscopy (XPS). The XPS

survey spectrum is presented in Fig. S18. The pristine ZnS, ZnS:Ni(0.1%), ZnS:Ni (0.2%) and ZnS:Ni (4.0%) are selected as the representative samples of the undoped, optimal doped, light doped andheavier doped one. As shown in Fig. 6c, the high-resolution XPS spectrum of Zn 2p reveals that there is a pair of peaks located at 1022.4 eV
and 1045.4 eV binding energy, which are assigned to Zn 2p3/2 and Zn
2p1/2 of undoped ZnS colloidal nanocrystals. When Ni was doped in theFig. 6. (a) ESR spectra and (b) PL spectra of undoped ZnS and ZnS:Ni nanocrystals at room temperature. The 425 nm emission represents the sulfur vacancy
signature. High-resolution XPS spectra of (c) Zn 2p and (d) S 2p, respectively of the undoped, ZnS:Ni (0.1%), ZnS:Ni (0.2%) and ZnS:Ni (4.0%) nanocrystals.
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Fig. 7. Ni K-edge XANES (a) and Fouriertransform of EXAFS spectra (b) for Ni metal foil
(thickness 5 μm, black line), NiS (5.6 mg, blueline) and fresh ZnS:Ni (2.0%) (50 mg, red line).
The XRD pattern of the as-prepared NiS is
shown in Fig. S21. k3-weighted EXAFS oscillation with best-fit results in k-space and Rspace for first-shell (Ni-S) of ZnS:Ni is seen in
Fig. S22. (For interpretation of the references
to colour in this figure legend, the reader isreferred to the web version of this article).

ZnS at 0.1%, the binding energy of the Zn 2p peaks and S 2p peak
maintained their original position, suggesting the small doping will not
obviously affect the oxidation states of the elements. But the major Zn
2p3/2 peak shifts to 1022.2 eV and to 1022.1 eV for ZnS:Ni (0.2%) and
ZnS:Ni (4.0%), equivalent to a negative shift of ∼0.2 eV and ∼0.3 eVwith respect to the undoped counterpart, respectively. It is reasonable
to deduce that Ni doping leads to a decrease of the electron density ofthe positively charged Zn ion adjacent to the Sv and hence a lower
binding energy. From Fig. 6d, we can observe the S 2p peaks slightly
shifts from 162.0 eV toward lower binding energy of 162.0 eV and
161.9 eV for ZnS:Ni (0.2%) and ZnS:Ni (4.0%), respectively. The lowering of S 2p binding energy also demonstrates the weakened Zn-S
bonds near the dopant site caused by Ni-S bond formation.
As XPS measurement failed to show any signal of Ni because thephotoelectron energy was not enough to penetrate the nanocrystal eventhough we adjusted the doping amount to 4.0% (Fig. S19) and Raman
spectra could not show obvious difference, either (Fig. S20), the synchrotron-based XAFS was collected over ZnS:Ni (2.0%) to establish theoxidation state and local structure of the Ni dopant.
In the normalized Ni K-edge X-ray absorption near edge structure(XANES) (Fig. 7a), the structure of ZnS:Ni differs from that of the Ni foil
and NiS reference, excluding the existence of Ni aggregation or theformation of NiS. The white line position of the absorption edge at
8350.1 eV, well correlated with that of the NiO and NiS, indicates Ni2+
dominates as the oxidation state. When k range was extended up to 8.2
Angstrom−1 and the k3-weighted EXAFS oscillation was fitted in a kspace and R space (Fig. S22), the best fit of the scattering interaction
can be obtained with a one-shell fit of Ni-S in the EXAFS spectrum(Fig. 7b). The derived coordination number of the Zn-S shell is 3.96,
reflecting the local coordination of Ni is tetra-coordinated by four sulfur
atoms. As the Zn-S distance (2.34 Å) is slightly shorter than the Ni-S
bond length (2.47 Å) [48], the smaller magnitude in the transform of
ZnS:Ni than NiS suggests a contraction and disorder of the lattice when
Ni is doped. Overall, the edge data and EXAFS data confirm thedominant Ni2+ status and the bonding coordination in the as-prepared
ZnS:Ni. However, careful observation in the pre-edge area (See Fig.
S23) and main peak found a slight shift towards lower energy withrespect to the NiS, indicating an increased charge density of Ni. The
Bader charge calculation further demonstrate the electronic configuration of Ni with 10.00 valence electrons changes to 9.12 valence
electrons in the ZnS:Ni with part of 3d9, s = 1/2 configuration. The restof two electrons after one sulfur atoms escape are easily trapped by theNi dopant if the doping amount is excessive. This might be the reason
why sulfur vacancies could be reduced by Ni dopant.
From the discussion above, large amount of sulfur vacancies in thebulk within the pristine undoped ZnS nanocrystals will bring about thesevere recombination of charge carriers and suppress the reduction
activity. As Ni doping are apt to localize the electrons from sulfur vacancies and reduce the Sv, light doping can regulate the Sv to the optimal abundance and induce a significant enhancement in activity;

