Electron Deficient Monomers that Optimize Nucleation and Enhance the Photocatalytic Redox Activity of Carbon Nitrides

Article abstract of DOI:10.1002/anie.201908322

Polymeric carbon nitride (PCN) is usually synthesized from nitrogen-rich monomers such as cyanamide, melamine, and urea, but is rather disordered in many cases. Now, a new allotrope of carbon nitride with internal heterostructures was obtained by co-condensation of very electron poor monomers (for example, 5-amino-tetrazole and nucleobases) in the presence of mild molten salts (for example, NaCl/KCl) to mediate the polymerization kinetics and thus modulate the local structure, charge carrier properties, and most importantly the HOMO and LUMO levels. Results reveal that the as-prepared NaK-PHI-A material shows excellent photo-redox activities because of a nanometric hetero-structure which enhances visible light absorption and promotes charge separation in the different domains.

Full text of DOI:10.1002/anie.201908322

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Accepted Article
Title: Electron Deficient Monomers Optimizes Nucleation and
Enhances Photocatalytic Redox Activity of Carbon Nitrides
Authors: Guigang Zhang, Minghui Liu, Tobias Heil, Spiros Zafeiratos,
Aleksandr Savateev, Markus Antonietti, and Xinchen Wang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201908322
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10.1002/anie.201908322
Angewandte Chemie International Edition
COMMUNICATION
Electron Deficient Monomers Optimizes Nucleation and Enhances
Photocatalytic Redox Activity of Carbon Nitrides
Guigang Zhang,* Minghui Liu, Tobias Heil, Spiros Zafeiratos, Aleksandr Savateev, Markus Antonietti
and Xinchen Wang*
Abstract: Polymeric carbon nitride (PCN) is usually synthesized
from nitrogen-rich monomers such as cyanamide, melamine and
urea, but is rather disordered in many cases. Here, a new allotrope
of carbon nitride with internal heterostructures was obtained by co-
condensation of very electron poor monomers (e.g., 5-amino-
some recent reviews.[16-17] Recent progress from our group and
other groups reveal that the use of suitable molten salts to
mediate the polymerization significantly decreases the number
of defects and improves the photo-catalytic performance.[16-21]
Molten salts (e.g., LiCl/KCl, NaCl/KCl) dissolve monomers and
tetrazole and nucleobases) in the presence of mild molten salts (e.g., intermediates and give a liquid medium for polycondensation,
NaCl/KCl) to mediate the polymerization kinetics and thus modulate
the local structure, charge carrier properties, and most importantly
the HOMO and LUMO levels. Results reveal that the as-prepared
NaK-PHI-A material shows excellent photo-redox activities because
thus improving the local order. Salt melt synthesis provides
further the opportunity to synthesize carbon nitride as new
allotropes and iso-forms, many coming with higher oxidation
potential. Recently, we were able to show that a more “noble”
carbon nitride with highly crystalline structure could be obtained
when electron deficient triazole/tetrazole based monomers were
polycondensed in the LiCl/KCl salt melts.[22-25] The as-prepared
polymers bear much better oxidation activity thanks to the
positively shifted HOMO (VB=2.5 V, vs. RHE). However, here
the simultaneous downshift of the LUMO (CB=-0.1 V, vs. RHE)
results in a weakening of the photocatalytic reduction activity.[24]
Interestingly, excellent reduction ability rather than oxidation
activity could be obtained when the tetrazole was polymerized in
an alternative NaCl/KCl salt melt.[26] It is thus exciting to search
rationally for other and even more active carbon nitrides, namely
more stable conjugated system with both strong oxidation power
and good reduction ability for photocatalytic redox reactions, and
here we use copolymerization with more electron deficient
monomers in diverse molten salts which will turn out to nucleate
different allotropes.
The systems are synthesized by copolymerization of
electron deficient aminotetrazole in NaCl/KCl molten salts, while
we use nucleobases as natural comonomers to introduce local
D-A junc-tions in spurious amounts (~0.3 wt. %). Interestingly,
already that little comonomer obviously changes the local
electron density, optical absorption, charge carrier transfer
process and photocaalytic performance of the as-prepared
heptazine based carbon nitrides.
More information about the materials synthesis is found in
experimental section (supporting information). In brief, the
melon-based carbon nitrides as references were synthesized by
thermal polymerization of the monomers dicyandiamide,
melamine and urea (denoted as CND, CNM, and CNU). NaK-
PHI-based carbon nitrides were obtained by copolymerization of
5-amino-tetrazole (1) without or with nucleobases (2-5) in salt
melts (denoted as NaK-PHI, NaK-PHI-A, NaK-PHI-G, NaK-PHI-
T, and NaK-PHI-C, respectively).
of
a nanometric hetero-structure which enhances visible light
absorption and promotes charge separation in the different domains.
