Single-atom nickel terminating sp2and sp3nitride in polymeric carbon nitride for visible-light photocatalytic overall water splitting

Article abstract of DOI:10.1039/d0sc07093a

Polymeric carbon nitride (PCN) has been widely used as a metal-free photocatalyst for solar hydrogen generation from water. However, rapid charge carrier recombination and sluggish water catalysis kinetics have greatly limited its photocatalytic performance for overall water splitting. Herein, a single-atom Ni terminating agent was introduced to coordinate with the heptazine units of PCN to create new hybrid orbitals. Both theoretical calculation and experimental evidence revealed that the new hybrid orbitals synergistically broadened visible light absorptionviaa metal-to-ligand charge transfer (MLCT) process, and accelerated the separation and transfer of photoexcited electrons and holes. The obtained single-atom Ni terminated PCN (PCNNi), without an additional cocatalyst loading, realized efficient photocatalytic overall water splitting into easily-separated gas-product H₂and liquid-product H₂O₂under visible light, with evolution rates reaching 26.6 and 24.0 μmol g^?1h^?1, respectively. It was indicated that single-atom Ni and the neighboring C atom served as water oxidation and reduction active sites, respectively, for overall water splittingviaa two-electron reaction pathway.

Full text of DOI:10.1039/d0sc07093a

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2
3
Single-atom nickel terminating sp and sp nitride
in polymeric carbon nitride for visible-light
photocatalytic overall water splitting†
Cite this: Chem. Sci., 2021, 12, 3633
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
a
a
b
b
c
Yanrui Li, Yiqing Wang, Chung-Li Dong,
Yu-Cheng Huang, Jie Chen,
a
d
d
a
a
Zhen Zhang, Fanqi Meng, Qinghua Zhang, Yiliang Huangfu, Daming Zhao,
d
a
Lin Gu
and Shaohua Shen
*
Polymeric carbon nitride (PCN) has been widely used as a metal-free photocatalyst for solar hydrogen
generation from water. However, rapid charge carrier recombination and sluggish water catalysis kinetics
have greatly limited its photocatalytic performance for overall water splitting. Herein, a single-atom Ni
terminating agent was introduced to coordinate with the heptazine units of PCN to create new hybrid
orbitals. Both theoretical calculation and experimental evidence revealed that the new hybrid orbitals
synergistically broadened visible light absorption via a metal-to-ligand charge transfer (MLCT) process,
and accelerated the separation and transfer of photoexcited electrons and holes. The obtained single-
atom Ni terminated PCN (PCNNi), without an additional cocatalyst loading, realized efficient
Received 30th December 2020
Accepted 17th January 2021
photocatalytic overall water splitting into easily-separated gas-product H
2
1
2 2
and liquid-product H O
ꢀ
ꢀ1
under visible light, with evolution rates reaching 26.6 and 24.0 mmol g
h
, respectively. It was
DOI: 10.1039/d0sc07093a
indicated that single-atom Ni and the neighboring C atom served as water oxidation and reduction
rsc.li/chemical-science
active sites, respectively, for overall water splitting via a two-electron reaction pathway.
a quantum efficiency of almost unity for overall water splitting
Introduction
9
over SrTiO , which is the highest value of photocatalytic water
3
Solar-driven water splitting based on semiconductors is a fasci- splitting reported so far, but is still limited in the ultraviolet
nating technology for converting earth-abundant solar radia- region. Other than these well-investigated metal-based semi-
1,2
tion energy into clean and renewable hydrogen fuel. In the conductors, signicant progress has also been achieved in
10
past few decades, numerous semiconductors including metal metal-free polymeric semiconductors. Among them, poly-
oxides, suldes and nitrides have been developed for efficient meric carbon nitride (PCN) has attracted the greatest attention
photocatalytic hydrogen evolution in the presence of sacricial because of its low cost, facile preparation and environmental
3
–5
11,12
reagents as hole scavengers.
splitting using a particulate photocatalyst in the absence of ting via visible light photocatalysis.
Photocatalytic overall water benignity,
which could be activated for overall water split-
13,14
However, PCN still
sacricial reagents serves as a simpler, more cost-effective suffers from fast charge carrier recombination and insufficient
approach for large-scale solar-fuel conversion, but still suffers driving force for photoexcited electron–hole separation, result-
from low efficiency. Up to now, some metal-based semi- ing in relatively poor photocatalytic activity for overall water
conductor particulates have been developed for photocatalytic splitting. It is therefore important to explore strategies and
6–8
overall water splitting. For example, Takata et al. achieved methodologies to prompt the charge carrier kinetics of PCN
photocatalysts.
PCN possesses a molecular structure of 1D heptazine-based
polymer chains conjugated by hydrogen bonding in the planar
a
International Research Center for Renewable Energy, State Key Laboratory of
Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an 710049,
arrangement, with 2D melon-based sheets stacked along the c-
China. E-mail: shshen_xjtu@mail.xjtu.edu.cn
b
15
direction with weak interlayer van der Waals forces. Upon
photoexcitation, the electrons generated in the heptazine units
Department of Physics, Tamkang University, 151 Yingzhuan Rd, New Taipei City
2
c
5137, Taiwan
Division of Physical Science and Engineering, King Abdullah University of Science and could migrate to the extended heptazine rings in three ways
Technology, Thuwal, 23955-6900, Saudi Arabia
d
including, interplanar migration along the p-stacking direction,
transporting from one heptazine unit to another with different
symmetric planes in the same polymer chains, or intrachain
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics,
Chinese Academy of Sciences, Beijing, 100190, China
†
Electronic supplementary information (ESI) available: Detailed experimental
migration along the polymer chains via bridging N atoms
procedures, additional structural characterization, photocatalytic measurements
and DFT calculations for reaction pathways. See DOI: 10.1039/d0sc07093a
16,17
(
Fig. S1 in the ESI†).