heavier doping leads to a great loss of Sv and a low performance adversely despite an extended light absorption in visible region.
4. Conclusion
Solar-driven CO2 reduction was successfully realized over ZnS:Nicolloidal nanocrystals with an optimal doping amount of 0.1%. Loadingwith Cd2+, the HCOOH production over ZnS:Ni colloidal nanocrystals
has reached up to 427 μmol·h−1 under 300 W Xe lamp and138 μmol·h−1 (selectivity ∼99%) under 1 sun. The AQY at 340 nm in
the UV region and at 420 nm in the visible light region are 59.1% and5.6%, respectively, at the front ranks in the currently reported photocatalytic CO2RR systems. The reason why photoconversion of CO2 is
optimized at a small doping amount (0.1%) of Ni is explained as aproper balance between the increased light absorption and decreased
Sv. Demonstrated by PL, ESR and XPS, it was found the Ni dopants canmodulate the Sv amount in the loose-structured ZnS:Ni colloidal architecture. Light doping of Ni will maintain the abundant Sv, suppressing the charge recombination in the bulk; heavier doping of Ni willcause the diminishing of Sv, unfavorable for the CO2RR. Thus, only
appropriate doping amount can give rise to the optimal performance ofthe photocatalytic CO2RR. The discovery offers a unique insight into thedoping strategy and modulation of anion vacancies on the photocatalysts towards CO2 reduction.
Conflict of interest
The authors declare no conflict of interest.
Acknowledgements
The work received the financial support from the World PremierInternational Research Center Initiative (WPI Initiative) on Materials
Nanoarchitectonics (MANA), MEXT, Japan, the National Basic Research
Program of China (973 Program, 2014CB239301), the National Natural
Science Foundation of China (21633004 and 21703065) and JSPS
KAKENHI Grant Number JP18H02065. X-ray absorption experiments
were conducted under the approval of the Photon Factory Proposal
Review Committee (2016G577).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcatb.2018.12.010.
References[1] J.L. White, M.F. Baruch, J.E. Pander Iii, Y. Hu, I.C. Fortmeyer, J.E. Park, T. Zhang,
K. Liao, J. Gu, Y. Yan, T.W. Shaw, E. Abelev, A.B. Bocarsly, Chem. Rev. 115 (2015)
12888–12935.
[2] T.J. LaTempa, S. Rani, N. Bao, C.A. Grimes, Nanoscale 4 (2012) 2245.

