Nano Energy 76 (2020) 105077

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Nano Energy
journal homepage: http://www.elsevier.com/locate/nanoen

Targeted removal of interfacial adventitious carbon towards directionalcharge delivery to isolated metal sites for efficient photocatalytic
H2 production
Yunxiang Li a, b, Shengyao Wang b, e, *, Pei Wang e, Yu He b, Xusheng Wang b, Kun Chang b,
Huiwen Lin b, Xing Ding e, Hao Chen e, Hongwei Zhang f, Yasuo Izumi f, Tetsuya Kako b,
Jinhua Ye a, b, c, d, **aGraduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo, 060-0814, Japan
International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japanc
TJU-NIMS International Collaboration Laboratory, School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, PR ChinadCollaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin, 300072, PR ChinaeCollege of Science, Huazhong Agricultural University, Wuhan, 430070, PR Chinaf
Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-Ku, Chiba, 263-8522, Japanb

A R T I C L E I N F O

A B S T R A C T

Keywords:
Interface engineering
Cleared interface
Adventitious carbon
Single-atom catalyst
Photocatalytic H2 generation

Creation of clean interfaces without contamination of adventitious carbon and robust single active sites arehighly desirable for delivery and utilization of electrons in sunlight-driven hydrogen production but still remains
challenging in part owing to the lack of understanding in junction nature. Herein, we tackle this challenge bytargeted removing of interfacial adventitious carbon between photoharvester CdS and novel single-cobalt cocatalyst (Co-NC). It’s found that surface-trapped electrons can readily migrate to the closely attached Co-NC
across the cleared interface between CdS and Co-NC. The small-resistance interfacial carrier path and therobust single-cobalt sites work in a cooperative way and hence achieve a superior visible-light driven H2 gen­eration activity with a rate of 4.34 mmol/h, an apparent quantum yield (AQY) of 63.9% at 400 nm and a ul­
trahigh turnover frequency (TOF) of up to 16714.7 h 1. Our finding will motivate future work in creating cleaninterfaces and unique single active sites for high performance photocatalysis.

1. Introduction
Efficient conversion of renewable solar energy into clean H2 via
photocatalytic water splitting has been widely regarded as an attractive
approach to solve the energy crisis and environment problem [1–3].
However, it still remains a great challenge to construct a low cost, robust
system for highly efficient photocatalytic H2 evolution despite enormous
progress in this field during the past decades [4]. Most photocatalysts
lack active sites on the surface for targeted reactions, leading to the fastrecombination of excited carriers and poor performance [5,6]. Loading
co-catalysts with robust active sites can not only lower the activationenergy of reactants but also accelerate the electron-hole separation at
the semiconductor/co-catalyst interface [7,8].

To date, great achievement has been made in exploring noble-metalfree co-catalysts to replace precious metal for constructing efficientphotocatalytic system [9–15]. However, several major issues related tothe intrinsic properties of current co-catalysts still exist for H2 genera­tion: (i) low atom utilization efficiency of the metal in co-catalysts
because they usually exist as aggregated status; (ii) lack of surfacefunctionalities to form robust connection with semiconductors towards
fast transfer of carriers across junction; (iii) unfavorable Gibbs free en­ergy for H2 generation. Hence, it is highly imperative to design andconstruct favorable active sites with merits of cost-effective, high utili­zation efficiency and high turnover frequency for realizing efficient solar
energy conversion.
Single-site catalysts, with isolated metal sites supported on solid* Corresponding author. International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba,
Ibaraki, 305-0044, Japan.
** Corresponding author. Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo, 060-0814, Japan.
E-mail addresses: wangshengyao@mail.hzau.edu.cn (S. Wang), Jinhua.YE@nims.go.jp (J. Ye).
https://doi.org/10.1016/j.nanoen.2020.105077
Received 17 October 2019; Received in revised form 20 May 2020; Accepted 11 June 2020
Available online 21 June 2020
2211-2855/© 2020 Elsevier Ltd. All rights reserved.

