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Stabilizing Atomically Dispersed Catalytic Sites on
Tellurium Nanosheets with Strong Metal–Support
Interaction Boosts Photocatalysis
Li Shi, Xiaohui Ren, Qi Wang, Yunxiang Li, Fumihiko Ichihara, Hongwei Zhang,
Yasuo Izumi, Long Ren, Wei Zhou, Yang Yang, and Jinhua Ye*

The utilization of appropriate supports for constructing single-atom-catalysts is of vital importance to achieve high catalytic performances, as thestrong mutual interactions between the atomically dispersed metal atoms
and supports significantly influence their electronic properties. Herein,
it is reported that atomic cobalt species (ACS) anchored 2D tellurium
nanosheets (Te NS) can act as a highly active single-atom cocatalyst forboosting photocatalytic H2 production and CO2 reduction reactions undervisible light irradiation, wherein Te NS serves as the ideal support material
to bridge the light absorbers and ACS catalytic sites for efficient electrontransfer. X-ray absorption near-edge structure spectroscopy reveals that the
ACS are built by a Co center coordinated with five CoO bonding, which
are anchored on Te NS through one CoTe bonding. The strong mutualinteraction between the Te NS and ACS alters the electronic structure of
Te NS, inducing the introduction of intermediate energy states, which
act as trap sites to accommodate the photogenerated electrons for promoting photocatalytic reactions. This work may inspire further capability
in designing other Te-based single-atom-catalysts for highly efficient solar
energy conversion.

Dr. L. Shi, X. Ren, Q. Wang, Dr. Y. Li, F. Ichihara, Prof. J. Ye
International Center for Materials Nanoarchitectonics (WPI-MANA)
National Institute for Materials Science (NIMS)
1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
E-mail: Jinhua.YE@nims.go.jp
X. Ren, Q. Wang, Dr. Y. Li, F. Ichihara, Prof. J. Ye
Graduate School of Chemical Sciences and Engineering
Hokkaido University
Sapporo 060-0814, Japan
H. Zhang, Prof. Y. Izumi
Department of Chemistry
Graduate School of Science
Chiba University
Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan
Dr. L. Ren
Australia Institute for Superconducting and Electronic Materials (ISEM)
Australian Institute for Innovative Materials (AIIM)
University of Wollongong
Wollongong, NSW 2500, Australia

1. Introduction
The conversion of abundant and renewable solar energy into valuable chemical
fuels is highly desirable to meet current
and future energy demand.[1] As a promising strategy to harvest and convert solar
energy, photocatalysis has attracted considerable attention, because it can produce clean and sustainable fuels in anenergy-saving way.[2,3] The exploration of
appropriate materials as photocatalysts
historically guided the development ofphotocatalysis. Although significant progress has been achieved during the past
decades, it still remains challenging toconstruct highly efficient materials forphotocatalysis, largely because of the main
intrinsic limitation: fast recombination ofphotogenerated charge carriers.[4,5] The
photocatalytic performances can be greatly
improved with the assistance of cocatalysts
via lowering the overpotential or activation

Prof. W. Zhou
Department of Applied Physics
Tianjin Key Laboratory of Low Dimensional Materials Physics
and ­Preparing Technology
Faculty of Science
Tianjin University
Tianjin 300072, P. R. China
Prof. Y. Yang
NanoScience Technology Center
Department of Materials Science and Engineering
Energy Conversion and Propulsion Cluster
University of Central Florida
12424 Research Parkway Suite 423, Orlando, FL 32826, USA
Prof. J. Ye
TJU-NIMS International Collaboration Laboratory
School of Material Science and Engineering, and Collaborative­Innovation Center of Chemical Science and Engineering (Tianjin)
Tianjin University
Tianjin 300072, P. R. China

The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202002356.

