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Single Cobalt Atom Anchored Black Phosphorous
Nanosheets as an Effective Cocatalyst Promotes
Photocatalysis
Xiaohui Ren,[a, b] Li Shi,*[b] Yunxiang Li,[a, b] Shuang Song,[b, c] Qi Wang,[a, b] Shunqin Luo,[a, b]Long Ren,[d] Hongwei Zhang,[e] Yasuo Izumi,[e] Xinsheng Peng,[f] Davin Philo,[a, b]
Fumihiko Ichihara,[a, b] and Jinhua Ye*[a, b, g, h]Exploring novel cocatalysts with sufficient active sites and rapid
photogenerated carrier separation remains challenge for boosting photocatalytic reactions. Recently, two-dimensional (2D)
black phosphorus (BP) has been developed and recognized to
be an ideal platform for anchoring metal ions through lone pairelectrons. Herein, we propose a new structure that cobalt
atomically dispersed on BP nanosheets (BP Co), and apply it ascocatalyst for photocatalytic H2 evolution and CO2 reduction
under visible light (> 420 nm) irradiation. Both scanning transmission electron microscope and X-ray absorption near edge
structure verified the existence of phosphorous-coordinated Co
sites. The interaction between Co and P brings greatly improve-

ment on photocatalytic activities, and 18-fold enhancement canbe achieved on CdS when taking BP Co as a noble-metal-freecocatalyst for H2 evolution, which is higher than that of Pt
under the same conditions. Besides, BP Co can also be used as
an effective cocatalyst to promote photocatalytic CO2 reductionin the presence of photosensitizer [Ru(2, 2’-bipyridy)3]Cl2 with a
CO evolution rate of 88.6 μmol h 1, superior to those obtained
from the state-of-the-art cocatalysts. It is anticipated that thecurrent work might provide basic understanding as well as newopportunities on 2D BP nanosheets for versatile photocatalytic
reactions.

1. Introduction

nation and promote surface reactions are commonly used invarious photocatalytic reactions such as water splitting, CO2
reduction, etc.[3] Up to now, the searching for efficient
cocatalysts becomes the utmost priority. Noble metals havebeen extensively investigated and metal Pt shows more favorite
performance than other metals owing to its optimized work
function and overpotential for H2 evolution.[4] Unfortunately,
considering the rarity and expensive cost of noble metals, thePhotocatalytic reaction that can directly drill solar power isessential for next-generation portable and renewable energy
applications.[1] To develop a high-efficient photocatalytic system, both photogenerated electron-hole pair separation efficiency and activation energy barrier on catalyst surface should
be carefully taken into consideration.[2] Toward this end,
cocatalysts with the abilities to retard charge carriers recombi-[a] X. Ren, Dr. Y. Li, Q. Wang, S. Luo, D. Philo, F. Ichihara, Prof. J. Ye
Graduate School of Chemical Sciences and Engineering
Hokkaido University
Kita 8, Nishi 5, Kita-ku
Sapporo 060-0814 (Japan)
E-mail: Jinhua.YE@nims.go.jp[b] X. Ren, L. Shi, Dr. Y. Li, S. Song, Q. Wang, S. Luo, D. Philo, 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: SHI.Li@nims.go.jp[c] S. SongDepartment of Environmental Science and Engineering
School of Architecture and Environment
Sichuan University
Chengdu 610065 (P. R. China)[d] Dr. L. Ren
Australia Institute for Superconducting and Electronic Materials (ISEM)
Australian Institute for Innovative Materials (AIIM)
University of Wollongong
Wollongong NSW 2500 (Australia)

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[e] H. Zhang, Y. Izumi
Department of Chemistry
Graduate School of Science
Chiba University
Yayoi 1–33, Inage-Ku
Chiba 263-8522 (Japan)[f] Prof. X. Peng
State Key Laboratory of Silicon Materials
School of Materials Science and Engineering
Zhejiang University
Hangzhou 310027 (P. R. China)[g] Prof. J. Ye
TJU-NIMS International Collaboration Laboratory
School of Material Science and Engineering
Tianjin University
Tianjin 300072 (P. R. China)[h] Prof. J. Ye
Collaborative Innovation Center of Chemical Science and Engineering(Tianjin)
Tianjin 300072 (P. R. China)
Supporting information for this article is available on the WWW underhttps://doi.org/10.1002/cctc.202000546
This publication is part of a Special Collection on “Phosphorus in Catalysis”.
Please check the ChemCatChem homepage for more articles in the collection.