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substrates, especially on N-rich carbon (NC), are emerging as a highly
attractive class of catalysts because of their unusual electronic struc­tures, extraordinary catalytic properties, etc. [16–21] These materials
feature diverse moieties such as rich N species [22] on surface, tunable
metal sites with flexible coordination [23–25] and maximum utilizationof metal-site [26] especially when those single sites are anchored on
ultrathin support. These attractive characteristics of single-atomembedded carbon-based family might be utilized for solving the issues
mentioned above and render it to be highly promising as co-catalysts.
However, such kind of materials have rarely been used as co-catalysts
in photocatalytic system, especially for achieving outstanding perfor­
mance in photocatalysis.
The co-catalysts with elaborate single-site can generate the targeted
product upon capturing the energetic photoelectrons; however, it also
inevitably brings a contaminated interface between photohavester andco-catalyst caused by the adventitious carbon at the junction. The in­terfaces, where electrons shuttle, are extremely crucial for efficientphotochemistry process. A contaminated interface will severely block
the electron transfer to the co-catalytic single-site and such negative
effect might also exist for various advanced heterostructures [27–31]. It
is worth stressing that surface adventitious carbon widely exists on
various materials, such as TiO2, carbon nitride and black phosphorus[32–35]; upon forming heterostructure, these carbon species will locate
at the interface and lead some substantial and underlying effects on thephotocatalytic process. However, the detailed roles of those adventitious
carbon at the surface or interface in photocatalysis are often neglected
but should arise our attention. Recently, Diebold et al. disclosed thatTiO2 selectively adsorbs carboxylic acids on its surface from air/solution
and further pointed out the potential effect of the surface carboxylatemonolayers on photocatalysis [32]; Medford et al. revealed that thesurface adventitious carbon could strongly interact with N2 moleculesand promote photocatalytic N2 fixation [33]. Further clarifying theexact effect of these carbon species located interfaces in heterostructuresare quite essential for constructing robust photocatalytic system but has
not been explored at present.
Motivated by the aforementioned key issues, we seek to create aclean heterostructured interface aiming at speeding up the carriers flow
to the robust single-site co-catalysts for constructing photocatalytic
system with record activity and metal utilization efficiency. Herein, we
report a rational design and novel synthesis of single Co implanted ul­trathin N-rich carbon nanosheets (Co-NC) and create a cleared junction
between photoharvester CdS and co-catalyst Co-NC through calcinationinduced elimination of interfacial adventitious carbon. The cleared in­
terfaces collaborate with the robust isolated Co–N4 sites to achieve an
outstanding visible-light-driven H2 evolution activity with a rate of 4.34
mmol/h, an AQY of 63.9% at 400 nm. Remarkably, the optimal systemachieves an ultrahigh TOFCo of up to 16714.7 h 1. This new strategy
constructs an efficient system for H2 photogeneration via creating robust
single-atom co-catalyst Co-NC and simultaneously clearing the interface
between CdS and Co-NC. This built-in system not only clarifies the sig­
nificant blocking effect of widely existed surface adventitious carbon on
charge transport, but also highlights both the essential roles of clearedinterface and single-atom based materials in photocatalysis.

product.
Subsequently, the above powder precursor underwent a two-stage
pyrolysis and carbonization process (retained at 650 � C for 1h, then�
increased to at 950 � C and held for 3 h with a ramping rate of 3 C/min)in tube furnace under argon atmosphere. After cooling down to the room
temperature, the obtained black product was leached with 0.5 M HCl toremove metal particles and unstable species and successively washedusing distilled water. Afterwards, the obtained sample was further
crushed into small-sized ultrathin nanosheets by ball milling treatment,
the final resultant sample was denoted as Co-NC. A typical wet-milling
was performed on a planetary ball mill (Fritsch, P-7) and conducted at
400 rpm for 15 h using distilled water as the solvent.
Other (M)-NC(NS) (M ¼ Fe, Ni) were synthesized using similar
procedure, except employing different metal precursors and different
solvents for the preparation of corresponding supermolecular pre­cursors. In detail, 0.1 mmol Fe(NO3)3⋅9H2O and 0.1 mmol NiCl2⋅6H2O
severed as the metal source for precursor fabrication of Fe-NC and NiNC. In the case of Ni-NC, 10 mL ethanol and 90 mL methanol was
employed as the solvent to prepare the precursor. For NC, only 10 mmol
Zn(OAc)2 was used to prepare the precursor.
Preparation of CdS/Co-NC composites: well-dispersed (M)-NC inaqueous solution was added into the dispersion of commercial CdS withstirring, after stirring for 5 min, the mixture was filtered and dried at 30
�
C under vacuum. A series of CdS/Co-NC samples with different Co-NC
content (0.5, 1, 1.5, 2.0, 2.5 wt %) were prepared. To reinforce theinterface interaction between CdS and (M)-NC, the dried sample wasannealed at different temperature (400, 450, 500, 550 � C) for 1 h under
Ar atmosphere. The CdS/Co-NC(RT) and CdS/Co-NC(500) refers to thehybrid CdS/Co-NC without and with annealing at 500 � C under Ar at­mosphere, respectively. The composites with different loading amountor treated at different temperature will be specifically stated.
2.2. Material characterization
XRD patterns of the as-prepared samples were collected with apowder X-ray diffractometer (Cu Kα radiation source, X’pert powder,
PANalytical B.V., Netherlands). UV–visible (UV–vis) diffuse reflectancespectra of the samples were recorded on an UV–visible spectropho­
tometer (UV-2700, Shimadzu, Japan) equipped with an integrating
sphere, then converted into absorption spectra by Kubelka Munktransformation. Nitrogen adsorption–desorption isotherms, and surface
area were recorded using a BEL SORP-mini II (BEL Japan INC., Japan),and were calculated via the Brunauer-Emmett-Teller (BET) method. The
Photoluminescence (PL) properties were investigated using a fluores­
cence spectrophotometer (Fluorolog-3, HORIBA Scientific, America). Xray photoelectron spectroscopy (XPS) measurements were carried out
on XPS instrument (Escalab 250, Thermo SCIENTIFIC, America). Allbinding energies were referenced to the C 1s peaks of the surface
adventitious carbon at 284.8 eV. The morphologies of samples werecharacterized with Field Emission Scanning Electron Microscope (JSM6701F, JEOL Co., Japan) and Transmission Electron Microscope (TEM)(JEM-2100F, JEOL Co., Japan). Energy-dispersive X-ray spectroscopy(EDS) mapping was also recorded with same TEM instrument. The STEM
characterizations were performed on a JEOL ARM-200F field-emission
transmission electron microscope. Raman spectra were collected using a
Horiba Jobin Yvon LabRAM system with a 532 nm excitation laser. The
electron spin resonance (ESR) measurements were performed on an ESR
spectrometer (JES-FA200, JEOL Co., Japan) at liquid nitrogen temper­
ature (77 K). The X-ray absorption fine structure spectra were performed
at 1W1B station in Beijing Synchrotron Radiation Facility (BSRF). The
XAFS data were recorded in fluorescence excitation mode using a Lytle
detector. Co foil, CoO and Co3O4 were used as the reference. The ac­quired XAFS data were analyzed by Athena and Artemis interfaces to the
IFFEFIT software, according to the standard procedures.