During migration, the photocarriers
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would be dissipated by electron–hole recombination during irradiation (l > 420 nm). It was demonstrated that the hybrid-
intrachain migration along the polymer chains, or the fast ization of single-atom Ni with the aromatic conjugated ring
recombination of photoexcited electron–hole pairs conned in created a new highest occupied molecular orbital (HOMO) and
the heptazine units, or the interrupted charge transport by the new lowest unoccupied molecular orbital (LUMO) in PCNNi,
hydrogen bonds. To overcome these limitations, representative synergistically broadening the optical absorption range via the
solutions include the design of plane heterostructural carbon MLCT process and accelerating the separation and transfer of
ring (Cring)–C
3 4
N nanosheets to build an in-built electric eld photoexcited electrons and holes generated in the heptazine
18
for fast spatial transfer of photoexcited electrons and the units. Theoretical calculations and electrochemical experi-
integration of dual co-catalysts to spatially separate H
evolution reactions.
2
and O
2
ments further revealed that the single-atom Ni and the neigh-
boring C atom served as oxidation and reduction active sites for
19
Recently, constructing atomically dispersed active sites on overall water splitting into H O and H respectively, through
2
2
2,
PCN has been considered as an ideal modication strategy by a two-electron reaction pathway. This study presents an alter-
bringing together the ne-tunable molecular structure and the native approach to engineer the atomic and electronic struc-
single-atom active sites of PCN to promote charge transfer and tures of PCN for great performance enhancement in
20,21
accelerate surface reactions.
For instance, Wei et al. photocatalytic overall water splitting, with liquid H
2 2
O and
conned single Co –P sites on the PCN matrix by a facile gaseous H
phosphidation method. The photoexcited charges generated the explosive gaseous mixture of O
in the conjugate ring could be rapidly separated by the neigh-
1
4
2
generated as easily-separated products, rather than
22
2
2
and H .
boring Co –P active sites and the carrier lifetime was prolonged
1
4
Results and discussion
Morphological and structural properties of PCNNi
by about 20 times relative to pristine PCN. Xiong et al. designed
2
+
2 6
PCN–Pt coordination by mixing dicyandiamide and H PtCl
via one-step thermal polymerization. This coordination unit It has been well documented that three kinds of nitrogen atom
would act as a new light absorption center through a metal-to- exist in the heptazine units of PCN, i.e., triangular edge nitrogen
ligand charge transfer (MLCT) process, resulting in the (N
improved visible-near-infrared photocatalytic performance.
A
), amino groups/bridge nitrogen (N
nitrogen (N ) as described in Scheme 1a. Both sp N
Wu et al. anchored single-atom Pt onto PCN via a mild liquid- possess lone-pair electrons, providing coordination sites to
B
) and central tertiary
23
2
3
C
A B
and sp N
32
phase route by mixing dicyandiamide and H PtCl in solu- conne metal atoms. It could be proposed that N atoms in
2
6
A
tion. It was demonstrated that the intrinsic change of the the aromatic conjugate rings are coordinated by Ni atoms
surface trap states of PCN induced by the single Pt atoms (Scheme 1b, coordination structure I). These Ni atoms exist
24
enhanced the photocatalytic water reduction performance.
between the melon polymer chains, breaking the hydrogen
Note that the thermal polymerization of nitrogen-rich precur- bonding (N–H/N) formed by the terminal –NH2 or –NH–
sors for the synthesis of PCN will produce intermediates such as groups (Scheme 1c and d). Meanwhile, Ni atoms also coordinate
25
26
B
the heptazine ring, melem and low-dimensional C–N poly- with N atoms in the heptazine units (Scheme 1b, coordination
27
mers, which all possess nitrogen with lone-pair electrons (see structure II), terminating amidogen polycondensation in the
the polymerization pathway in Fig. S2 in the ESI†). During the polymerization process thus interrupting the heptazine chains,
thermal polymerization process, metal atoms could coordinate which brings about numerous defective structures. To
2
3
with these intermediates containing sp or sp hybridized N conclude, single-atom Ni could act as a terminating agent that
25,28,29
atoms,
which may alter the stacking mode of conjugate coordinates with N atoms to break the hydrogen bonding and
A
rings in PCN and manipulate the molecular and electronic with N atoms to interrupt the polymerization process, which
B
23,30,31
structures.
Therefore, it is essential to investigate the would trigger remarkable alterations in the molecular structure
chemical coordination between single metal atoms and inter- of PCN. In this study, a series of single-atom Ni coordinated
mediates during the thermal polymerization process, which PCN was synthesized by thermal polymerization of melamine
could regulate the ne structure of the as-synthesized PCN, with and nickel chloride as precursors under a nitrogen atmosphere
an engineered band structure for extended light absorption and (see the Experimental section in the ESI†). The samples are
coordinated active sites for promoted charge transfer and named PCNNi-1, PCNNi-2, PCNNi-3, and PCNNi-4 with Ni
catalytic reactions. High photocatalytic performances can thus contents determined to be 0.3 wt%, 0.5 wt%, 1.0 wt%, and
be anticipated.