DOI: 10.1002/smll.202002356

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Full Paper

energy of the targeted reactions and retarding the recombination of electron–hole pairs.[6,7] Noble-metals, such as Au, Pt, andRu, are extensively investigated as effective cocatalysts to boost
photocatalysis, however, their large-scale energy production isgreatly limited by their scarcity and high cost.[8,9] In recent decades, transition metal-based materials, particularly cobalt andits derivative oxide, phosphide, sulfide, and boride have been
proposed as efficient and nonprecious cocatalysts alternative
to noble-metals.[6,7] The high activity of cobalt-based cocatalysts can be due to their intrinsic catalytic active sites that canlower the overpotential of photocatalytic reactions, as well as
the strong interfacial interactions between the semiconductor
and the cocatalyst that expedite the charge transfer.[6] Reducing
the size and dimension of cocatalysts to clusters or even single
atoms provides an effective way to further boost the photocatalytic performances by maximizing the amount of exposed
active sites, which is emerging as a new research frontier.[10–14]
The atomically dispersed single metal atoms have high surface
energy and are easy to mobile and aggregate during the preparation process, thus an appropriate support material with available anchor sites is required to coordinate/stabilize them.[15,16]
From a perspective of fully exploiting the performances ofsingle atom cocatalysts, two prime factors are of vital importance: i) the efficiency of electron transfer from photocatalysts to atomically dispersed catalytic sites and ii) the mutual
metal–support interactions between atomically dispersed
metals and supports.[10,17] The strong metal–support interactions have significant influences on the electronic structure of
single-atom-catalysts, guarantee the charge transfer, and finally
improve the catalytic activities and the stability.[18,19] Thus, theselection of supports that can produce strong metal–­support
interactions and facilitate the electron transfer between thephotocatalysts and atomically dispersed catalytic sites is significant for achieving efficient photocatalysis. An ideal support
material for constructing single atom cocatalysts owns two
main features: i) flexible surface and available coordinationsites to stabilize atomically dispersed metals and ii) high carrier mobility to guarantee the charge transportation.[15] Metal
oxide, metal sulfide, graphene, metal–organic frameworks,
and g-C3N4 have been intensively investigated as appropriate
support materials for anchoring single metal atoms based on
MO, MS, MC, and MN bonding interactions (M refers
to metal), which strengthen the stability and improve the catalytic performances.[10] However, the recently developed supports
cannot fulfill the entire requirements of catalysis community.
The exploration of novel supports for producing strong metal–
support interactions and the systematic study of the related
mechanism are of vital importance to the practical applicationsof single-atom-catalysts but remain challenging.
Tellurium (Te) has received tremendous attention as
an emerging material because of its intriguing properties,
including high carrier mobility, excellent thermo/piezoelectricity, high electronic conductivity, and fast photoresponse.[20,21]
Those properties guarantee enormous potential applications of
Te in electronic devices.[22] Te displays a typical chiral-chain-likecrystalline structure, which is built with helical Te chains along
the direction of the c-axis. Every Te atom is covalently bondedwith two Te atoms in the same chain, while individual chainsare bound together via van der Waals’ force into a 2D hexagonal

Small 2020, 16, 2002356

elementary cell.[20] Due to its unique crystal structure, 2D hexagonal Te nanosheets were reported to be obtained via eitherhydrothermal process or liquid-phase exfoliation method.[23,24]
In sharp contrast to phosphorene and other environmentsensitive 2D materials, Te exhibits pronounced stability, which
renders it more attractive in practical applications.[25] Recent
studies have revealed that 2D Te displays p-type semiconducting
property with tunable bandgaps from 0.35 eV in bulk state to
1 eV in a monolayer.[21] The thickness-dependent bandgap andhigh photoconductivity enable 2D Te promising application in
optoelectronics. In addition, 2D Te with abundant surface Te
sites is a promising support to readily immobilize other elements, such as transition metals, to form functionalized 2D Te
materials. The utilization of 2D Te or its functionalized materials as cocatalysts in photocatalysis is promising but has notbeen explored yet.
Herein, we demonstrate a facile method to stabilize theatomic cobalt species (ACS) on the Te NS, and apply it as anovel and highly active cocatalyst for boosting photocatalytic
H2 production and CO2 reduction reactions. The extended
X-ray absorption fine structure (EXAFS) spectra confirm thehomogeneous dispersion of ACS on the Te NS. The coordination between the Te NS support and ACS can not only strongly
immobilize monodispersed ACS but also affect the electronic
structure of Te NS. The functionalization of Te NS by ACS
introduces intermediate energy states below the conduction
band position of Te NS, which act as trap sites to accommodate the photogenerated electrons, and promote charge separation and transfer. The photocatalytic performance evaluations
indicate that ACS functionalized Te NS (Te NS-ACS) shows adramatically enhanced visible-light photocatalytic H2 production rate of 130.8 µmol h−1 in the presence of Eosin Y (EY)
as light absorber, and CO production rate of 52.3 µmol h−1
in the p­resence of Tris(2,2′-bipyridyl)dichlororuthenium(II)([Ru(bpy)3]Cl2) as light absorber, representing the state-ofthe-art noble-­metal-free cocatalyst.