2. Experimental section
2.1. Sample preparation
Synthesis of (M)-NC: Co/Zn-supermolecular precursor was firstprepared by a facile and scalable wet-chemical method. Typically,melamine (1.26g, 10 mmol), Zn(OAc)2 (1.82g, 9.9 mmol) and Co(OAc)2⋅4H2O (24.91 mg, 0.1 mmol) was added to 100 mL methanol withvigorous stirring, then the dispersion was transferred to an oil bath andrefluxed at 70 � C for 3 h. The obtained light pink solid was centrifuged
and washed with ethanol for two times, then dried at 70 � C under vac­uum. The yield wad estimated to be 85.8% with 2.66 g light pink powder
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2.3. Photocatalytic activity evaluation(Ushio, USR-40, Japan). The AQY at each monochromatic wavelength
was calculated by following the equation below:

The photocatalytic H2 evolution test was carried out in a glass reactor
with a closed gas circulation system. In a typical test, 150 mg photo­catalyst was firstly dispersed with a magnetic stirrer in 280 mL 20 vol %lactic acid solution (the pH of the solution was adjusted by NaOH in
advance). After vacuumizing, H2 evolution over the photocatalyst was
carried out under visible light irradiation (light source, 300 W Xe lamp
equipped with an L42 cutoff filter, λ > 400 nm). The evolved H2 wasanalyzed by online gas chromatography (GC-8A; Shimadzu Corp.,
Japan) equipped with a thermal conductivity detector. The photo­catalytic H2 generation rates over as-prepared samples was fitted withthe four points ranging from 0.5 h to 2h.
The apparent quantum efficiency (AQY) was measured under similar
conditions as described above except for the light intensity and thewavelength regions of the irradiation light. A series of band-pass filters
were adopted to control the wavelength regions of incident light. A
water filer was also used together with the band-pass filter to avoid thepossible damage caused by heating effect of Xe lamp. The average in­tensity of irradiation was determined by using a spectroradiometer

AQY for H2 evolution ¼ N(H2) * 2 / N(photons) * 100%
Note: N(H2) represents the molecular number of evolved H2 and N(photons) means the number of incident photons.
3. Results and discussion
3.1. Synthesis and characterization of co-catalyst Co-NC
The ultrathin single atom co-catalysts ((M)-NC; M ¼ Co, Fe, Ni) weresynthesized mainly via two steps as illustrated in Fig. 1a: (i) pyrolysis of
a supramolecular precursor; (ii) downsizing the obtained hierarchicalsheets into small-sized ultrathin nanosheets by ball milling treatment. Xray powder diffraction (XRD) pattern of Co-NC (Fig. S1a) exhibits a
broad peak centered at ~26.0� , assigned to the (002) plane of graphitic
carbon. Raman spectra (Fig. S1b) show two typical signals belonging to
defect-rich carbon, i.e., D band (1355.9 cm 1) and G band at (1591.6

Fig. 1. Synthesis and characterizations of Co-NC co-catalyst. (a) Scheme of the formation process. (b, c) TEM images. (d) EDS elemental mapping. (e) Aberrationcorrected HAADF-STEM image.
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cm 1). The Co-NC possesses a specific surface area of 222.7 m2/g
(Fig. S2) and presents as uniform 2D sheets with a planar size of ~200
nm (Fig. 1b and Fig. S3) and an ultrathin thickness of ~1.5 nm (Fig. S4).
The high-resolution transmission electron microscopy (HRTEM) image(Fig. 1c) reveals some typical fringes, corresponding to few-layer carbon
sheet. In particular, no signal of Co clusters or nanoparticles are detectedon the ultrathin nanosheets by scanning electron microscopy (SEM),
TEM and XRD. The TEM-EDS (energy disperse X-ray) mapping results(Fig. 1d) reveal that Co, N and C distribute homogeneously across theentire architecture. Moreover, atomic-resolution aberration-corrected
high-angle dark-field scanning TEM (HAADF-STEM) was used to directly
distinguish the Co species with atomic scale. As clearly revealed inFig. 1e, isolated Co sites, appeared as individual bright spots with anaverage size of ~0.2 nm, are well dispersed on the N-rich carbon
nanosheets.
The chemical composition and valence states of Co-NC were exam­
ined using X-ray photoelectron spectroscopy (XPS) (Fig. S5) measure­ment. The isolated Co sites exhibit a peak at 780.6 eV, corresponding to
the Co–N species (Fig. S5a) [36]. The atomic content of Co in Co-NC
determined by XPS is 0.77 at % (3.5 wt %, close to 3.4 wt % exam­
ined by inductively coupled plasma atomic emission spectroscopy). Fourtypes of N species (Fig. S5b), including pyridinic-N (398.8 eV),pyrrolic-N (400.0 ev), graphitic-N (401.3 eV) and oxidized-N (402.8 eV),can be clearly distinguished, among which the pyridinic N serves as theanchor sites for stabilizing the single Co. This assumption was supportedby the shift of pyridinic N peak towards higher binding energy after
introducing Co species while other kinds of N species remain almost
unchanged, revealing the binding formation between Co and pyridinicN.