1.9 wt%, respectively, by inductively coupled plasma mass
Bearing the favorable coordination between metal atoms spectrometry. As shown in Table S1,† the Ni contents could be
and polymerization intermediates, herein, we utilized single- well controlled in PCNNi synthesized in different batches.
atom Ni as a terminating agent to coordinate with triangular Pristine PCN presents a bulky morphology (Fig. S3a†), similar to
2
3
edge (sp ) nitrogen and amino groups/bridge (sp ) nitrogen in the previous reported PCN obtained from the thermal poly-
33
heptazine units during the polymerization reaction for PCN merization process. In comparison, all the PCNNi samples
synthesis. The single-atom Ni terminated PCN (PCNNi), without display nanosheet-like structures (Fig. 1a and S3b–d†), which
an additional cocatalyst loading, exhibited much increased should be due to the single-atom Ni interrupting the polymer-
photocatalytic activity for overall water splitting, with gas- ization process to produce small and thin pieces of PCN. One
product H
2
and liquid-product H
2
O
2
generation rates reaching can hardly observe any Ni-based nanoparticles in these PCNNi
ꢀ1
ꢀ1
26.6 and 24.0 mmol g
h , respectively, under visible light samples, even in PCNNi-4 with the maximum content of Ni
3634 | Chem. Sci., 2021, 12, 3633–3643
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Scheme 1 (a) Molecular structure of heptazine-based PCN. (b) Proposed schemes of thermal polymerization of heptazine units with Ni–N
coordination. Regional schemes of (c) PCN with hydrogen bonding between terminal –NH and –NH– groups, and (d) PCN with broken
2
hydrogen bonding with single-atom Ni as the terminating agent.
(
Fig. S4†), implying the possibility of single-atom Ni existing in Fourier transform infrared (FT-IR) spectroscopy was further
the PCN matrix, which will be conrmed in the following conducted to investigate the inuence of the single-atom Ni
sections. terminating agent on the thermal polymerization process and
The crystalline phases of the PCNNi samples were deter- on the molecular structure of PCN. As shown in Fig. 1c, the
ꢀ1
mined from the X-ray powder diffraction (XRD) pattern. As characteristic bands in the region of 1100–1600 cm belonging
shown in Fig. 1b, both pristine PCN and PCNNi display two to the ngerprint of heptazine rings are very similar for PCN
ꢁ
ꢁ
13
diffraction peaks at 2q ¼ 13.1 and 27.3 , corresponding to the and PCNNi, suggesting that the single-atom Ni as a termi-
100) and (002) planes of PCN, which should be related to the in- nating agent does not change the basic structure of the hepta-
plane structural repeating units of heptazine and the interlayer zine units during polymerization. However, the bands in the
(
ꢀ1
stacking of the conjugated aromatic rings for the graphitic-like region of 3000–3400 cm typical for hydrogen bonding display
structure of PCN, respectively. Noteworthily, both the diffrac- a progressive decrease in intensity, whilst the characteristic
tion peaks show gradually decreased intensity with the bands of NH – or –NH– in the region of 3400–3600 cm are
2
34
ꢀ
1
increasing Ni contents in PCNNi. Furthermore, as presented in gradually strengthened, with the increasing Ni contents in
Table S2,† the full width at half maximum (FWHM) of the peak PCNNi. For more detailed analyzation, the FTIR spectrum of the
ꢁ
ꢁ
ꢁ
27
at 2q ¼ 27.3 increases from 1.86 for PCN to 2.36 for PCNNi-4, melamine precursor is shown as reference (Fig. 1d). The bands
ꢀ1
indicating the loss of the ordered structure in PCN. No peak in the region of 500–1800 cm correspond to the ngerprint of
assigned to Ni-based species is observed in PCNNi, again sup- melamine. The two sharp bands featuring in the region of 3400–
porting the possible existence of single-atom Ni. The thermal 3500 cm (centered at 3428 cm and 3482 cm ) are assigned
ꢀ
1
ꢀ1
ꢀ1
stability of PCNNi was then examined by thermogravimetric to the symmetric and asymmetric stretching of NH –, respec-
2
analysis (TGA) under nitrogen (Fig. S5†). It is observed that the tively, which shows that plenty of amidogens exist in the
ꢁ
onset temperature of mass loss decreases from 480 C for melamine precursor. One will observe that these bands are
ꢁ
pristine PCN to 380 C for PCNNi-4. Interestingly, the onset stronger for PCNNi than PCN, with intensities increasing
temperature is proportional to the increasing Ni contents in depending on the increasing Ni contents in PCNNi (Fig. 1c and
PCNNi. This reduced thermal stability should be related to the d). This conrms our hypothesis that single-atom Ni coordi-
increasing ratio of small and thin pieces of PCN with a disor- nates with N in heptazine units, interrupting the polymeriza-
B
dered structure, which were created during the polymerization tion reaction with more NH – or –NH– groups created in PCNNi.