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exploration of earth-abundant materials as the cocatalyst
becomes indispensable.
To date, earth-abundant transition metals such as Co, Ni, Cu
and their cluster show promising cocatalytic performance invarious photocatalytic reactions.[5] Differing from other nonnoble metals, Co possesses less 3d-orbital electrons andemerges as a promising catalyst for energy conversion andstorage especially for H2 evolution.[6] The existed cobalt can
incorporate with semiconductor, by collecting electrons andproviding effective active sites to enhance the ultimate photocatalytic performance in thermodynamics and kinetics aspects
during catalytic process.[7] Recent advances indicate atomicallydispersed metals (noted as single-atom metal) demonstrate
remarkable catalytic activity owing to the strong size effect andenlarged surface exposed area.[8] It was reported that only small
amount of single-atom metals loaded on C3N4 are necessary to
induce a dramatically improvement of photocatalytic
performance.[9] The high effectivity can be contributed to longer
lifetime of photogenerated electrons in addition to favorable
surface trap states because of the existence of single-atom
metals.
Despite numerous merits of single-atom metals, thetendency of aggregation is usually unavoidable during fabrication processes.[10] Until now, several attempts successfully makean optimization to achieve stable single-atom dispersion as wellas high photocatalytic activity,[11] while the strategy that applies
coordinatively unsaturated material as supporting platform isconsidered to be the most feasible way in current situation.
Recently, newly developed graphene-like semiconductor black
phosphorous (BP) has gained considerable interests for photorelated applications.[12] In the atomic structure of BP, only threeof the five electrons in the outermost orbital are covalently
bonded to each other, leaving the lone pair electrons to interact
with metal ions. Apart from that, the relatively high carrier
mobility in association with tunable direct bandgap together
make it a promising substrate for stable catalyst dispersing.[13]Recent progress achieved by Bai and co-workers provided solid
evidence on BP/Pt heterostructure for demonstrating enhanced
stability as well as accelerated charge separation under the
influence of strong BP Pt interaction.[14] Meanwhile, Yuan et al.
indicated the important role of Co P bonds to facilitate chargecarrier transfer for efficient H2 production.[15] Thereby, theadoption of BP nanosheets as the support material is deemed
to trigger high stability and efficient electron-hole separation
for atomically dispersed metals, prolonging the charge carrier
lifetime and improving the photocatalytic reduction efficiency
in consequence. In this regard, the integration of atomicallydispersed metal Co with BP nanosheets substrate can be
expected for realizing excellent photocatalytic activity. However, no related work has been reported as we known.
Herein, we prepared coordinated Co on BP nanosheets(BP Co) as a cocatalyst and applied it into photocatalytic H2evolution and CO2 reduction. The H2 production performance
evaluation reveals that BP Co can dramatically improve thephotocatalytic performance of CdS, reaching an attractive H2
production rate of 345.4 μmol h 1 (18 times higher than that ofpure CdS). Apart from that, BP Co can also be used as an
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effective cocatalyst to promote photocatalytic CO2 reduction in
the presence of photosensitizer [Ru(2, 2’-bipyridy)3]Cl2 ([Ru]).
The CO evolution rate reaches up to 88.6 μmol h 1, superior tothose results obtained from the state-of-the-art cocatalysts. We
attribute the enhanced photocatalytic reduction performance
into the atomically dispersed cobalt active sites and improved
electron-hole separation efficiency through additional Co P(O)charge transfer channel. By disclosing the ability of BP nanosheets for atomically dispersed cobalt anchoring, our results will
provide a meaningful way for designing advanced BP basedmaterials for efficient photocatalysis.