To further examine the coordination configuration and chemical
state of the single Co sites, X-ray absorption fine structure (XAFS)
measurements were performed. The results reveal that the Co species
exist in an isolated state and stabilized by nitrogen in the form of Co–N4.
Fig. 2a presents the X-ray absorption near-edge structure (XANES)
spectra of Co-NC in comparison to standard spectra of Co foil, CoO andCo3O4. The absorption edge of Co-NC locates between those for CoO andCo3O4, and close to that of CoO, clearly suggesting the Co sites deliver aslightly higher oxidation state than Co2þ. More detailed structural in­formation of Co sites can be obtained based on extended X-ray absorp­
tion fine structure (EXAFS) analysis (Fig. 2b). The Co-NC shows adominant Co–N peak at 1.42 Å, which is shorter than Co–O signal at
1.71 Å in CoO and 1.49 Å in Co3O4. Additionally, no Co–Co peak can beobserved at 2.12 Å for Co-NC, corroborating the atomic dispersion of
single Co sites in Co-NC. We further investigate the coordination of Co
sites by EXAFS curve fitting analysis. The surrounded N number of Co
center is estimated to be four according to the fitting results (Table S1).
In further electron spin resonance (ESR) analysis (Fig. 2d), the similar
spectra with close g value for Co-NC and commercial cobalt (II) phtha­
locyanine (CoPc) at 77 K reveal that the Co sites in Co-NC exhibits
similar coordination with four N sites and valence state (Co2þ) withthose in CoPc, which coincides well with the XAFS analyses (Fig. 2a,b,c).
Note that the signal pointed by an arrow with the g value of 2.2097 is
attributed to the low spin Co (II) in CoPc [37].

Fig. 2. Site coordination analyses of Co-NC co-catalyst. (a) Co K-edge XANES spectra of Co-NC and reference samples. (b) Fourier transformed k3-weighted χ(k)function of the EXAFS spectra for Co K-edge. (c) Corresponding EXAFS fitting curve for Co-NC. Inset: the proposed Co–N4 configuration. (d) ESR spectra of Co-NC andreference CoPc.
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3.2. CdS/Co-NC(500) is about 289 times more active than bare CdS

sample CdS/Co-NC(500) can even exhibit an activity of 814 μmol/hwith excellent stability under the irradiation of simulated sunlight witha light intensity of 100 mW cm 2. To further judge the advantage ofultrathin morphology of support in H2 evolution activity, anothersingle-Co (Co–N4) co-catalysts with rhombic dodecahedron morphology(Co-NCrd) was fabricated according to the previous report [24] (Fig. S11
and Fig. S12) and further anchored on CdS (Fig. S13) followed by heat
treatment at 500 � C. Under the same conditions, the CdS with ultrathin
Co-NC (Fig. S14; 4.34 mmol/h; 4801.7 h 1) achieves a ~3 times H2
evolution rate and a ~2 times TOF those of CdS/Co-NCrd(500) (1.51mmol/h; 2249.6 h 1), respectively.

As single Co sites based catalysts have been reported as promising
electrocatalysts for hydrogen evolution [38], enlightened by this, here
we constructed a system composed of CdS/(M)-NC forvisible-light-driven hydrogen generation, wherein the CdS serves as thephoto-harvester and (M)-NC acts as the co-catalysts for water reduction.
The CdS/(M)-NC(RT) and CdS/(M)-NC(500) represent the composites
without and with annealing treatments, respectively; the loading
amount of co-catalysts for these hybrids is 1 wt% unless otherwise
stated. These ultrathin (M)-NC co-catalysts can be uniformly and facilely
grafted on the surface of CdS via simple mixing process as identified by
the SEM analyses, UV–vis spectra, and TEM elemental mapping analyses(Figure S6, S7 and S8). As shown in Fig. 3a, bare CdS shows a poor
photocatalytic hydrogen evolution activity of ~15 μmol/h and sole
Co-NC is inactive for hydrogen evolution. Remarkably, the coupling of
Co-NC with CdS drastically boost the activity of CdS, reaching up to
4.34 mmol/h under optimized conditions (Fig. S9) and achieve anapparent quantum yield (AQY) of 63.9% at 400 nm (Fig. 3b), repre­
senting the currently achieved state-of-the-art H2 generation activity
among all the CdS-based H2 generation system (Table S2). Such an ac­
tivity of CdS/Co-NC(500) even far exceeds that of CdS/1 wt%Pt underappropriate conditions (Fig. 3a). Besides that, the isolated Co sites in thissystem can afford an ultrahigh turnover frequency (TOF) reaching up to16714.7 h 1 (the number of moles of generated H2 divided by the
number of moles of Co in CdS/Co-NC(500) when we altered amount of
catalysts (Fig. 3c). Such a high TOF value far surpasses that of advanced
Co-based species on CdS (600 h 1) by a factor of 27.9 times [12]. In
addition, the CdS/Co-NC(500) exhibits considerable stability as identi­
fied by the relatively stable H2 generation in the four-cycle test
(Figs. S10a and b; at pH ¼ 6 and 7). Fig. 3d shows that the optimal