2. Results and Discussion
The rationale for the proposed concept of using 2D Te as a support material is that the abundant surface Te sites can coordinate readily with the Co atom through CoTe bonding in orderto prevent the ACS aggregation. Moreover, Te with high electronic conductivity (2 × 102 S m−1)[26] can serve as an electron
relay to facilitate the electron transfer between ACS catalytic
sites and light absorbers, which fulfils the criteria for ideal supports. Figure 1a illustrates the synthetic process of Te NS-ACS.
First of all, the Te NS was prepared from the liquid exfoliation of bulk Te in 1-methyl-2-pyrrolidinone (NMP) solution.
As characterized by atomic force microscopy (AFM, Figure S1,
Supporting Information), the measured 2D Te NS has size distributions in a broad range from around 20 to 70 nm. Muchsmaller Te NS with lateral dimensions in several nanometersis also observed. The measured thickness of Te NS varies from
2.0 to 8.2 nm. The transmission electron microscopy (TEM)
image in Figure S2a of the Supporting Information further confirms the nanoscale size of Te NS. The high-resolution transmission electron microscopy (HRTEM) image in Figure S2b of

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Figure 1. a) The illustration of the process to fabricate Te NS-ACS. b) AFM and the corresponding thickness distribution of randomly measured Te
NS-ACS. c) TEM and HRTEM images of Te NS-ACS. d) XRD patterns, e) Raman spectra, and f) UV–vis absorption spectra of bulk Te, Te NS, and Te
NS-ACS.

the Supporting Information shows a lattice spacing of 0.32 nm,
which is ascribed to the (101) plane of Te.[24] Then, the Te NS
reacted with CoCl2⋅6H2O to obtain Te NS-ACS. The loading
amount of Co on Te NS is 1.5 wt% measured by ICP-AES.
Figure 1b shows the AFM image and its corresponding thickness distribution of six randomly measured Te NS-ACS. It canbe seen that the randomly measured thickness of nanosheets
varies from 1.5 to 7.3 nm, which is similar to the pristine Te NS,
indicating the thickness of Te NS remains almost unchanged
after ACS functionalization. The as-obtained Te NS-ACS showsa diameter of tens of nanometers, and the lattice spacing of0.32 nm can be attributed to the (101) plane of Te (Figure 1c;
Figure S3, Supporting Information).
X-ray diffraction (XRD) patterns were measured to investigate the crystal structures of samples. As shown in Figure 1d,bulk Te shows a standard hexagonal crystalline structure,
matching well with the references.[24,27] The as-prepared 2D Te
NS and Te NS-ACS exhibit the same hexagonal structure, confirming that the crystal structure of 2D Te is unchanged after
exfoliation and ACS functionalization. No peaks corresponding
to CoO or Co are observed. The Raman spectra in Figure 1eshow that the bulk Te exhibits vibration peaks centered at119.2 and 145.1 cm−1, ascribing to the A1 and E2 phonon modes,
respectively.[28] Te NS shows red-shift of E2 and blue-shiftof A1 compared with bulk Te, reaching 140.7 and 122.1 cm−1,
respectively. The red-shift of the E2 mode is attributed to thelength elongation of the covalent bond between two adjacent
Te atoms and weakening of the interatomic interactions, while

Small 2020, 16, 2002356

the blue-shift of A1 is caused by the equilateral triangle projection increasing on basal plane, which is perpendicular to thec-axis of the Te chain.[28] For the Te NS-ACS, the A1 and E2 peaks,
located at 122.2 and 141.9 cm−1, respectively, exhibit minor blueshifts as compared to the pristine Te NS, further confirmingthat the hexagonal crystalline structure of 2D Te is unchanged
after ACS functionalization. The optical properties of samples
were recorded by ultraviolet–visible (UV–vis) diffuse reflectancespectra. As shown in Figure 1f, bulk Te exhibits a very broad
absorption from the UV to near-infrared (NIR) region with atypical absorption peak located at 250.9 nm, which is due tothe p-bonding to p-antibonding bands interband transitions.[29]The Te NS shows an improved light absorption ability than its
bulk phase, with a very strong absorption tail from the visible
to NIR region, which is due to the weaker electronic band dispersion of Te NS than that of bulk Te.[29] The bandgaps of samples were calculated according to transformed Kubelka–Munkfunction as shown in Figure S4 of the Supporting Information. The bandgap of bulk Te is 0.37 eV, which is similar to thereported value of bulk Te.[21] The bandgap of Te NS increases to
0.71 eV, which is ascribed to the quantum confinement effectinduced by reducing the layer numbers in Te.[21] The calculated
bandgap of Te NS-ACS decreases to 0.61 eV as compared to
the Te NS, owing to the increase in the carrier concentrationby the introduction of ACS and the creation of energy levelsin the bandgap. The flat-band (FB) potentials for Te NS and
Te NS-ACS are +0.22 V versus NHE and +0.13 V versus NHE,
respectively (Figure S5, Supporting Information). Considering