2. Results and Discussion
BP nanosheets are produced from bulk BP crystal material byliquid exfoliation in 1-Methyl-2-pyrrolidone (NMP) solution as
illustrated in Figure 1(a). Scanning electron microscopy (SEM)
images show the exfoliated few-layer BP nanosheets preserved
its layer structure while the thickness and size decreaseobviously when compared to that of bulk BP crystals (Figure S1). X-ray diffraction (XRD) patterns (Figure S2) show BP
bulk and BP nanosheets can be indexed into orthorhombic BP
crystalline structure in consistent with JCPDS No. 731358. A
new diffraction peak (111) appears in the result of as-preparedBP nanosheets which formerly cannot be found in pristine bulk
structure, and the peak intensity ratio between (021) and (040)also changes from (0.496/1) to (1.027/1). These slight deviations
possibly resulted from the cleaving along with {010} facets of
BP crystal during exfoliation process, as reported previously byother papers.[16] From High-resolution transmission electron
microscopy (HRTEM) images (Figure S3 and Figure S4), thelattice spacing (d-spacing) of 0.33 nm and 0.246 nm corresponds to (021) and (040) plane of few-layer BP nanosheets,
respectively. In addition, atomic force microscopy (AFM)
analyses in Figure S5 confirm the lamellar structure which canbe observed from TEM measurements. The thickness of asprepared BP nanosheets ranges from 4 nm to 10 nm, which iscorresponding to the value of few-layers BP nanosheets.[17]Average line analysis is preformed based on three-dimensional
AFM height image, and the extracted height profile ofexfoliated few-layer BP nanosheets is approximately 5 nm,
indicating the successful preparation of BP nanosheets.
Figure 1(b) illustrates the proposed strategy to attach atomically dispersed Co onto the surface of BP nanosheets (Figure 1(c)). To further confirm the interaction between Co and BP,
characterizations including TEM, high-angle annular dark-field
STEM (HADDF-STEM), AFM and Zeta-potential are investigated(see supporting information Experimental Section for detail
information). The low resolution TEM image (Figure 2(a)) reveals
the typical morphology of layer BP nanosheets. HAADF-STEM
image (Figure 2(b)) of BP Co shows the high density brightdots (highlighted by red circles), which clearly depicts that the
Co is atomically dispersed on the surface of BP, while the
HRTEM image of the related area demonstrated in Figure 2(c)indicates good crystallization of BP nanosheets. Besides that,
EDX spectroscopy (Figure S6) and mappings (Figure 2(d) and

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Figure 1. (a) Schematic illustration of NMP liquid solution assisted probe sonication exfoliation route in ice water bath to prepare few-layer BP nanosheets. (b)Proposed strategy to obtain uniform dispersed Co on BP nanosheets substrate. (c) Atomic structure of single atom dispersed BP nanosheets.