3.3. Cleared interfaces and robust Co–N4 moieties for high performance
We further explored the reason that the CdS/Co-NC(500) delivers
superior photocatalytic hydrogen evolution efficiency. Generally, tofully exploit the activity of a photocatalytic system, three requirements
should be met: (i) excellent photohavester with suitable band level andband gap; (ii) high mobility of light-induced carriers from the photo­
absorber to active centers; (iii) outstanding active sites to collect thephotoelectrons and subsequently transform the reactants into the
products. To clarify the exact role of Co-NC in the highly efficient H2generation system CdS/Co-NC(500), a contrast experiment was con­ducted. Besides co-catalyst NC, we also prepared co-catalysts Fe-NC andNi-NC with isolated Fe and Ni centers (Fig. S15) respectively, in a similar
way to fabrication of Co-NC. As illustrated in Fig. 3e, the co-catalysts NC,
Fe-NC and Ni-NC loaded CdS exhibit poor and similar activity of ~1.0mmol/h, which are much lower than that of Co-NC modified one.
Apparently, the isolated Co center in Co-NC with unique properties
mainly acts as the active site for superior H2 evolution. The slightdeactivation of CdS/Co-NC(500) at pH ¼ 5 compared with the excellent

Fig. 3. Photocatalytic performance of CdS/Co-NC(500) and reference samples. (a) A comparison of H2 evolution activity of as-prepared samples. (b) Wavelengthdependent apparent quantum yield (AQY) of H2 generation from CdS/1%Co-NC(500). (c) Turnover frequency (TOF) calculated based on the single Co sites in CdS/
Co-NC(500). (d) H2 evolution activity of CdS/1%Co-NC(500) under the irradiation of AM 1.5 with a light intensity of 100 mW cm 2. (e) Comparison of H2 evolution
activity over CdS coupled with different co-catalysts (M)-NC (M ¼ Fe, Ni and Co). (f) Comparison of H2 generation activity of as-prepared samples with andwithout annealing.
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stability at pH ¼ 7 (Fig. S10) lends further support to the essential role ofisolated Co sites in H2 generation. The acid solution (pH ¼ 5) mightslightly etch CdS and generate some free S2 species, which could poison
the single Co sites and account for the slight decrease in activity. The
stable performance of CdS/NC(500) without discrete Co sites (Fig. S16)
at pH ¼ 5 further verifies above assumption. Moreover, the annealing
process are also quite significant for boosting the performance of CdS/NC and CdS/Co-NC. For both co-catalysts NC and Co-NC, a dramaticenhancement can be observed (Fig. 3f) when heat treatment was
adopted. Note: annealing process causes no changes on CdS phase as
revealed by XRD patterns (Fig. S17). The annealing treatment may
strengthen the interface interaction between CdS and the attached cocatalysts and thus accelerate the charge transfer from CdS to cocatalysts. In brief, the efficient H2 evolution activity attributes to both
the distinct single Co sites and the potential interface structure evolution
caused by annealing considering above contrast experiments. Therefore,
we try to gain deeper insight into the underlying mechanism from these
two aspects.
First, to unravel the root of the excellent co-catalytic H2 production
performance of Co–N4 sites embedded in co-catalyst Co-NC, thehydrogen adsorption free energy change ΔGH on various sites including
graphitic-C, graphitic-N, pyridinic-N and metal sites coordinated withfour pyridinic-N (the models for calculation are provided in Tables S3
and 4 and Figs. S18,19,20), were employed as a descriptor to reflecthydrogen evolution activity. Usually, the ideal value of ΔGH is 0 eV,
which means hydrogen is bound neither too strongly nor too weakly andthus favors both the hydrogen adsorption and consequent desorption.
The diagram in Fig. 4a demonstrates the isolated Co sites bonded withfour N atoms exhibit the nearest value to 0 eV compared with Fe, Co sites
with identical coordination environment and other possible sites. This
result indicates that the isolated Co sites are the most favorable sites forH2 evolution among all the studied sites and mainly account for the H2generation for CdS/Co-NC(500).