2
process interrupted by the single-atom Ni terminating agent. Hence, the N to C ratio of PCNNi was increased compared with
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Fig. 1 (a) TEM image of PCNNi-3. (b) XRD patterns of PCN and PCNNi with different Ni contents. (c) FTIR spectra of PCN and PCNNi with
different Ni contents. (d) FTIR spectra of melamine, PCNNi-3 and PCN.
that of PCN (Table S3†). Furthermore, the melamine precursor eld scanning transmission electron microscopy (HAADF-
ꢀ
1
displays two obvious peaks in the region of 3000–3400 cm
(
STEM) was conducted on PCNNi. The HAADF-STEM image
ꢀ
1
ꢀ1
centered at 3134 cm and 3327 cm ), which is the typical clearly displays the isolated bright dots of atomic size in white
indicator of hydrogen bonding (N–H/N) formed between as highlighted by the red circles (Fig. 2a), which can unambig-
melamine molecules. However, this ngerprint of hydrogen uously determine the atomic distribution of Ni in PCN. A
bonding in PCNNi is found to be much weaker than that of PCN quantitative analysis of the Ni atom positions in the HAADF-
(Fig. 1c and d), revealing that the hydrogen bonding is broken STEM image indicates that the measured nearest-neighbor
by the coordination between single-atom Ni and N /N atoms in distances of the bright dots are in accordance with the Ray-
A
B
heptazine units. To conclude, FTIR analysis provides direct leigh distribution for a random distribution of single-atom Ni
evidence from the viewpoint of the molecular structure that over the support of PCN. X-ray absorption structure spectros-
2
3
single-atom Ni terminates sp and sp nitrogen with lone-pair copy (XAS) is sensitive to local atomic/electronic structures and
electrons in the heptazine units during the thermal polymeri- provides atomic-level structural information. For more insights
zation of the melamine precursor, leading to diminishing of into the chemical and bonding situations of the atom-
hydrogen bonds between the neighboring melon chain and distributed Ni in PCNNi, X-ray absorption near-edge structure
increasing residual NH
from the nitrogen adsorption and desorption isotherms along with Ni foil and NiO as references. As shown in Fig. 2b,
Fig. S6†), the Brunauer–Emmett–Teller (BET) specic surface the spectral prole of the Ni K-edge of PCNNi is apparently
2
– or –NH– groups in PCN. As calculated (XANES) spectra at the Ni K-edge were monitored over PCNNi
(
2
areas (SBET) were determined to be 5.1, 6.1, 7.3, 7.5 and 8.0 m
g
different from that of Ni or NiO. The shoulder-like feature at the
ꢀ1
for PCN, PCNNi-1, PCNNi-2, PCNNi-3 and PCNNi-4, K-edge has been interpreted as 1s to d transition, which is
respectively. This slight increase in SBET should be due to the usually forbidden according to the Fermi golden rule. However,
small and thin pieces of PCN produced via interrupting the this transition will be allowed, if Ni 2p–3d rehybridization
polymerization process by the single-atom Ni, which should not happens with energy band formation in Ni foil. Thus, the
be responsible for the great improvement in the photocatalytic absence of the shoulder-like feature in PCNNi suggests that the
activity of PCNNi.
band structure of single-atom Ni is different from that of Ni foil.
To evidence the atomic dispersion of Ni as speculated in Additionally, the intensity of the Ni K-edge of PCNNi is higher
previous analysis, atomic-resolution high-angle-annular-dark- (lower) than that of Ni foil (NiO), revealing that the single-atom
3636 | Chem. Sci., 2021, 12, 3633–3643
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Fig. 2 (a) HAADF-STEM image of PCNNi-3. (b) Ni K-edge XANES spectra of PCNNi-3 and PCN. The inset in (b) shows the outer electron
configuration of single-atom Ni in PCNNi-3. (c) Fourier Transformed (FTs) amplitude of the extended X-ray absorption fine structure (EXAFS) of
the Ni K-edge spectra for PCNNi-3, Ni foil and NiO. (d) XPS Ni 2p spectrum of PCNNi-3. (e) N K-edge and (f) C K-edge XANES spectra of PCNNi-3
and PCN. (g) Atomic model of the heptazine units in PCN, and N 1s XPS spectra of PCN and PCNNi.
35,36
Ni possesses an oxidation state higher (lower) than that of assigned to the Ni–N bond is detected,
again suggesting that
metallic Ni (fully oxidized NiO) and holds more (less) unoccu- the Ni species are well isolated as single-atom sites and coor-
pied states than metallic Ni (fully oxidized NiO). Considering dinated with N atoms in the framework of PCN.
2
+
8
that Ni in the NiCl2 precursor possesses a 3d electron
X-ray photoelectron spectroscopy (XPS) measurements were
conguration, which will coordinate with N atoms in heptazine also conducted to investigate the valence state of single-atom Ni
units, electron transfer from N atoms to the unoccupied orbitals coordinated with PCN. It was previously recorded that the Ni 2p
1
9,20
2+
of Ni atoms
Ni in PCNNi, in comparison to NiO with 3d (4s electron 873.6 eV.
transfers to oxygen sites) electron conguration. Note that ed for PCNNi (Fig. 2d and S7a†) are shied to lower binding
gives rise to the reduced unoccupied orbitals of XPS peaks for Ni in NiO are located at 855.5 eV and
8
2
37,38
In comparison, these two Ni 2p XPS peaks identi-
8
2
8
39,40
metallic Ni has 3d 4s electron conguration while NiO has 3d
energies of 855.0 eV and 872.4 eV,
electron conguration; the single-atom Ni should possess XPS peak indexed to metallic Ni (853.2 eV) and NiN (852.7 eV)
a partially lled 4s orbital (Fig. 2b, inset), attributed to the is observed. These comparative results conrm that the
respectively. Note that no
0
3
41
electron transfer from neighboring N to Ni atoms. Fig. 2c shows oxidation state of single-atom Ni sites in PCNNi is lower than
2
+
the Fourier Transformed (FTs) amplitude of the extended X-ray that of Ni in the fully oxidized NiO but higher than that of
0
absorption ne structure (EXAFS) of Ni K-edge spectra. Ni foil metallic Ni , which well agrees with the Ni K-edge XANES
ꢀ
ꢀ
and NiO display strong peaks at 2.2 A and 2.4 A, attributed to analysis.