Figure S7) also indicate the existence of uniformly distributed
Co element on BP nanosheets, while unavoidable oxidation andthe use of CoCl2 in synthesis processes maybe lead to theappearance of O and Cl elements. We notice that the layered
structure and the lateral size of BP nanosheets remain intact
after Co loading from SEM images (Figure S8). Meanwhile, theAFM measurement of BP Co material reveals sheet-like structure and the thickness has nearly no change when compared tothat of original BP layers (Figure 2(e) and Figure 2(f)). Zetapotential measurements in Figure 2(g) indicate the surface
charge on BP in solution, and exfoliated BP nanosheets shows
the lowest average value of
26.6 mV, suggesting welldistribution status than that of bulk BP. Meanwhile, zetapotential becomes positive after Co modification, and increase
from 26.6 � 0.48 mV for BP nanosheets to + 21.9 � 0.26 mV for
BP Co. Both freshly prepared BP nanosheets and BP Co
materials show well dispersion in water and obvious Tyndall
effect (inset image in Figure 2(g)). Those results emphasize thesuccessful immobilization of Co onto the negatively charged BP
nanosheets.
XRD patterns in Figure 3(a) shows no difference between
fresh BP nanosheets and BP Co, except for the ratio between(040) and (111) peaks appeared at 34.2° and 35° changes little.
We suppose this phenomenon can reflect the interaction
between lone pair electrons on the surface of exposed BP
nanosheets and Co ions. Raman spectra of BP nanosheets areshown in Figure 3(b). BP belongs to the Cmca (No. 64) spacegroup, and in this situation both out-of-plane vibration Ag1mode as well as in-plane vibration B2g and Ag2 modes can beobserved through Raman spectra analysis.[18] The position of
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Raman peaks Ag1, B2g and Ag2 of as-exfoliated few-layer BP
nanosheets downshift from its bulk counterpart position(Table S1) about 0.9 cm 1, 2.1 cm 1, and 3.3 cm 1 due to thickness decreasing as previously reported.[19] The light absorption
capacities of BP and BP Co have been determined by UV-VisNIR absorption spectra (Figure S9), the threshold value of
1078 nm and 1512 nm from intrinsic absorption edge of BP and
BP Co spectra corresponding to the bandgap of 1.15 eV and
0.82 eV, respectively. The narrowed band gap is believed to be
induced by the increased carrier concentration from cobalt ionsand the creation of defect levels (or mid-gap state) in the
bandgap,[20] which acting as trap sites to accommodate thephotogenerated electrons and promote charge carrier migration in consequence. The band energy position of BP isdetermined by Mott-Schottky plot (Figure S10), and the flatband potential is determined to be 1.12 V vs. Ag/AgCl( 0.92 V vs. NHE). After coordinated with Co, the flat band
potential will further downshift to be 1.01 V vs. Ag/AgCl( 0.81 V vs. NHE) (Figure S10). Considering the small difference
between the flat band potential and conduction band (CB)position for n-type semiconductors, the band positions of BP
and BP Co are estimated (Figure S11). It indicates that CB
positions of BP and BP Co are much higher than H2 production
potential ( 0.41 V vs. NHE). Also, the on/off photoresponse
behaviors in Figure S12 show the photocurrent density of
BP Co is 10 times higher than that of BP nanosheets, and amuch longer response time of BP Co means a longer photogenerated carrier lifetime after Co coordination. EIS investigations (Figure S13) also reveal a smaller semicircular radius for
BP Co which represents improved conductivity and lower

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Figure 2. (a) Low resolution TEM (scale bar 100 nm), (b) HADDF-STEM (scale bar 5 nm) image (Co atoms are marked using red circles), (c) HRTEM image (scalebar 5 nm) with d-spacing 0.246 nm of BP Co material. (d) EDS mapping images of as-prepared BP Co material (scale bar 100 nm). (e) Tapping-mode AFM
image (scale bar 500 nm) and (f) the related cross-sectional analysis (line 1 to line 4) of BP Co. (g) Zeta-potential of BP bulk, freshly prepared BP nanosheets
and BP Co tested in ethanol solution. (Inset image: Observation of Tyndall effect of freshly prepared BP nanosheets and BP Co dispersed in water.)charge transfer barrier than that of BP nanosheets. Thus,
introduce the Co may greatly shorten diffusion distance andrealize efficient electron-hole separation to induce favorite
photocatalytic reduction kinetics.
The chemical states of BP Co as well as electronicinteractions are studied by using X-ray photoelectron spectroscopy (XPS) analysis. For as-prepared BP nanosheets, binding
energies at 129.8 eV, 131.0 eV and 133.7 eV can be assigned toP 2p3/2. P 2p1/2 and PxOy (P O species) on the surface,respectively.[21] In comparison, the three predominated P 2p
peaks (Figure 3(c)) shift to lower binding energies 129.3 eV,
130.6 eV and 133.1 eV, which indicate strong interaction
between BP and Co. High resolution core-level Co 2p spectrum(Figure 3(d)) of BP Co material can be fitted with spin-orbit split
2p3/2 and 2p1/2 main peaks and two satellites. The binding
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energies of 781.7 eV, 786.0 eV, 797.0 eV and 803.4 eV can be
attributed into Co2 + and Co3 + components due to unavoidable
surface oxidation, the peak observed lower than 781.7 eV may
suggest a new bond Co P formed due to interaction between
Co and P.[22] In all situations, standard C 1s peaks are used forcalibration (Figure S14 and Figure S15).
In order to further understand the atomic structure of
BP Co, X-ray absorption near-edge structure (XANES) spectroscopy and Co K-edge extended X-ray absorption fine structure(EXAFS) spectrometry are used to probe the local structure of
the Co element in BP Co material. The Co K-edge XANES
spectra of Co metal foil, CoO, CoP, and BP Co are demonstrated
in Figure 3(e). Different from the spectra of metallic Co and CoO
reference materials, the as-fabricated BP Co exhibits near edge
structure which is very similar to that of CoP reference sample.