Furthermore, the giant H2 evolution activity enhancement by theannealing process ignited our interest in disclosing how the interfacial
structure evolve upon annealing and how it affects the charge flow
across the junction between CdS and Co-NC. A series of characteriza­
tions including HRTEM, XPS, photoelectrochemical method, in situ FTIR (Fourier transform infrared spectroscopy) and photoluminescence(PL) spectroscopy were adopted to approach the detailed charge flow
process and interfacial structure evolution. High-resolution TEM image(Fig. 4b) of CdS/Co-NC reveals an intimate interfacial heterostructurecomposed of ultrathin Co-NC tightly wrapping on a bulk CdS. Such a
compact contact between CdS and ultrathin Co-NC could enable the fastelectron transfer through the interface. The analyses of chemical valenceof Cd, S, C and N contained in the composites allow us to understand theflow direction of electrons. High-resolution Cd 3d (Fig. 4c) and S 2p XPS
spectra (Fig. S21) show that both the binding energy of Cd and S in­creases after loading Co-NC, clearly suggesting the electron donating
feature of CdS and thus the electron accepting nature of the Co-NC in
CdS/Co-NC obtained without annealing. More interestingly, heat
treatment at 500 � C under Ar can further promote charge flow from CdS
to Co-NC as evidenced by the larger binding energy of Cd and S in CdS/
Co-NC after annealing, and cause dramatically enhanced H2 generation
activity from ~1.0 to 4.3 mmol/h (Fig. 3f). In relative to the NC, the CoNC possesses stronger ability for collecting the electron from photo­
havester CdS as identified by the higher binding energy of Cd and S for
CdS/Co-NC compared to CdS/NC (Fig. 4c and Fig. S21).
Additionally, the anchored co-catalysts Co-NC, as the electrons
acceptor, should present as an electron-rich state. The high-resolution N
1s spectra reveal (Fig. 4d) that all N species including pyridinic N co­
ordinated with Co sites, pyrrolic N and graphitic N in Co-NC could
accept the donated electron from CdS as verified by the shifted binding
energy towards low energy side. Beyond that, the further increased
binding energy of pyridinic N sites bonded with Co centers caused byannealing process clearly indicates that the heat treatment can further

Fig. 4. Calculation of isolated Co sites for hydrogen adsorption and characterizations of the relatively clean interface and their promotional effects in chargetransport. (a) Calculated Gibbs free energy diagram for hydrogen adsorption on various sites. (b) High resolution TEM image of the interface between CdS andultrathin Co-NC. (c,d) Cd 3d and N 1s XPS spectra of samples. (e) In situ FT-IR spectra of CdS/Co-NC measured at increased temperature. (f) Steady-state photo­
luminescence spectroscopy of the samples.
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enrich the electrons on these N sites and then transfer to the active Co
sites for H2 generation. Unfortunately, the content of Co in the com­posite CdS/Co-NC(500) is too low to detect using XPS method (Fig. S22),
thus it’s difficult to directly observe the electron-rich state of Co sites
currently. However, according to the remarkably enhanced activity
induced by Co-NC (Fig. 3e) in relative to NC, Fe-NC and Ni-NC, it’srational to conclude that the Co sites can accept the electrons for robust
H2 evolution.
To explore the origin of aforementioned annealing-promoted elec­
trons transfer from CdS to Co-NC and visualize the interfacial structure
evolution in the system CdS/Co-NC with annealing, in-situ FT-IR was
conducted under elevated temperature and Ar atmosphere, in which
CdS/Co-NC(RT) was examined and the detected signal at 100 � C wasused as the background. It’s worth noting that surfaces of commercial
CdS in our case are covered with abundant adventitious carbon species(Fig. S23), similar to the widely studied TiO2 [32] and carbon nitride[35]; upon forming heterostructures, the interfaces will be enrichedwith these carbon species. The calcination-time dependent infrared
spectra (Fig. 4e) of CdS/Co-NC(RT) present gradually decreased absor­
bance at ~1000 (O–H, out-of-plane bend), ~1177–1232 (C–O, stretch),– O, stretch) with increasing~1405 (C–H, bend), and ~1626 cm 1 (C–temperature, clearly indicating the decomposition of the adventitious
carbon. The distinct signals of contrast samples CdS and Co-NC
compared with hybrid CdS/Co-NC(RT) indicates that, in CdS/Co-NC– O, C–O and OH groups originate from(RT), most of those reduced C–
the decomposition of adventitious hydrocarbon confined at the in­terfaces between CdS and Co-NC rather than on the naked surfaces of
CdS and Co-NC (Fig. S24 and relevant discussions). Therefore, the fully
calcined sample CdS/Co-NC(500) should contain no or very limited
interfacial adventitious carbon, which is evidenced by the in-situ FT-IR
spectra of fully annealed CdS/Co-NC(500) (Fig. S25 and relevant dis­cussions). In brief, annealing process can effectively remove the inter­
facial adventitious carbon for CdS/Co-NC(RT), which is further
supported by the declined C and O content in the calcined CdS/Co-NC(500) detected with XPS method (Fig. S26). As a result, a muchcleaner interface is created, which could enable the smoother electrontransport from CdS to Co-NC. Interestingly, an extra peak appears inhigh resolution N 1s spectrum for annealed CdS/Co-NC(500) (Fig. 4d) incontrast to CdS/Co-NC obtained at room temperature. This additionalsignal may come from the N–S bonds at the interfaces between CdS and
Co-NC because of the elimination of interfacial adventitious hydrocar­
bon after heat treatment. Such an extra bond might act as a new inter­
facial electron shuttle channel to mediate the electron transfer from CdS
to Co-NC. Benefitting from the cleaner junction and the possible newly
formed bonds between CdS and Co-NC induced by the increased
annealing temperature, the activity of CdS/Co-NC was improved pro­
gressively (Fig. S9a), which further corroborates the effective
annealing-induced clearing of the junction between CdS and Co-NC.
The cleared interfaces enable the fast charge transfer from CdS to CoNC as identified by in situ FT-IR and XPS analyses. We further focus on
the promoted kinetics of photocarriers benefitted from this point bycomparing a group of CdS-based composites. Steady-state photo­
luminescence (PL) spectroscopy characterization, as a powerful tool tomonitor the excitonic processes, i.e., radiative recombination of photo­
generated electron-hole pairs in photocatalyst, was carried out for aseries of samples. As shown in Fig. 4f, pristine CdS presents two distinct
emission peaks in the range of 385–503 nm and 503–670 nm, assigned
to the excitonic emission of CdS and surface states related carriers
recombination, respectively [39]. Upon loading Co-NC on CdS without
further annealing, the PL peak centered at ~547 nm is substantiallysuppressed meanwhile the one centered at ~417 nm remains almost
unchanged. It signifies that the Co-NC can collect the energetic electrons
trapped by surface defect level of CdS although adventitious carbon
layer exists between CdS and Co-NC (Scheme 1a). After removing theorganic layer by annealing the composite at 500 � C, Co-NC captures
most photogenerated electrons from CdS side as evidenced by thefurther quenched emission peak in the range of 503–670 nm (Scheme1b). Consequently, those massively collected electrons will power therobust isolated Co sites for highly efficient hydrogen evolution. Addi­
tionally, compared with pristine CdS and CdS/Co-NC(RT) without
heating, the obvious blue shift of emission peak related to surface states
for annealed CdS/Co-NC(500) could be ascribed to the strong interfacialinteraction after eliminating the adventitious carbon at the junctions.
Photoelectrochemical characterizations lend further support to the