the Ni–Ni bond and Ni–O bond, respectively. In comparison,
In addition to the characterization of the metal Ni site,
both the peaks are invisible for PCNNi, implying the absence of XANES measurements on the non-metal N and C K-edge were
ꢀ
Ni–O and Ni–Ni bonds. Yet, a peak located at about 1.6 A carried out to monitor the local atomic and electronic structure
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changes of PCNNi vs. PCN. In the N K-edge XANES spectra early stage of thermal polymerization (see Fig. S9 and S10 and
Fig. 2e), both PCNNi and PCN display two typical p* reso- related discussion in the ESI†).
(
nances at 399 eV and 402 eV, associated with the triangular edge
nitrogen (N , see the inset of Fig. 2e) and the bridge nitrogen
A
42
Optical properties and band structure of PCNNi
(N
B
, see the inset of Fig. 2e), respectively. The N
A
peak intensity
of PCNNi is stronger than that of PCN, due to the coordination The optical properties of the as-obtained PCNNi samples were
19
between single-atom Ni and N
A
atoms. Thereaer, the more determined using UV-visible diffuse reectance spectra (DRS).
unoccupied 2p states at N atoms and the greater p* charac- As compared to pure PCN, the optical absorption of PCNNi
A
teristics, which could generally accelerate the electron transfer redshis to longer wavelengths with increasing Ni contents
from N to single-atom Ni and strengthen the hybridization of (Fig. 3a). The Tauc plots translated from UV-vis spectra by the
A
triangular C–N]C and single-atom Ni in PCNNi, respectively, Kubelka–Munk equation showed two band gaps at 2.73 and
benet photocatalysis. Meanwhile, the intensity of peak N is 2.3 eV for PCNNi, respectively (Fig. 3b). The bandgap of 2.73 eV
B
decreased, suggesting the structure breaking of bridge nitrogen, is attributed to p / p* transition, i.e., band-to-band transition
caused by the introduction of the single-atom Ni terminating from the HOMO to LUMO in the conjugated electron system of
41
45
agent during the polymerization reaction. As the N 2p orbital PCN. The bandgap of 2.3 eV in PCNNi should be related to the
is strongly hybridized with the C 2p orbital in the heptazine Ni–N coordination structure, in which electron transition
unit, complementary C K-edge spectra are provided to illustrate happens between the metal and ligand, i.e., the metal-to-ligand
the hybridization of single-atom Ni with heptazine units charge transfer (MLCT) process, with transition energy lower
46
(
Fig. 2f). The overall spectral prole of PCNNi resembles that of than that of band-to-band transition, contributing to optical
PCN, indicating that the C atoms in the heptazine units are not absorption in visible light.
disturbed by the single-atom Ni, which is consistent with the
The band structures and electronic structures of PCNNi were
unchanged C 1s XPS spectra and (see the detailed analysis in further explored using XPS VB spectra and density-functional
Fig. S7b in the ESI†). Interestingly, the peak related to p*C–N–C at theory (DFT) calculations. The HOMO tails of PCNNi ends at
ca. 287.5 eV is higher in PCNNi than in PCN, indicating that the 0.8 eV (Fig. S11†), similar to that of pure PCN, indicating that
heptazine unit anchored with single-atom Ni has a greater p* the HOMO maximum remains unchanged with the Ni–N coor-
characteristic, as evidenced in the N K-edge spectra. These dination. This result can be proved by the DFT calculation
analyses well support that single-atom Ni acts as a terminating results. As shown in Fig. 3c, the new HOMO in PCNNi is at the
agent to coordinate with N atoms with lone pair electrons in the same energy level as the HOMO of the pristine PCN. Interest-
heptazine units during the thermal polymerization of PCN, ingly, a new LUMO state in PCNNi is created below the original
which agrees well with the FTIR results.
LUMO of the pristine PCN, which should be ascribed to the
We further carried out N 1s XPS measurements to detect the hybridization of single-atom Ni with the heptazine unit
local chemical environment of single-atom Ni and N atoms. (namely, M–L unit). The densities of states (DOS) were calcu-
The N 1s spectrum of PCN could be deconvoluted into three lated for the pure PCN and the M–L unit, as displayed in Fig. 3d.
peaks at 398.4 eV, 399.6 eV, and 400.89 eV (Fig. 2g), which are The LUMO of pure PCN is composed of the 2p hybridization
assigned to triangular edge nitrogen (C]N–C) (N , see the inset states of C and N atoms, while the HOMO is dominated by the
A
of Fig. 2g), amino groups (C–NH ) (N , see the inset of Fig. 2g) 2p states of N atoms. In contrast, the orbital distributions of
x
B
and central tertiary nitrogen (N–(C)
Fig. 2g), respectively.