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Figure 3. (a) XRD analysis of BP nanosheets and BP Co before and after annealing process. (b) Raman spectra of BP bulk, BP nanosheets and BP Co. (c) P 2p
and (d) Co 2p binding energy spectra. (e) Co K-edge XANES spectra and (f) Fourier transform magnitudes of the experimental Co K-edge EXAFS spectra of
BP Co and reference samples.

The EXAFS data of BP Co was Fourier transformed over R-space
range (R range) up to 4.0 Å as shown in Figure 3(f), the Co metal
foil and CoO exhibit typical Co Co shell peak at 2.2 Å and Co( O )Co shell peak at 2.6 Å (both peaks phase shift uncorChemCatChem 2020, 12, 3870 – 3879

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rected). For BP Co sample, three peaks appeared in the R range
between 0 and 4 Angstrom. While peak positions due to Co O
(1.34 Å) and Co P (1.68 Å) matched well with corresponding
peaks of CoO and CoP reference samples, a peak at 2.06 Å does

3874

© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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not match the peaks observed for Co metal, CoO, or CoP
references and should be related to Co Cl coordination fromsurface remained CoCl2.[23] Both the XANES and EXAFS information reveal that the Co is coordinated with P after annealing,
and no Co Co peaks are observed which indicates the singleatom status of Co. The coordination number has been extracted
from curve fit analyses as shown in Figure S16. In detail, thecoordination number of cobalt to oxygen is calculated to be
3.9, and the value of cobalt to phosphorous atoms is close to
1.5 (Calculation methods are provided in supporting information). Considering the fact that P O species existed on thesurface of BP nanosheets, we predict one isolated Co atom iscoordinated with one or two phosphorous atoms as well asfour oxygen atoms to form Co P(O) species in BP Co atomic
structure.
An evidential improvement of photocatalytic reduction
capability has been provided based on our experiments. The asprepared BP Co is chosen to combine with CdS for photocatalytic investigations. After combination, the absorption
ability of CdS BP Co materials has been explored to NIR region,
as shown in Figure S17. As shown in Figure 4(a), pure CdS
possesses low photocatalytic H2 production performance which
is only 18.5 μmol h 1. After mechanical mixing CdS with 0.5 wt%
BP Co, 5 times improvement can be achieved. The most
optimized ratio between CdS and BP Co is 1 wt%, and theaverage H2 production rate is determined to be 345.4 μmol h 1,

Figure 4. (a) Averaged H2 evolution rate of pure CdS, CdS-0.5 wt% BP Co, CdS-1 wt% BP Co, CdS-1 wt% BP, CdS-2 wt% BP Co, CdS-1 wt% Pt, and CdS-2 wt%
Pt. (Catalyst amount: 20 mg). Schematic illustration on the (b) band structure and (c) surface interaction mechanism of photocatalytic reaction on CdS BP Co.