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Figure 2. a) Co K-edge XANES spectra and b) the associated Fourier transform k3-weighted EXAFS spectra for Co metal, CoO, CoTe2, and Te NS-ACS.
Best-fit results in k- and R-space of c) CoO and d) CoTe for Fourier-filtered EXAFS data of Te NS-ACS.

the small difference between the FB potential and the higher
valence band edge for p-type semiconductor, the estimatedband positions of Te NS with and without ACS functionalization are shown in Figure S5 of the Supporting Information,
which indicates that the conduction band minimum potentials
of Te NS and Te NS-ACS can reach the H2 evolution energy
level.
To probe the chemical environment of Co element in the Te
NS-ACS, the Co K-edge EXAFS and X-ray absorption near-edge
structure (XANES) spectroscopies were measured. Figure 2ashows the normalized XANES spectra collected at Co K-edge
for Co metal, CoO, CoTe2, and Te NS-ACS. The Co K-edge of Te
NS-ACS sample displays a similar near edge structure to thatof CoO, indicating the presence of bivalent Co. The detailed
coordination environment of Co was further investigated by
the Fourier transform of EXAFS spectra. The first major peakin the EXAFS spectra of Te NS-ACS (Figure 2b) shows that Co
atoms are coordinated with O nearest neighboring shell locatesat 1.57 Å, referring to the peak of CoO bonding in CoO. The
second major peak can be attributed to CoTe bonding that Co
atoms are coordinated by Te nearest neighboring shell at 2.37 Å,
which is well consistent with data for CoTe2 reference sample,
indicating that Co atoms interact with Te NS through CoTe
bonding. As compared to the EXAFS spectra of Co metal foil,
the signal at 2.19 Å is absent in Te NS-ACS, which excludes
the existence of CoCo coordination bonding. Moreover, comparing with CoO, the CoOCo coordination bonding is alsoabsent in the Te NS-ACS sample, demonstrating that the Co

Small 2020, 16, 2002356

atom exists as the single-atom structure to bond with O and
Te. The coordination numbers of the nearest-neighbor O andTe atoms surrounding the single Co atom are estimated to be
4.8 and 0.92, respectively, according to the EXAFS curve fitting
analysis (Figure 2c,d; Table S1, Supporting Information). Basedon above information, the atomic geometry structure of Te
NS-ACS is built by the density functional theory (DFT) simulation, in which the Co center is coordinated with five CoO
bonding and one CoTe bonding, and the optimized atomic
structure model is described in Figure 3a.
DFT calculations were further performed to reveal the influence of ACS functionalization on the electronic structure of Te
NS. Figure 3a,b shows the calculated density of states (DOS) of
Te NS-ACS and Te NS, respectively. It can be seen that the total
DOS of Te NS-ACS exhibit an upward shift of valence band
minimum (0.08 eV) toward the Fermi level with respect to thatof Te NS, leading to a narrower bandgap and easier excitationof charge carriers. The ACS are anchored on Te NS throughCoTe bonding, and due to the low orbital level of ACS, theorbital hybridization between the ACS and Te atoms would
result in new intermediate energy states in the bandgap of Te
NS. As shown in Figure 3a, intermediate energy states composed of Te 5p, Co 3d, and O 2p orbits appear below the conduction band (CB) of Te NS-ACS. The intermediate energy states
act as trap sites to accommodate the photogenerated electrons,
promoting charge separation and transfer. The charge density
distributions of Te NS-ACS interface are shown in Figure 3c,d.
ACS and CoTe bonding exhibit stronger charge density than

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Figure 3. a) The optimized atomic structure model for Te NS-ACS, and the corresponding calculated DOS. b) The single-layer model structure of Te,
and the corresponding calculated DOS. c) 3D and d) 2D charge density distributions of Te NS-ACS.