3
) (N
The N 1s spectra of PCNNi could be single-atom Ni with the heptazine unit. For the M–L unit, the 4s
tted with three peaks analogous to PCN, suggesting the well- orbitals of single-atom Ni and the 2p orbitals of N atoms are
C
, see the inset of PCNNi are signicantly changed upon the coordination of
43,44

preserved heptazine-based structure in PCNNi. According to hybridized, which contributes to the new HOMO. The LUMO of
the detailed assignment of the N 1s peak (Table S4†), both peak the M–L unit is mainly composed of the C 2p state, while N 2p
NA and peak NB shi toward higher binding energies, which and Ni 4s states contribute very slightly. Based on the above
indicates that the electron densities at N and N are reduced, analysis, the band structures of PCN and PCNNi could be
A
B
30
likely due to charge transfer from N atoms to single-atom Ni.
schematically proposed in Fig. 3e. It was reported that PCN
This result also conrms the coordination between single-atom possesses the LUMO at a potential of ca. 0.9 eV higher than the
+
Ni and N atoms (N
A B
2
and N ) by Ni–N chemical bonding, in good H /H redox potential, and the HOMO at a potential of ca. 0.6 eV
47
agreement with the FTIR and XAS results. Furthermore, in the lower than the H O/O redox potential, giving rise to the
2
2
1
3
Solid-state C CP-MAS NMR spectra (Fig. S8†), the lower bandgap of ꢂ2.73 eV. In comparison, a narrower bandgap of
chemical shis of PCNNi than PCN indicate the reduced elec- 2.3 eV was observed for PCNNi, implying that electron transi-
tron densities of aromatic conjugated rings, which could again tion occurs between the new HOMO and LUMO of the M–L unit.
evidence the coordination between single-atom Ni and the Considering that the HOMO position of PCNNi was unchanged
heptazine network.
relative to PCN, the narrowed bandgap should have resulted
Detailed investigation into the chemical interaction between from the downward shi of the LUMO by 0.43 eV. Although
single-atom Ni and coordinated N sites in intermediate prod- shied, the LUMO and HOMO positions of PCNNi still guar-
ucts obtained at different temperatures indicates that single- antee enough potential to drive water reduction and oxidation
atom Ni coordinates with N sites in heptazine formed at the reactions, respectively. Encouragingly, the built-in electric eld
created by the new LUMO of the M–L unit and the original
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Fig. 3 (a) UV-vis diffuse reflectance spectra of PCN and PCNNi. (b) Tauc plots of PCN and PCNNi calculated by the Kubelka–Munk method. DFT
calculations of the (c) band structures of PCN (left) and the M–L unit (right) and the (d) projected density of states of PCN (top) and the M–L unit
(
down). (e) Band structure diagrams of PCN and PCNNi.
LUMO of PCN would provide a driving force to steer the is gradually increased, with a highest H evolution rate of 26.65
2
ꢀ1
ꢀ1
migration of photoexcited electrons from the unhybridized mmol g
h
achieved over PCNNi-3. However, further increase
heptazine unit to the M–L unit.
in Ni contents would decrease the photocatalytic activity of
PCNNi-4. The apparent quantum yields (AQY) of PCNNi-3 at
380 nm, 420 nm and 500 nm were examined to be 1.53%, 1.12%
Photocatalytic performance for overall water splitting into H
and H O
2
and 0.05%, respectively, which depends on the optical absorp-
tion prole of PCNNi (Fig. S12†). It is clear that PCNNi could
photocatalyze overall water splitting at a longer wavelength of
2
2
The photocatalytic water splitting performances of the as-
synthesized PCNNi samples were evaluated under visible light
5
00 nm, which indicates that the extended optical absorption
should contribute to the improved photocatalytic activity.
Interestingly, rather than O , H was detected as the water
(l > 420 nm) without any sacricial reagents. As shown in
2
2 2
O
Fig. 4a, the pristine PCN can hardly produce H from pure water
2
oxidation product over PCNNi (see raw GC data in Fig. S13†),
indicating that a two-electron process is responsible for pure
water splitting (Fig. 4b). For all the PCNNi photocatalysts, the
under visible light. In comparison, PCNNi steadily produces H
from pure water without loading additional cocatalysts. With
the increasing Ni contents in PCNNi, the photocatalytic activity
2
Fig. 4 (a) Photocatalytic hydrogen generation from pure water under visible light (l > 420 nm), (b) photocatalytic overall water splitting into
hydrogen and hydrogen peroxide without any sacrificial reagent under visible light (l > 420 nm) with activity marked by the error bar, (c) the
measured Pt disk current and Pt exchange current and corresponding electron transfer number.
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: H
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H
2
O
2
2
molar ratios are measured to be ꢂ0.9 : 1, slightly bonding orbital (Fig. 5a, le). As shown in Fig. S15,† PCN shows
smaller than the stoichiometric ratio of 1 : 1 for overall water strong photoluminescence (PL) emission at ca. 460 nm, which is
splitting into H O and H , possibly due to the decomposition related to the electron transition between the LP orbital and p*
2
2
2
48
of H O during the reaction. Moreover, PCNNi shows quite antibonding orbital. In comparison, all the PCNNi samples
2
2
good photocatalytic stability for overall water splitting under show a signicant decrease in PL emission intensity, which
visible light, with H and H evolution rates remaining implies the inhibited charge carrier recombination in PCNNi
2
2 2
O
almost unchanged during the 3-cycle (24 h) reaction (Fig. S14a with the introduction of single-atom Ni. Interestingly, PCNNi
and b†), which exhibits very good reproducibility as conrmed has PL emissions located at almost the constant position
in different batches of PCNNi (Fig. 4b). The well maintained (Fig. 5b, taking PCCNi-3 as an example), when excited by
single-atom Ni–N coordination structure aer the photo- photons with different energies higher than the measured
catalytic reaction (Fig. S14c and d†) further conrms the good bandgap of PCN (e.g., light with wavelengths varying from
chemical stability of PCNNi for photocatalytic overall water 310 nm to 370 nm). This observation suggests that excited
splitting. By carrying out rotating Pt disk–Pt ring electrodes electrons in PCNNi would relax from s* orbitals to the same p*
49
(
RRDE) measurements, the electron transfer number is calcu- antibonding orbital through intersystem crossing (ISC).