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which is 18 times higher than that of pure CdS. The H2
production performance of the optimized CdS- 1 % BP Co canretain 90 % from the first stage after three cycles (Figure S18).
The promotion is basically from synergetic effect between BP
and Co, for 1 wt% loading of BP will only achieve 3 times
enhancement. In Figure 4(a) and Figure S19, the H2 production
rate of CdS with 1 wt% BP Co is even higher than that of Pt at
the same loading amount, and the performance is comparablewith other reported BP based material in recent state(Table S2).[16,21,24] However, no H2 evolution was observed overbare BP or BP Co without the assistance of CdS, which we think
it is because of the fast electrons and holes recombination in
the situation of both BP and BP Co (Figure S20). Our findings
not only suggest the importance of BP nanosheets as a
substrate for atomic metal loading, but also great potential of
BP Co as substitute for Pt cocatalyst for photocatalytic H2evolution.[25]Based on our experimental results, the mechanism thatviews BP Co in both band structure as well as surface
interaction has been proposed in Figure 4(b) and (c). From the
aspects of band structure of CdS BP Co, the CB edge of CdS( 1.03 eV vs. NHE) is higher than that of BP nanosheets( 0.92 eV vs. NHE) and BP Co ( 0.81 eV vs. NHE), a thermodynamic driving force will drive photogenerated electrons transferfrom excited CdS to BP and Co P(O) sites in the end. From the
aspects of surface interaction, the intimate and large contact
interface of CdS, BP and Co P(O) give a new insight on theenhanced H2 evolution activities. Generally, two different surface contact modes (face contact and point contact) playdifferent roles in the charge carrier transport and chemical
reactions. The face contact between BP nanosheets and CdS
particles promotes fast charge transfer, improving electron-hole
separation efficiency.[26] Meanwhile, the point contact between
CdS and Co P(O) leads to antenna effect, collecting electrons
and lowering the energy barrier of photocatalytic reduction
reactions. The active sites of BP Co are further revealed byusing photosensitizer Eosin Y to exclude the influence fromsemiconductor CdS. (see supporting information for detail
discussion, Figure S21–26) The H2 production performances ofBP, BP Co and BP-CoCl2 (BP Co2 +) in Eosin Y system have been
investigated. BP Co demonstrates the highest activity andgood stability (90 % from its pristine stage even after 1-weekduration) amongst all reference samples. Thus, it is reasonable
to believe the interaction between Co and P(O) on the surface
may support uniform distribution of Co and provide additionalcharge transfer channel from CdS to Co P(O) active sites toachieve improved photocatalytic performance.
We further investigate the CO2 reduction performance(Figure 5(a)) of BP Co in CH3CN, TEOA and water mixing
solution saturated with atmospheric CO2 in the presence of [Ru]as photosensitizer under visible light (λ > 420 nm). In detail,bare BP in the presence of [Ru] depicts excellent performanceon water reduction to H2, and nearly no CO can be detectedmeans the CO2 reduction is kinetically much harder than water
reduction for bare BP. After Co loading, the H2 reduction
performance of BP slightly decreases, whereas the CO2 reduction ability increases dramatically to 88.6 μmol h 1, superior to
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those obtained with the state-of-the-art noble-metal-freecocatalysts (Table S3).[27] The CO2 reduction performance wehave achieved is 88.6 μmol h 1 for BP Co with H2 production of
61.1 μmol h 1 at the same time (Figure 5(b)), and the selectivityof CO is calculated to be 59.2 % in consideration of thecompetition between water and CO2 reduction. The excellent
photocatalytic reduction performance of BP Co may betriggered by the interaction between Co and P, which has beenreported to show electron-rich property for boosting photocatalytic reactions.[28] The electron-rich property of BP Co maylead to higher surface electron density for promoting photogenerated electrons to participate into the reactions withmultiple electrons involved.[29] No CO evolution was observed
over bare BP or BP Co in the absence of [Ru], indicating the
cocatalytic effect of BP and BP Co for photocatalytic CO2
reduction. Isotope experiments are carried out to validate thesource of generated CO by using 13CO2 as carbon source toreplace 12CO2 during our experiments. As shown in Figure 5(c),the total ion chromatographic peak around 6.2 minutes can be
assigned to CO, while O2 and N2 are confirmed from air thatremain in the sampling needle. The analysis of CO only obtains
the m/z = 29 in mass spectra (Figure 5(c)), which is attributed to
the 13CO, confirming that the product CO originates from CO2
reduction. The proposed mechanism for photocatalytic CO2
reduction over BP Co has been depicted in Figure 5(d). The
photosensitizer [Ru] is excited under visible light to the [Ru]*state, and transferring generated electrons to the CB and the
Co P(O) active sites of BP Co. Then, the CO2 molecules and H +are reduced to CO and H2. The oxidized state of the [Ru] +receives electrons from sacrificial electron donor TEOA torecover back to [Ru].