Te NS, indicating the strong mutual interaction between ACS
and Te NS, and the generated CoTe bonding induces theefficient charge transfer from Te NS to ACS. Figure S6 of the
Supporting Information shows the electrochemical impedance spectrum of Te NS and Te NS-ACS, which experimentally
proves the decreased electron transfer resistance of Te NS after
ACS functionalization.
The photocatalytic performances of Te NS with and without
ACS functionalization were first investigated for H2 evolution under visible light irradiation using EY light absorber
and triethanolamine (TEOA) sacrificial agent. As shown in
Figure 4a,b, the H2 evolution is not observed for pure Te NS
with and without ACS functionalization in the absence of EY
under visible light. Pure EY shows a very moderate H2 evolution rate of 10.1 µmol h−1, while the H2 evolution activity isenhanced upon adding Te NS into the reaction system, suggesting the cocatalytic effect of Te NS for photocatalytic H2evolution. Te NS is a more effective cocatalyst than bulk Te forpromoting photocatalytic H2 evolution in the presence of EY(Figure S7, Supporting Information). When using Te NS-ACS
as a cocatalyst in the system, the photocatalytic H2 evolution
activity can be significantly improved to an evolution rate of
130.8 µmol h−1. In addition, Te NS-ACS is much more effective than Te NS-Co2+ for promoting photocatalytic H2 evolution(Figure S8, Supporting Information). These results indicatethat the ACS play a dominant role for improving the photocatalytic H2 evolution. The measured CB position of Te NS-ACS
is slightly above the potential required for H2 evolution, thus
Small 2020, 16, 2002356

the electrons on the CB position of Te NS-ACS have very weak
reduction ability to produce H2, and the high activity of Te
NS-ACS is mainly ascribed to the atomic cobalt species active
sites, which can accumulate electrons to promote H2 evolution.
The obtained photocatalytic H2 evolution rate by Te NS-ACS
cocatalyst is superior to those of the majority of reported noblemetal-free cocatalysts in EY sensitized photocatalytic reaction
systems (Table S2, Supporting Information).[30–35] The photo­catalytic stability of Te NS-ACS for H2 evolution was also
examined. As shown in Figure 4c, continuous H2 production
is observed with no obvious decrease in the subsequent runs,
indicating that the Te NS-ACS acts as a stable cocatalyst forphotocatalytic H2 evolution.
Noting that the charge transfer is a crucial fact that contributes to the improvement of the photocatalytic performance,
photoluminescence (PL) spectroscopy is first utilized to reveal
the charge transfer kinetics. As shown in Figure S9 of the Supporting Information, EY in aqueous solution shows a strong
emission peak at 580.3 nm upon excitation at 400 nm. Afterdispersing Te NS and Te NS-ACS into EY solution, the PL issignificantly quenched, indicating a promoted electron transferfrom photoexcited EY to Te NS and Te NS-ACS. The PL intensity follows the order: pure EY > EY/Te NS > EY/Te NS-ACS,
revealing that the electron transfer from EY to Te NS-ACS is
more efficient than that from EY to Te NS. The time-resolved
PL decay measurements were further employed to study theelectron transfer kinetics. The decay spectra together with thecorresponding lifetime calculated by the biexponential analysis

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Figure 4. a) Time-dependent H2 evolution and b) comparison of H2 evolution activities under different conditions. c) Cycling test of EY/Te NS-ACS forphotocatalytic H2 evolution. d) Proposed mechanism of EY/Te NS-ACS for photocatalytic H2 evolution.

are shown in Figure S10 of the Supporting Information. The
PL lifetime follows in the order: pure EY > EY/Te NS > EY/Te
NS-ACS. A shortened average lifetime is observed for EY in
the presence of Te NS and Te NS-ACS, demonstrating the electron transfer from excited EY to Te NS and Te NS-ACS. It also
indicates that the electron transfer from EY to Te NS-ACS is
more efficient than that from EY to Te NS, as EY/Te NS-ACS
shows the shortest PL lifetime. The improved electron transfer
efficiency is largely due to the fact that the ACS sites can serveas trap sites to capture electrons from photoexcited EY. The
mechanism of photocatalytic H2 production for the EY-photosensitized Te NS-ACS system is shown in Figure 4d based on
the above results and previous reports of EY-sensitized systems.[34,35] Under visible light irradiation, the EY can be excited
to produce EY1* (singlet excited state), followed to generate theEY3* (lowest-lying triplet excited state) through an intersystemcrossing process, yielding (EY+• EY−˙) radical pairs. Then, EY+•
is reductively quenched back to EY by electron donor TEOA.
The redox potential of EY1∗/EY+• is −1.1 V versus NHE,[36] which
is much higher than the CB potential of Te NS-ACS, enabling
the electrons transfer from photoexcited EY to the CB andthe ACS intermediate energy level of Te NS-ACS, and finallyinitiate the H2 evolution.
In an attempt to extend the applications of the constructed
Te NS-ACS cocatalyst to other photocatalytic reactions, the­photocatalytic CO2 reduction reactions were evaluated. The reactions were conducted under mild reaction conditions (ambient
temperature and pressure), and a typical visible-­light-active
photosensitizer [Ru(bpy)3]Cl2 was chosen as the light absorberand charge generator.[37,38] The setup of the system is shown in
Figure S11 of the Supporting Information. The main p­ roducts