lated to be 2 for water oxidation (Fig. 4c), which well explains Hence, similar to PCN, PCNNi has PL emission originating from
the production of H rather than O during photocatalytic the radiative recombination of electrons in the p* antibonding
2
O
2
2
water splitting over PCNNi. To reveal the great enhancement in orbital and holes in the LP orbital (Fig. 5a, le). As previously
photocatalytic performances for overall water splitting, the analyzed (Fig. 3c–e), the M–L unit creates a new LUMO in the
charge transfer processes are systematically investigated on band structure of PCNNi with the introduction of the single-
PCNNi in the following sections.
atom Ni terminating agent (Fig. 5a, right). It could then be
deduced that the new LUMO could act as the trap site to capture
electrons from the unhybridized heptazine unit, resulting in
inhibited charge carrier recombination, which could be well
evidenced by the decreased PL emission of PCNNi (Fig. S15†). As
further evidenced in electrochemical impedance spectroscopy
Mechanism analysis of photocatalytic overall water splitting
49
3
As previously reported for the band structure of PCN, C–N sp
hybridization gives rise to high energy s and s* molecular
2
orbitals, while C–N sp hybridization constitutes a conjugated
(EIS), PCNNi exhibits much decreased charge-transfer resis-
network, forming low energy p bonding and p* antibonding
orbitals (Fig. 5a, le). Besides, the unbonded lone pairs (LP) of
electrons on pyridinic N atoms create a LP orbital above the p
tances, as compared with PCN, suggesting that the single-atom
Ni could greatly improve charge transfer ability for the surface
Fig. 5 (a) Illustration of the charge transfer processes in the unhybridized heptazine unit (left) and the M–L unit (right). (b) Steady-state fluo-
rescence spectra of PCNNi-3 at different excitation wavelengths. (c) Time-resolved transient PL decay spectra of PCN and PCNNi-3 with the
excitation wavelength of 337 nm. (d) C K-edge and (e) N K-edge XANES spectra of PCN and PCNNi-3 with or without illumination. The insets in
(
d) and (e) show the spectral difference between in the dark and under illumination, respectively. (f) Differential charge density mapping of the
2
M–L unit (single-atom Ni coordinated with sp NA atoms). Red and yellow represent electron accumulation and electron depletion regions,
respectively.
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water redox reaction, especially for PCNNi-3 (Fig. S16†). unhybridized heptazine unit to the M–L unit, resulting in the
Furthermore, the time-resolved PL spectra (Fig. 5c) demonstrate inhibited charge carrier recombination. As a result, the broad-
that PCNNi-3 possesses an average carrier lifetime (5.922 ns) ened optical absorption and the improved charge transfer
much longer than that of PCN (1.759 ns). This again proves the behaviors synergistically contribute to great enhancement in
inhibited charge recombination and then the improved charge photocatalytic performances for overall water splitting into H
transfer ability, due to the migration of electrons from s* and and H . As proposed in the DFT calculations (Fig. 5f and
2
2 2
O
p* antibonding orbitals of the unhybridized heptazine unit to S17†), holes are enriched at the single-atom Ni and electrons are
the new LUMO created in the M–L unit. In addition, a low- aggregated at the adjacent C and N atoms. DOS results (Fig. 3d)
energy tail was observed in the PL curve of PCNNi, and the indicate that the 2p orbital of the C atom is the major
peak can be tted with three Gaussian peaks centered at contributor to the HOMO states of the M–L unit to accept the
4
60 nm, 480 nm, and 525 nm (Fig. 5b, inset). Considering the photoexcited electrons from LUMO states. Thus, single-atom Ni
absorption spectrum with photoexcitations of 2.73 eV and and the adjacent C atoms are favorable to act as water oxidation
.3 eV, the emission peaks at 460 nm and 525 nm should and reduction active sites, respectively. Then, the reaction
2
originate from the electron transitions of the p* / LP orbital in pathways for water reduction and oxidation were further theo-
the heptazine unit and the LUMO / HOMO in the M–L unit, retically calculated for PCNNi with single-atom Ni coordinated
2
3
respectively. The emission peak at 480 nm may be related to with sp N
A
and sp N
B
atoms, with free energy diagrams shown
47
other mid-states due to the structural defect in PCNNi.