Scheme 1. Origin and mechanism of the efficient photocatalytic system CdS/Co-NC. (a) Illustration of carriers separation and transfer in CdS/Co-NC(RT) systemwith interfacial adventitious carbon. (b) Scheme of fast electron separation and transfer across cleared interface created by facile calcination treatment for CdS/CoNC(500).
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Nano Energy 76 (2020) 105077

essential role of smooth carrier transport channel created by removing
the interfacial adventitious carbon. Nyquist plot of CdS/Co-NC(500)under irradiation presents smallest semicircle in comparison with thatof the pristine CdS and CdS/Co-NC(RT) (Fig. S27a), indicating thesmallest resistance across the interface between CdS and Co-NC for
annealed CdS/Co-NC(500). Meanwhile, a marked boosted photocurrent(Fig. S27b), benefitting from the smoother junction, is also observed for
CdS/Co-NC(500) compared to that of CdS and CdS/Co-NC(RT).
According to the analyses above, the outstanding photocatalytic
performance of CdS/Co-NC system originates from two reasons: (i) the
distinctive single Co sites with favorable ability for robust H2 genera­tion; (ii) the smooth carrier channel created by eliminating the interfa­
cial adventitious carbon species between CdS and Co-NC as illustrated in
Scheme 1.

Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.nanoen.2020.105077.
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4. Conclusions
In summary, our study demonstrates an outstanding visible-lightdriven H2 evolution system CdS/Co-NC(500), which achieves a superb
performance with an activity of 4.34 mmol/h, an AQY 63.9% at 400 nm
and an remarkably high TOFCo of up to 16714.7 h 1. The excellent
performance mainly attributes to two reasons as disclosed by experi­
ment analyses and theoretical investigation. First, eliminating theinterfacial adventitious hydrocarbon layer by annealing process enables
directional and fast delivery of electron from CdS to Co-NC; second, theisolated Co site coordinated with four N owns favorable hydrogen
adsorption ability and thus can fully utilize the collected electrons fromphotosensitizer for H2 evolution. This study clarifies the significant
negative role of widely existed surface adventitious carbon on materials
in photocatalytic process and highlights both the great potential ofcleared interface and single-atom based materials in photocatalysis.
Thus, our results offer new opportunities for the exploration and designof high-performance system for efficient solar energy-driven conversion.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
CRediT authorship contribution statement
Yunxiang Li: Conceptualization, Investigation, Writing - originaldraft, Visualization. Shengyao Wang: Supervision, Investigation,
Writing - review & editing, Visualization. Pei Wang: Formal analysis.
Yu He: Investigation, Writing - review & editing. Xusheng Wang:
Investigation. Kun Chang: Investigation. Huiwen Lin: Investigation.
Xing Ding: Investigation. Hao Chen: Investigation. Hongwei Zhang:Investigation, Writing - review & editing. Yasuo Izumi: Investigation,
Writing - review & editing. Tetsuya Kako: Investigation. Jinhua Ye:
Supervision, Writing - review & editing, Funding acquisition.
Acknowledgement
This work received financial support from the World Premier Inter­national 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 (Nos. 21633004, 51902121), Fundamental
Research Funds for the Central Universities (Nos. NE2019103,
2018QD011) and the Natural Science Foundation of Hubei Province(No. 2019CFB322). Y. L. thank Dr. Shuxin Ouyang for his great support
as always. The authors are grateful to Beijing Synchrotron Radiation
Facilities staff for beam time and for their technical assistance in XAS
test.

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Nano Energy 76 (2020) 105077
Yunxiang Li received his B.S. degree in 2014 and M.S. degreein 2017 from Tianjin University and his Ph.D. degree in 2020
from Hokkaido University joint with National Institute forMaterials Science. His research interests focus on the design andfabrication of carbon-based and single-atom-based materials
for photocatalysis and electrocatalysis.

Kun Chang received his Ph.D. from Zhejiang University in2012, and joined the University of Western Ontario(2012–2013) and National Institute for Materials Science(2013–2018) for postdoc research. Now, he is a full professor in
Nanjing University of Aeronautics and Astronautics. His
research interests include design and synthesis of graphenebased and graphene-like materials, and application ofgraphene-like transition metal disulfides on hydrogen
evolution.