Small 2020, 16, 2002356

detected in this system are CO and H2, and no CH4 or liquid
products evolution is observed, which is the same with thepreviously reported systems.[38] The time-dependent photocatalytic activity of Te NS-ACS photosensitized by [Ru(bpy)3]
Cl2 is shown in Figure S12 of the Supporting Information.
The evolution rates for CO and H2 decrease after 1 h continuous testing, which is largely ascribed to the photobleaching of
the [Ru(bpy)3]Cl2 photosensitizer.[37–39] Figure 5a summarizes
the CO and H2 evolution performances from photocatalytic
CO2 reduction under various reaction conditions for 2 h. The
bare [Ru(bpy)3]Cl2 only exhibits the CO evolution amount of
4.5 µmol and H2 evolution amount of 6.7 µmol, and the bare
Te NS is inactive in photocatalytic CO2 reduction reaction.
When Te NS was added into the system, the evolution amountof H2 slightly increased while the generation of CO was suppressed. This result i­ndicates that the Te NS is not a promising
cocatalyst for photocatalytic CO2 reduction, largely due to the
low CB position (−0.49 V vs NHE, pH = 7) that cannot trigger theCO2 to CO conversion reaction (CO2 + 2H+ + 2e− → CO + H2O,
E0redox = −0.53 V vs NHE at pH 7). Interestingly, when Te
NS-ACS was introduced into the system, dramatically improved
CO and H2 evolutions were observed, reaching 104.5 and84.9 µmol within 2 h, respectively. The obtained CO evolution
rate outperforms the state-of-the-art catalysts under comparable
conditions (Table S3, Supporting Information).[37–43] The CO
and H2 evolutions are completely terminated in the absence of[Ru(bpy)3]Cl2, indicating the cocatalytic effect of Te NS-ACS in
the photocatalytic CO2 reduction reaction. Apparently, it is the
ACS that acts as the active site for superior CO and H2 evolution. Even though the reduction potential of the CO formation is as negative as −0.53 V versus NHE at pH 7, which is2002356 (6 of 10)© 2020 Wiley-VCH GmbH

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Figure 5. a) CO and H2 evolution from photocatalytic CO2 reduction under different conditions (the reaction time is 2 h). b) Cycling test of [Ru(bpy)3]
Cl2/Te NS-ACS for photocatalytic CO2 reduction, and each cycle is conducted for 2 h. c) GC-MS of 13CO generated in the photocatalytic reduction of
13CO over [Ru(bpy) ]Cl /Te NS-ACS. d) Proposed mechanism for photocatalytic CO reduction over [Ru(bpy) ]Cl /Te NS-ACS.
2
3
2
2
3
2

not energetically accessible for Te NS-ACS (CB position locatesat −0.48 V vs NHE at pH 7), we propose that the ACS intermediate energy states with abundant localized electrons transferred from [Ru(bpy)3]Cl2 are beneficial for lowering the energy
barrier to activate CO2 molecules. Similar mechanisms werereported in other photocatalyst systems such as TiO2, WO3, andBi-based semiconductors with defective energy states for the
reduction of CO2 and N2.[44–46]
To reveal the photocatalytic CO2 reduction process in[Ru(bpy)3]Cl2/Te NS-ACS system, in situ Fourier transform
infrared spectroscopy (FTIR) was employed. As is known toall, photocatalytic reduction of CO2 to CO proceeds throughthe generation of a formate intermediate (COOH*) via the

Small 2020, 16, 2002356

reaction equation CO2* + H+ + e− → COOH*.[40] As shown in
Figure S13a of the Supporting Information, the strong peaklocated at 2331 cm−1 is attributed to the absorbed CO2.[47] The
broad absorption peak centered at 1520 cm−1 corresponds
to the carbonate-type species, which are derived from theadsorbed formate (COOH*) intermediate.[47] In addition, thebroad absorption peak centered at 1725 cm−1 is ascribed tothe asymmetric stretching of OCO, which is also derived
from the formate species.[48] The intensities of these peaks
gradually increase with the increase of light irradiation time,demonstrating the generation of formate intermediate over the[Ru(bpy)3]Cl2/Te NS-ACS photocatalytic CO2 reduction system.
The formate intermediate originates from the CO2 activation