1019

Applied Catalysis B: Environmental 244 (2019) 1013–1020

H. Pang et al.
[3] S. Wang, X. Wang, Appl. Catal. B: Environ. 162 (2015) 494–500.
[4] G.A. Olah, G.K.S. Prakash, A. Goeppert, J. Am. Chem. Soc. 133 (2011)
12881–12898.
[5] J.J. Concepcion, R.L. House, J.M. Papanikolas, T.J. Meyer, Proc. Natl. Acad. Sci.
109 (2012) 15560–15564.
[6] G. Zhao, W. Zhou, Y. Sun, X. Wang, H. Liu, X. Meng, K. Chang, J. Ye, Appl. Catal. B:
Environ. 226 (2018) 252–257.
[7] G. Zhao, H. Pang, G. Liu, P. Li, H. Liu, H. Zhang, L. Shi, J. Ye, Appl. Catal. B:
Environ. 200 (2017) 141–149.
[8] G. Chen, R. Gao, Y. Zhao, Z. Li, G.I.N. Waterhouse, R. Shi, J. Zhao, M. Zhang,
L. Shang, G. Sheng, X. Zhang, X. Wen, L.Z. Wu, C.H. Tung, T. Zhang, Adv. Mater. 30(2018).
[9] Y. Hao, A. Steinfeld, Sci. Bull. 62 (2017) 1099–1101.
[10] H. Pang, T. Masuda, J. Ye, Chem. Asian J. 13 (2018) 127–142.
[11] X. Chang, T. Wang, J. Gong, Energy Environ. Sci. 9 (2016) 2177–2196.
[12] J. Qin, S. Wang, X. Wang, Appl. Catal. B: Environ. 209 (2017) 476–482.
[13] Y. Su, Z. Zhang, H. Liu, Y. Wang, Appl. Catal. B: Environ. 200 (2017) 448–457.
[14] Z. Sun, J.M.T.A. Fischer, Q. Li, J. Hu, Q. Tang, H. Wang, Z. Wu, M. Hankel,
D.J. Searles, L. Wang, Appl. Catal. B: Environ. 216 (2017) 146–155.
[15] R. Shi, G.I.N. Waterhouse, T. Zhang, Solar RRL 1 (2017) 1700126.
[16] S. Xie, Q. Zhang, G. Liu, Y. Wang, Chem. Commun. 52 (2016) 35–59.
[17] H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J. Ye, Adv. Mater. 24 (2012)
229–251.
[18] R. Zhou, M.I. Guzman, J. Phys. Chem. C 118 (2014) 11649–11656.
[19] X. Hao, Y. Wang, J. Zhou, Z. Cui, Y. Wang, Z. Zou, Appl. Catal. B: Environ. 221(2018) 302–311.
[20] X. Meng, Q. Yu, G. Liu, L. Shi, G. Zhao, H. Liu, P. Li, K. Chang, T. Kako, J. Ye, Nano
Energy 34 (2017) 524–532.
[21] M. Kanemoto, T. Shiragami, C. Pac, S. Yanagida, J. Phys. Chem. 96 (1992)
3521–3526.
[22] H. Pang, G. Zhao, G. Liu, H. Zhang, X. Hai, S. Wang, H. Song, J. Ye, Small 14 (2018)
e1800104.
[23] A. Kudo, Y. Miseki, Chem. Soc. Rev. 38 (2009) 253–278.
[24] G. Yang, D. Chen, H. Ding, J. Feng, J.Z. Zhang, Y. Zhu, S. Hamid, D.W. Bahnemann,
Appl. Catal. B: Environ. 219 (2017) 611–618.
[25] X. Meng, G. Zuo, P. Zong, H. Pang, J. Ren, X. Zeng, S. Liu, Y. Shen, W. Zhou, J. Ye,
Appl. Catal. B: Environ. 237 (2018) 68–73.
[26] Y. Ji, Y. Luo, J. Am. Chem. Soc. 138 (2016) 15896–15902.
[27] S. Gao, B. Gu, X. Jiao, Y. Sun, X. Zu, F. Yang, W. Zhu, C. Wang, Z. Feng, B. Ye, Y. Xie,