Shengyao Wang received his Ph.D. degree from Huazhong
Agricultural University in 2017. Later he worked in the Collegeof Science, Huazhong Agricultural University, China, as an
Associate Professor. He joined Prof. Jinhua Ye’s group as a
Guest Researcher in 2017. His research interests focus on thedesign and synthesis of nanostructured materials and theirapplication in photocatalysis.

Huiwen Lin received his bachelor’s degree in 2010 from Uni­
versity of Science and Technology of China and his Ph.D. degreein 2018 from Hokkaido University joint with National Institutefor Materials Science. He is currently a postdoctoral fellow in
Nanjing University of Aeronautics and Astronautics. His
research interests include the design and fabrication of Siliconbased and two-dimensional materials for energy storage andsolar energy conversion.

Pei Wang received her Ph.D. degree from the College ofChemistry and Chemical Engineering, Xiamen University in2017, supervised by Prof. Huiling Wan and Prof. Gang Fu. Latershe joined the Advanced Materials and Green Catalysis Group,
College of Science of Huazhong Agricultural University from2018. Her research interests mainly focus on understanding themechanism of heterogenous catalysis, theoretically. Especiallyin field of ammonia synthesis, CO2 reduction reaction, andHydrogen evolution reaction.

Xing Ding is currently an associate professor in the College of
Science at Huazhong Agricultural University. She obtained her
B.S. degree in 2010 and Ph.D. in 2015 from the Key Laboratoryof Pesticide & Chemical Biology of Ministry of Education,
College of Chemistry at Central China Normal University in2015. Her main research interests include the development ofphotocatalytic nanomaterials and their applications in the fieldof environmental remediation.

Yu He received her B.S. degree in the School of Materials Sci­
ence and Engineering, Tianjin University in 2014. Now, she is a
Ph.D. candidate in Material Science under the supervision of Dr.
Yida Deng and Dr. Xiaopeng Han at Tianjin University. She
joined Prof. Jinhua Ye’s research group in National Institute ofMaterials Science (NIMS) as a visiting student from 2018. Her
research interests focus on designing electroactive transition
metal-based materials electrocatalysis.

Hao Chen received his B.S. degree in 1985 and M.S. degree in1988 from Wuhan University, China. Then he obtained his Ph.
D. degree from Huazhong Agricultural University in 2007. He is
now a Principle Investigator of Advanced Materials and Green
Catalysis Group at Huazhong Agricultural University, and a
Professor at College of Science, Huazhong Agricultural Uni­
versity, China. His research interests focus on the nano­structured materials.

Xusheng Wang got his Ph.D. at Fujian Institute of Research on
the Structure of Matter, Chinese Academy of Science, under thesupervision of Prof. Rong Cao in 2019. Later he joined Prof. Dan
Li group at Jinan University as a postdoctoral researcher andProf. Jinhua Ye group at National Institute for Materials Science(Japan) as a guest researcher. He specializes in Metal-Organic
Frameworks for photocatalysis, including hydrogen produc­tion, CO2 photoreduction, and photo-assistant N2 fixation.

Hongwei Zhang studied at Tianjin Chengjian University(Bachelor’s degree 2013) and Chiba University (Master’s de­gree 2018). He is currently pursuing his PhD degree under thesupervision of Prof. Yasuo Izumi at the Department of Chem­
istry, Chiba University. His research interests focus on photo­catalytic reduction of carbon dioxide over metal nanoparticles
loaded with semiconductor materials and highly selective cat­alysts for fuel products such as methane and methanol havebeen developed. Explore the reaction mechanism of the catalyst
in heterogeneous reactions through in-situ spectroscopic
techniques.

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Nano Energy 76 (2020) 105077
Yasuo Izumi is a professor at the Graduate School of Science,
Chiba University, Japan. He obtained Doctor of Science at the
University of Tokyo and worked at Tokyo Institute of Tech­
nology. He has been studying in the fields of catalysis, X-ray
spectroscopy, and environmental chemistry. Recent interests
are sustainable solution by chemistry. A new carbon-neutral
cycle is completed by investigating photocatalysts to convert
CO2 into fuels using light as renewable energy source. During
the reaction, extended X-ray absorption fine structure isemployed not only to monitor the structural change of theactive site but optical and thermal behavior.

Jinhua Ye received her Ph.D. from the University of Tokyo in
1990, and joined National Research Institute for Metals (former
NIMS) in 1991. She is now a Principle Investigator and the Field
Coordinator of Nano-Power Field at International Center for
Materials Nanoarchitectonics (MANA), National Institute ofMaterials Science (NIMS), and a Professor of Joint Doctoral
Program in Graduate School of Chemical Science and Engi­
neering, Hokkaido University, Japan. She is also the appointed
director of TU-NIMS Joint Research Center, Tianjin University,
China. Her research interests focus on the research and devel­
opment of novel photocatalytic materials and their applicationsin the fields of environment remediation and solar to chemicalenergy conversion.

Tetsuya Kako received his B.S. degree, M.S. degree and PhD
degree from the University of Tokyo. He is now working as asenior researcher in National Institute for Materials Science(NIMS), Japan. His research mainly focuses on the developmentof visible light sensitive photocatalysts for energy and envi­
ronmental applications.

10