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during photocatalytic CO2 reduction reaction. The COOH*
intermediate is then converted to CO* according to the reaction equation COOH* +H+ + e− → CO* + H2O, and the final
product CO is produced from the dissociation of the CO*.[40]In sharp contrast, when Te NS is used as cocatalyst in the reaction system, no obvious peaks correspond to the formate intermediate are observed (Figure S13b, Supporting Information).
The in situ FTIR characterization clearly demonstrates thatthe intermediate energy states induced by ACS can lower theenergy barrier for activating CO2 molecules.
The stability of Te NS-ACS for photocatalytic CO2 reduction was then evaluated. As shown in Figure 5b, the recycled
Te NS-ACS cocatalyst retains 93.3% of the original activity after
four runs, indicating the good stability for photocatalytic CO2
reduction. The XRD pattern of Te NS-ACS after photocatalytic CO2 reduction is also recorded, which shows no obvious
change of crystal structure (Figure S14, Supporting Information). The evolution of CO is not detected under either Aror without light, showing that the CO product indeed comesfrom the photocatalytic CO2 reduction. To validate the carbon
source of the generated CO, isotopic experiment was conductedin the [Ru(bpy)3]Cl2/Te NS-ACS system by using 13CO2. The
generated gas products were analyzed by gas chromatogram(GC) and mass spectra (MS). As shown in Figure 5c, the CO
product is detected at the GC retention time of 6.15 min, andthe other two peaks before CO in GC spectrum are proved to
be O2 and N2, which come from the remained air in the sampling needle. The analysis of CO product gets the m/z = 29 inMS, which is ascribed to the 13CO, clearly confirming that the
CO generates from CO2 conversion. The proposed ­mechanism
for the [Ru(bpy)3]Cl2 related photocatalytic CO2 reduction­process is shown in Figure 5d. Under visible light irradiation,
the light absorber [Ru(bpy)3]Cl2 is promoted to the excitedstate [Ru(bpy)3]Cl2*, which is then directly quenched to produce electrons and [Ru(bpy)3]3+. The generated electrons can betransferred to the CB and the ACS intermediate energy levelof Te NS-ACS due to the high redox potential of [Ru(bpy)3]Cl2(E0[Ru(bpy)3]Cl2*/[Ru(bpy)3]3+ = −1.33 V vs SCE).[49] Then, theCO2 molecules and H+ absorbed on the surface of Te NS-ACS
are reduced to CO and H2. The oxidized state of the [Ru(bpy)3]3+
is simultaneously reduced back to [Ru(bpy)3]Cl2 by the sacrificial electron donor (TEOA).

3. Conclusion
In summary, Te NS-ACS was rationally prepared and investigated as a novel and efficient noble-metal-free cocatalyst toboost the photocatalytic H2 evolution and CO2 reduction performances. Specifically, a stable and high photocatalytic H2
production rate of 130.8 µmol h−1 was achieved by applying Te
NS-ACS as cocatalyst and EY as light absorber, outperformingthat of the majority of reported noble-metal-free cocatalysts ina similar system. Besides, the newly developed Te NS-ACS can
also be applied as an effective cocatalyst to promote the photocatalytic CO2 reduction reaction in the presence of [Ru(bpy)3]
Cl2 as light absorber with a CO evolution rate of 52.3 µmol h−1,outperforming that of the state-of-the-art catalysts undercomparable conditions. The high performance of Te NS-ACS

Small 2020, 16, 2002356

can be ascribed to the fact that the strong mutual interaction between the Te NS support and ACS alters the electronic
structure of Te NS, leading to the introduction of intermediate
energy states, which act as trap sites to accommodate thephotogenerated electrons, promoting charge separation andtransfer. This study adds a new class of 2D Te-based nanomaterials into the large family of noble-metal-free cocatalysts,
and opens up new possibilities in the emerging field of solar
energy conversion.