3. Conclusions
In summary, we have prepared two-dimensional BP Co nanostructure, where coordinated Co atoms are dispersed on thesurface of BP nanosheets. The as-prepared BP Co can be used
as cocatalyst to enhance photocatalytic ability for both H2
production and CO2 reduction. We find that BP Co loaded CdS
can deliver much higher H2 production performance than thatof Pt loaded CdS, indicating great potential of BP Co as
substitute for Pt cocatalyst for photocatalytic H2 evolution. On
the other hand, the CO evolution rate as high as 88.6 μmol h 1
has been realized from photocatalytic CO2 reduction over
BP Co in the presence of [Ru], superior to those obtained fromthe state-of-the-art noble-metal-free cocatalysts. Based on the
results of photocatalytic and photoresponse measurements, we
attribute the enhancement of photocatalytic reduction toadditional charge transfer channel and active sites derived fromCo P(O) bonding which can benefit photocatalytic conversion
kinetics for boosting performance. In our work, the Co P(O)interaction assist stable dispersion of single atom Cobalt on thesurface of BP, leading BP Co nano-materials with good activity
as well as promising future in many photo-related applications
including hydrogen generation, CO2 reduction, pollutant degradation and photo-thermal reactions.

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Figure 5. (a) Diagram of H2 generation and CO2 reduction performance of different catalysts. (b) Time dependent plot of BP Co for CO2 photocatalytic
reduction with selectivity 59.2 % in the presence of [Ru]. (c) GC-mass spectra of 13CO (m/z = 29) produced over BP Co/[Ru] from the atmosphere of 13CO2. (d)The schematic illustration about the mechanism of electron transfer in BP Co/[Ru] for photocatalytic CO2 reduction.

4. Methods
Materials. Black phosphorus (BP) bulk (99.5 %) was provided by
a commercial supplier (Nanjing XF NANO), 1-Methyl-2-pyrrolidinone (NMP) (99.5 %, anhydrous), ethanol (95 %) and CoCl · 6H2O
(99.0 %) were purchased from FUJIFILM Wako Pure Chemical
Corporation. Tris(2, 2’-bipyridyl) dichlororuthenium(II) ([Ru(2, 2’bipyridy)3]Cl2, 99.95 %) was purchased from Sigma-Aldrich
Corporation. The deionized water with a resistivity of
18.2 MΩ cm 1 was utilized in all experiments.
Preparation of few-layer BP nanosheets. Few-layer BP
nanosheets were obtained by using liquid exfoliation method.
In detail, 0.5 g bulk BP was used as received and grinded in an
agate mortar with the help of NMP. Then, a small portion of the
pre-treated BP solution was transferred into a conical tube
(50 mL) with totally 35 mL NMP added. The exfoliation
processes are carried out in the micro-tip probe ultrasonication(Branson Sonifier advanced 250) with output power level 5 andduty cycle 50 % at a temperature below 25 °C. After 3 hours’
sonic exfoliation, the dispersion contains BP nanosheets can be
harvested after several times centrifugation at 18 000 rpm inorder to remove the residual bulk BP. After that, the powder of
BP nanosheets was retrieved after washing for three times using