The direct conversion of solar energy into storable chemical fuel
molecule (e.g, H2) via photocatalytic water splitting has been
regarded as a feasible way to decrease the strong dependence
on traditional fossil fuels.[1-3] The key of this technique however
is the development of low-cost, stable and highly efficient
photocatalysts, which are able to catalyze the desired reactions
in an efficient manner. Among all the investigated photocatalysts,
polymeric carbon nitride (PCN) has drawn particular interests
owing to its inherent advantages including being metal-free,
ease of fabrication, chemical and thermal robustness, and
visible light activity.[4-9] However, direct photocatalytic splitting of
water by PCN is rather challenging because PCN in its pristine
form features only moder-ate water oxidation activity.[10-12]
In principle, PCN is usually prepared by thermal
polymerization of carbon and nitrogen containing monomers
(e.g., cyanamide, melamine, urea, and many more) at elevated
o
temperatures (usually 550 C).[13-15] The polymerization process
is at later stages hindered by the sluggish deamination kinetics
and thus generates PCN with incomplete condensation and
lower local order; it is however potentially still strongly oxidizing
(VB=1.82 V, vs. RHE). The created overpotential is however not
enough for cocatalyst-free water oxidation, which is -in spite of
its apparent simplicity- a remarkably complicated process. PCN
however oxidizes many organic compounds, as described in
Dr. G. Zhang, Dr. T. Heil, Dr. A. Savateev, Prof. M. Antonietti
Department of Colloids Chemistry
Max Planck Institute of Colloids and Interfaces
14476 Potsdam (Germany)
E-mail: guigang.zhang@mpikg.mpg.de
M. Liu, Prof. X. Wang
State Key Laboratory of Photocatalysis on Energy and Environment,
College of Chemistry, Fuzhou University
Email:xcwang@fzu.edu.cn
Fuzhou, 350116 (China)
Dr. S. Zafeiratos
ICPEES, Institut de Chimie et des Proc8d8s pour l’Energie,
l’Environnementet la Sant8, UMR 7515 CNRS
Universit8 de Strasbourg 25 rue Becquerel
67087 Strasbourg cedex (France)
The as-synthesized poly(heptazine imide) (PHI)-based
carbon nitrides were firstly characterized by XRD, FT-IR, and
XPS. As seen from Figure S1, such carbon nitrides show only
one diffraction peak located 27.4o, corresponding to the graphitic
layer stacking distance of the units.[26-28] The lack of the typical
peak at 13o, which is related to the periodic in plane repeat of
heptazine units, and the broad diffraction peaks of PHI-based
already indicate a local rearrangement of the packing units,
which will get more clear in the TEM images later. FT-IR (Figure
S2) of the samples reveals that both sample sets of carbon
nitrides present similar breathing and stretching modes of the
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201908322
Angewandte Chemie International Edition
COMMUNICATION
heptazine units at 700-850 cm-1 and 1200-1600 cm-1,
respectively. For the PHI-based carbon nitrides, evident
variations at 987 cm-1 and 1065 cm-1 are observed, revealing the
similar to J-aggregates of dye molecules. As J-aggregated
usually allows a red shift of the absorption band and higher
thermodynamic stability, while H-packing then correspond to the
usual PHI–packing, we would have a simple observation of the
observed electronic and contrast features. Electron-hole
separation would then simply occur between the different stacks,
thus greatly simplifying local charge separation and charge
transport in each well aligned molecular columns (see local D-A
junction as depicted in Scheme S2). We also find new optical
absorption transitions beyond the intrinsic π-π* electron
transitions of the standard heptazine stacks. The role of 0.3%
nucleobases in this structure is still unclear to us and deserved
further analysis, but we have to assume that primary
condensation structures are able to nucleate such bipolar stacks,
which then simply continue to grow, i.e. it is the classical role of
a nucleation agent.[32, 33]
formation of the symmetric and asymmetric variations of imide
24, 29]
bound metal ions (here Na/K-NC2 groups).[22,
The weak
absorption band of surface uncondensed amino groups at 3300-
3500 cm-1 and the strong variations of surface cyano groups at
2177 cm-1 reveal the higher extension of polycondensation in the
presence of molten salts in comparison with the reference
without salts.[4, 19, 22] Raman spectra were conducted, but both
melon and NaK-PHI-A show quite similar triazine/heptazine
structures (Figure S3).
C-N=C
N1s
(b)
(a)
melon
NaK-PHI
NaK-PHI-A
N-C3
NaK-PHI-A
2.14 eV
2.03 eV
1.92 eV
NaK-PHI
(a)
0.8 eV
402
melon
404
400
398
396
394
4,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
BE / eV
BE / eV
Figure 1. (a) High resolution N1s XPS analysis and (b) UPS analysis of melon,
NaK-PHI and NaK-PHI-A.