4. Experimental Section
Preparation of Te Nanosheets: Te nanosheets were prepared by using
a liquid exfoliation method. Typically, 0.5 g of bulk Te (purchased from
Sigma-Aldrich) was added into 50 mL NMP solution. The exfoliation
process was carried out on microtip probe ultrasonication (Branson
Sonifier advanced 250) at an output power level 5 and duty cycle 50%
cooled by ice water. After exfoliation for 2 h, the Te nanosheets solution
was obtained after being centrifuged at 4000 rpm for 5 min in order toremove the residual bulk Te. After that, Te nanosheets were obtainedby further centrifugation at 18 000 rpm for 10 min. The obtained Te
nanosheets were washed with ethanol for three times, and dried in
vacuum at 50 °C.
Preparation of Te NS-ACS: The obtained Te nanosheets were mixed
with CoCl2·6H2O (5 wt% Co) directly in 5 mL of ethanol. The mixture was
ground in agate mortar until dried, then the Te nanosheets CoCl2·6H2O
precursor sample was put into a quartz boat and heated in the quartz
tube furnace at 300 °C for 2 h in Ar. The final samples were collected
after cooling down to room temperature and washed with water forseveral times to remove the unreacted CoCl2·6H2O, and dried in vacuum
at 50 °C. The loading amount of Co on Te NS was 1.5 wt% measured by
ICP-AES.
Photocatalytic H2 Evolution: 10 mg of Te NS or Te NS-ACS cocatalysts,
30 mg of EY, and 280 mL aqueous solution with 30 mL of triethanolamine
were added in a glass reaction cell, and then the reaction cell wasconnected to a glass-closed reaction system. During the photocatalytic
reaction, a gas circulation pump was utilized to cycle the gas in thereaction system. Before light irradiation, the air in the reaction solution
was completely removed by evacuation. The visible light source
was applied by a 300 W Xe lamp coupled with a UV-light cutoff filter(L42, λ > 420 nm). The amount of H2 was determined by using a gas
chromatograph (GC-8A, Shimadzu Co., Japan) with pure Ar carrier gas.
The cycling experiment was carried out as follows. The used Te NS-ACS
cocatalyst was obtained by centrifugation and then washing with water
for three times, followed by redispersing in a fresh reaction solution(30 mg of EY, 280 mL solution containing 30 mL of triethanolamine) for
the next cycling test until a total of 4 cycles.
Photocatalytic CO2 Reduction: The photocatalytic CO2 reduction
reactions were conducted in a liquid–solid reaction mode in a
100 mL quartz reactor under ambient conditions, and the setup of
the experiment is shown in Figure S11 of the Supporting Information.
Typically, 10 mg of Te NS or Te NS-ACS cocatalysts, 30 mg of Tris(2,2′bipyridyl)dichlororuthenium(II) (commercially obtained from SigmaAldrich), 6 mL of water, 18 mL of acetonitrile (MeCN), and 6 mL oftriethanolamine were dispersed in a quartz reactor. The reaction solution
was sonicated for a while to obtain well-dispersed solution. Then thereactor was connected to a glass-closed reaction system, followed by
evacuating completely (no O2 or N2 can be detected). After that, theairtight system was filled with about 101 kPa pure CO2 gas. The visible
light source was applied with a 300 W Xe lamp coupled with a UV cutoff
filter (L42 filter, λ > 420 nm), and a cooling water filter was set between
light and reactor. The evolved amount of H2 and CO gas products
were detected and analyzed by using gas chromatography (GC-8A,
Shimadzu Co., Japan) and (GC-14B, Shimadzu Co., Japan), respectively.
The isotope-labeled experiment was carried out by using 13CO2 instead

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of 12CO2, while keeping other conditions unchanged, and the products
were detected and analyzed by GC-MS (JEOL-GCQMS, JMS-K9, and
6890N Network GC). The cycling experiment was carried out as follows.
The used Te NS-ACS cocatalyst was obtained by centrifugation and thenwashing with MeCN for three times, followed by redispersing in a fresh
reaction solution (30 mg of Tris(2,2′-bipyridyl)dichlororuthenium(II),6 mL of water, 18 mL of MeCN, and 6 mL of TEOA) for the next cycling
test until a total of 4 cycles.

Supporting Information
Supporting Information is available from the Wiley Online Library orfrom the author.

Acknowledgements
This work received financial support from the World PremierInternational Research Center Initiative (WPI Initiative) on Materials
Nanoarchitectonics (MANA), MEXT (Japan), JSPS KAKENHI(JP18H02065), the Photo-excitonix Project at Hokkaido University,
National Natural Science Foundation of China (21633004), andFundamental Research Funds for the Central Universities (No.
NE2019103).

Conflict of Interest
The authors declare no conflict of interest.

Keywords
cocatalysts, photocatalysis, single-atom-catalysts, tellurium nanosheets
Received: April 13, 2020
Revised: June 16, 2020
Published online: August 5, 2020

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