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acetone and ethanol, and drying under 120 °C overnight for
further characterizations.
Fabrication of BP-Co materials. In a typical experiment, theobtained BP nanosheets were mixed with CoCl · 6H2O directly.
After grinding the aforementioned materials in an agate mortar
for 15 minutes, the BP Co precursor will be collected andtransferred into a porcelain boat and located at the heating
center of the quartz tube furnace. Before heating, the sealed
quartz tube was pumped with argon gas for approximately 30
minutes to evacuate oxygen and air as much as possible. Afterthe argon gas washing was finished, the furnace was heated to
300 °C with heat speed 5 °C min 1 and maintain at this temperature for calcination for 2 hours. The final sample named as
BP Co was collected after it cooling down to room temperature
and washed by Ethanol and deionized water for 3 times.
Photocatalytic H2 evolution activity. Photocatalytic activities were evaluated by the hydrogen evolution from water
under visible light irradiation. A 300 W xenon arc lamp
equipped with a 420 nm cut-off filter (to remove light with awavelength lower than 420 nm) was used as the visible lightsource. Commercial CdS and freshly prepared BP Co were
physically mixing together, and 20 mg of mixed sample wasdissolved into 80 mL of 20 vol % lactic acids (electron donor)
solution by sonication for 5 minutes, after that the solution was

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transferred into a quartz glass reaction cell and connect to a
gas-closed system with a gas-circulated pump. The produced H2
was recorded and analyzed by an online gas chromatograph(GC-8A, Shimadzu Corp., Ar carrier, TCD, Japan) with argon as
the carrier gas. For evaluating the H2 production performanceof BP Co, 10 mg of catalyst, 20 mg Eosin Y (EY) and 80 mL of
25 vol % TEOA were added in a quartz glass reaction cell thatwas connected to a gas-closed system with a gas-circulated
pump. Before the irradiation, the reaction cell was evacuated toremove the air completely. The produced H2 was analyzed byan online gas chromatograph (GC-8A, Shimadzu Corp., Ar
carrier, TCD, Japan) with argon as the carrier gas.
Photocatalytic CO2 reduction activity. The CO2 photocatalytic reduction experiments were conducted in the samegas closed circulation system equipped with a glass reaction
cell (upside quartz window). Before the measurements, 1.5 mg
of catalyst powders and 30 mg [Ru(2, 2’-bipyridy)3]Cl2 were used
and uniformly dispersed into 30 mL of CH3CN/TEOA/water
mixing solution (CH3CN:TEOA:H2O = 3 : 1 : 1), the gas closed
reaction system was evacuated and then filled with 100 KPa ofpure CO2 gas. A 300 W Xe-lamp with a 420 nm cut-off filter was
employed as the visible light source to drive the CO2 photoconversion. The gaseous products formed in the reaction were
sampled and measured with a gas chromatograph (GC-14B,
Shimadzu) equipped with a flame ionization detector (FID)
according to the standard curves, while the generation of H2
was measured with an online gas chromatograph (GC-8A,
Shimadzu) with a TCD detector. Isotopic labeling experiment
was performed by replacing CO2 gas with 13C-labeled 13CO2 (The
Linde Group, 13C enrichment: > 99 atom%) under same test
conditions, and the final products were analyzed by gas
chromatography-mass spectrometry (GC-MS).

Acknowledgements
This work was financially supported by JSPS KAKENHI(JP18H02065), the World Premier International Research Center
Initiative (WPI Initiative) on Materials Nanoarchitectonics (MANA),MEXT (Japan), Photoexcitonix Project in Hokkaido University,
National Natural Science Foundation of China (Grant No.
21633004 and 51872091), National Key R&D Program of China(Grant No. 2018YFC190503), State Scholarship Fund by China
Scholarship Council (CSC) (201808430219 and 201806400044).
This research used the JEOL JEM-ARM200F funded by theAustralian Research Council (ARC)-Linkage, Infrastructure, Equipment and Facilities (LIEF) grant (LE120100104) located at the
UOW Electron Microscopy Centre.

Conflict of Interest
The authors declare no conflict of interest.
Keywords: two-dimensional · black phosphorus · single atom ·photocatalysis

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Manuscript received: March 30, 2020
Revised manuscript received: April 27, 2020
Accepted manuscript online: April 27, 2020
Version of record online: June 8, 2020

3879

© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

18673899, 2020, 15, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.202000546 by Chiba University, Wiley Online Library on [10/11/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

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