J. Am. Chem. Soc. 139 (2017) 3438–3445.
[28] X. Jiao, Z. Chen, X. Li, Y. Sun, S. Gao, W. Yan, C. Wang, Q. Zhang, Y. Lin, Y. Luo,Y. Xie, J. Am. Chem. Soc. 139 (2017) 7586–7594.
[29] S. Gao, Y. Lin, X. Jiao, Y. Sun, Q. Luo, W. Zhang, D. Li, J. Yang, Y. Xie, Nature 529(2016) 68–71.
[30] W. Tu, Y. Xu, J. Wang, B. Zhang, T. Zhou, S. Yin, S. Wu, C. Li, Y. Huang, Y. Zhou,
Z. Zou, J. Robertson, M. Kraft, R. Xu, ACS Sustain. Chem. Eng. 5 (2017) 7260–7268.
[31] B. Wang, X. Wang, L. Lu, C. Zhou, Z. Xin, J. Wang, X.-k. Ke, G. Sheng, S. Yan, Z. Zou,ACS Catal. 8 (2017) 516–525.
[32] Z. Geng, X. Kong, W. Chen, H. Su, Y. Liu, F. Cai, G. Wang, J. Zeng, Angew. Chem.
Int. Ed. 57 (2018) 6054–6059.
[33] C. Li, T. Wang, Z.J. Zhao, W. Yang, J.F. Li, A. Li, Z. Yang, G.A. Ozin, J. Gong,
Angew. Chem. Int. Ed. 57 (2018) 5278–5282.
[34] X. Chen, L. Liu, P.Y. Yu, S.S. Mao, Science 331 (2011) 746.
[35] M. Wang, L. Cai, Y. Wang, F. Zhou, K. Xu, X. Tao, Y. Chai, J. Am. Chem. Soc. 139(2017) 4144–4151.
[36] G. Li, G.R. Blake, T.T. Palstra, Chem. Soc. Rev. 46 (2017) 1693–1706.
[37] F. Wang, S. He, H. Chen, B. Wang, L. Zheng, M. Wei, D.G. Evans, X. Duan, J. Am.
Chem. Soc. 138 (2016) 6298–6305.
[38] Y. Zhao, G. Chen, T. Bian, C. Zhou, G.I. Waterhouse, L.Z. Wu, C.H. Tung, L.J. Smith,
D. O’Hare, T. Zhang, Adv. Mater. 27 (2015) 7824–7831.
[39] Y. Zhao, Y. Zhao, G.I.N. Waterhouse, L. Zheng, X. Cao, F. Teng, L.Z. Wu, C.H. Tung,
D. O’Hare, T. Zhang, Adv. Mater. 29 (2017).
[40] Z. Mei, B. Zhang, J. Zheng, S. Yuan, Z. Zhuo, X. Meng, Z. Chen, K. Amine, W. Yang,
L.W. Wang, W. Wang, S. Wang, Q. Gong, J. Li, F.S. Liu, F. Pan, Nano Energy 26(2016) 405–416.
[41] G. Murugadoss, J. Lumin. 132 (2012) 2043–2048.
[42] G. Wang, B. Huang, Z. Li, Z. Lou, Z. Wang, Y. Dai, M.H. Whangbo, Sci. Rep. 5 (2015)8544.
[43] A. Tetsuya, M. Teruyoshi, S. Koichi, Japn. J. Appl. Phys. 8 (1969) 1411.
[44] Z. Fang, S. Weng, X. Ye, W. Feng, Z. Zheng, M. Lu, S. Lin, X. Fu, P. Liu, ACS Appl.
Mater. Interfaces 7 (2015) 13915–13924.
[45] M. Surma, A. Zakrzewski, M. Godlewski, T. Surkova, Acta Phys. Pol. A 1 (1995)
221–224.
[46] H. Tang, G. Xu, L. Weng, L. Pan, L. Wang, Acta Mater. 52 (2004) 1489–1494.
[47] N.D.P.H. Borse, R.F. Shinde, S.K. Date, S.K. Kulkarni, J. Mater. Sci. (1999)
6087–6093.
[48] H. Hosokawa, K. Murakoshi, Y. Wada, S. Yanagida, M. Satoh, Langmuir 12 (1996)
3598–3603.

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