in Fig. 6 and S18,† respectively. For pure PCN, a free energy
C K-edge and N K-edge XANES spectra were collected in the difference of 0.0665 eV is required for the formation of the
dark and under illumination to further investigate the charge transition state (TS) for hydrogen evolution (Fig. 6a). In
transfer processes in PCNNi upon photoexcitation. As comparison, PCNNi holds a much lower free energy difference
mentioned in DFT calculations (Fig. 3d), the HOMO of the of 0.0015 eV (Fig. 6a), due to electron accumulation at the
M–L unit is derived from the hybridization of Ni 4s and N 2p reductive active sites of C atoms via the MLCT process, leading
orbitals, while the LUMO mainly consists of C 2p orbitals. It is to the reduced barrier energy for TS formation. For easy
well known that the electrons are excited and transferred understanding, the water reduction reaction pathway is sche-
from the LUMO to HOMO, contributing to the emerging matically depicted in Fig. 6c–f. According to the principle of
peaks in C and N K-edge spectra. As shown in Fig. 5d and e, minimum energy, with an initial model of the photocatalytic
the C K-edge and N K-edge spectra of both PCN and PCNNi water splitting reaction constructed at a reasonable charge
48
exhibit higher peak intensities under illumination than in distribution site, single-atom Ni rstly absorbs two H O
2
the dark. Moreover, the spectral deviation between illumi- molecules (Fig. 6c) and breaks the H–O–H bond into H and OH.
nated and dark conditions (the inset in Fig. 5d and e) is less Then, two H atoms tend to adsorb at the neighboring C atoms of
signicant in PCNNi, as compared to PCN, which should be the single-atom Ni (Fig. 6d), and subsequently the H–H bond is
related to the transfer of photoexcited electrons in M–L units formed as the intermediate between two H adatoms (Fig. 6e),
via the MLCT process. In detail, since the single-atom Ni has with H
2
releasing from PCNNi nally (Fig. 6f). In this study,
its non-vacant 4s orbital hybridized with the N 2p orbital, the H O rather than O2 is produced over PCNNi as the water
2
2
photoexcited electrons tend to transfer from the Ni 4s state to oxidation product during photocatalytic water splitting. Herein,
the 2p states of neighboring N or even C atoms through the the reaction pathways of water oxidation into H O (pathway 1)
2
2
9
MLCT process.
2
and O (pathway 2) were theoretically investigated for PCNNi
The differential charge density was calculated to further (Fig. 6b). The energy calculations show that the free energy
conrm the MLCT process in the M–L unit under illumination. difference of 0.5554 eV for TS2.1 is lower than that of 0.8330 eV
As shown in Fig. 5f and S17,† upon photoexcitation, the charge for TS2.2, indicating that HO–OH species rather than O–O
density in the M–L unit is redistributed, with electron transfer species are formed as the intermediates during the water
from single-atom Ni to the adjacent C and N atoms, resulting in oxidation reaction. Furthermore, the absorption energy of H O2
2
electron depletion at single-atom Ni and electron accumulation (0.1911 eV) is lower than that of O (0.2115 eV), resulting in the
2
at C and N atoms. Therefore, the single-atom Ni builds a bridge easier desorption of H O from the surface of PCNNi. Hence,
2
2
for photoexcited electrons to transfer between the adjacent the oxidation reaction is likely to generate H
2 2 2
O rather than O .
heptazine-based polymer chains, accelerating the separation As shown in Fig. 6c–f, with the H O molecules absorbed at
2
and transfer of photoexcited electrons and holes generated in single-atom Ni and broken into H and OH (Fig. 6c), two OH
PCNNi. In contrast, such a charge transfer process is always remaining at the single-atom Ni (Fig. 6d) tend to form the TS of
interrupted by the hydrogen bonds in the pristine PCN (see HO–OH species (Fig. 6e). Subsequently, H
2 2
O is formed and
charge migration pathways in PCN in Fig. S1 in the ESI†). released from the single-atom Ni (Fig. 6f). For PCNNi with
3
Based on the above characterization and analysis, new single-atom Ni coordinated with sp N atoms (see the reaction
B
HOMO and LUMO are created by the M–L unit in the band pathway and detailed analysis in Fig. S18 in the ESI†), the
structure of PCNNi with the introduction of the single-atom Ni calculated results also show that the barrier energy of PCNNi is
terminating agent. Electron transition happens via the MLCT lower than that of pure PCN for the water reduction reaction.
process in the M–L unit, broadening the optical absorption in And, the water oxidation reaction prefers to generate H O
2 2
2
visible light. Meanwhile, the downshied LUMO of the M–L rather than O over PCNNi.
unit can steer the migration of photoexcited electrons from the
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Fig. 6 Reaction pathways of (a) water reduction to H
2
and (b) water oxidation to H
2
O
2
(top) and O
2
(down) for PCNNi (single-atom Ni coor-
2
dinated with sp NA atoms). (c–f) Proposed mechanisms of photocatalytic pure water splitting into H
2
and H
O
2 2
for PCNNi (single-atom Ni
2
coordinated with sp NA atoms).
notch Young Professionals, the “Fundamental Research
Funds for the Central Universities”, the Project Supported by
Conclusion
We successfully introduced single-atom Ni as a terminating the Natural Science Basic Research Plan in Shaanxi Province of
2
3
agent to coordinate with sp or sp N atoms in heptazine units China (2018JQ2028), the China Postdoctoral Science Founda-
in PCN (PCNNi) to realize efficient photocatalytic overall water tion (2018M640981), and “The Youth Innovation Team of
splitting into easily-separated gas-product H
product H
under visible light, without an additional supporting this work under contracts MoST 107-2112-M-032-
2
and liquid- Shaanxi Universities”. C.L.D is grateful to MoST for nancially
2 2
O
cocatalyst loading. It was experimentally revealed that new 004-MY3 and 108-2218-E-032-003-MY3.
HOMO and LUMO states were created in PCNNi by the
hybridization of single-atom Ni with N atoms in the heptazine
ring, resulting in an altered band structure and electronic
structure. The superior overall water splitting activity ach-
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