XPS analysis provides more structural details of the
materials. Survey spectra (Figure S4) of the materials show that
melon comprises C, N, and O, while PHI-based carbon nitrides
are composed with C, N, O, Na and K. High resolution N 1s
analysis of the sample shows a clearly positive shift of the N-C3
species, indicating the stronger electron binding of a more stable
and extended conjugation of the PHI-based carbon nitrides, the
first indication of a more positive work function of the material.[30,
NaK-PHI-A
(b)
melon
N K-edge
(c)
K-L edge
σ*
C K-edge
C K-edge
π*
112.2 eV
σ*
N K-edge
π*
σ*
113.78 eV
area 2
π*
31]
σ*
The successful generation of carbon nitrides with enhanced
area 1
area 2
π*
“nobility” can be also proven by the positive shift of the valence
band positions, moving from 1.92 eV of melon to 2.14 eV of
NaK-PHI-A, as shown in UPS analysis (Figure 1b). This number,
in comparison with the previous reported values of PHI-K, is less
positive, but it keeps the strength for a successful photocatalytic
water reduction reaction, as certified by the slightly downshift in
LUMO (from -1.4 V of melon to -1.22 V of NaK-PHI-A, Mott-
Schotty plots of the materials are presented in Figure S5).
area 3
area 1
area 3
270
300
390
420
270
300
390
420
BE / eV
BE / eV
Figure 2. (a) Fourier filtered HR-TEM image of the selected area of NaK-PHI-
A; C-K edge and N-K edge EELS of (b) melon and (c) NaK-PHI-A.
The secret of this new structure nucleated from nucleobases
is revealed in AC-HRTEM, where we can observe domains with
distinct locally ordered structure, different from the weakly
ordered structure of melon (Figure S6). The lamellar repeat
period is unusually high (ca. 1.65 nm, d1+d2), which is of the
Interestingly, relevant electronic details of the materials
could be revealed by the C-K and N-K edges EELS of the
samples (Figure 2 b and c). It is clearly to observe that both
samples exhibit a typical carbon nitride structure comprising of
both π* and σ*-transitions, while however the σ*-transition of
carbon is much sharper in the new material than that of melon,
which potentially can be related to the extra C-T band also seen
in optical absorption. On top of that, the distance between the
carbon and the nitrogen EELS peaks is increased from 112.2 to
113.8 eV, and this positive change in the binding energy (~1.6
eV) is mostly due to stronger binding of elecrons also at the
nitrogen position, i.e. the overall electronic structure with its
positive change in the binding energy indicates a lower valanve
order of two heptazine units, i.e. we see
a periodic
superstructure, with the electron contrast presumably created by
the charge distribution within a D-A-structure. In some selected
parts of the picture, we even can see that the 1.65 nm repeat is
breaking up into two layer distances of ca. 0.67 nm and ca. 0.98
nm, (Figure 2a). The formation of new stacking distance of 0.67
nm (smaller than the traditional packing distance of about 0.92
nm in unmodified PHI) can indicates a tilted stacking between
neighboring heptazine units (tilt angle then around 40 °), very
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10.1002/anie.201908322
Angewandte Chemie International Edition
COMMUNICATION
band of a more stable polymorph.[24] This is in very good
accordance with the XPS and UPS data discussed above.
Another difference of the new and the reference carbon
nitrides is the surface morphology. SEM images of PHI-based
carbon nitrides (Figure S7) confirm the re-organization of the
crystals in the presence of salt melts. Especially with the
guanine containing samples we see in the surface texture well
facetted primary crystals of 20-50 nm in size, thus supporting
our nucleation hypothesis. These primary particles are however
inter-grown to a dense bulk, with planar gains boundaries as
depicted in Fig. 2a. BET analysis (Figure S9) of the samples
illustrate that the specific surface area of NaK-PHI-A is only 11
m2 g-1, which is slightly larger than that of weakly ordered melon
(e.g., CND, 5 m2 g-1) but much lower than that of a porous mpg-
CN (125 m2 g-1) because of the intergrowth of the primary
crystals. Besides, SEM elemental mapping (Figure S8) of the
NaK-PHI-A sample conform the even distribution of the C, N, Na,
and K elements.
lower PL emission intensity than melon, indicating suppressed
8, 35]
recombination of the radioactive charge carrier.[7,
Time
resolved PL (Figure S11) reveal that the mean radioactive
lifetimes (Aτ) of the recombining charge carriers of melon and
NaK-PHI-A were calculated to be 12.18 and 3.88 ns,
respectively. It is most likely the local heterostructure in the
crystalline polymer accelerates the charge transfer over the
interface.[36] The better charge carrier transfer can also been
revealed by transient photocurrent generation and the
electrochemical impedance spectroscopy (Figure 3c and d). A
steady and clearly enhanced photocurrent was generated for the
NaK-PHI-A. Meanwhile, the smaller EIS radius of the NaK-PHI-A
electrode than that of melon also suggests a faster charge
carrier transfer, which is beneficial to improve the photocatalytic
performance.[37]
(b)
A
(a)70
A
I-
I-
melon
H
G
H
P
I-
P
-
G
-
H
P
K
melon
I-
K
a
N
a
60
50
40
30
20
10
0
H
-
8
6
4
2
0
N
P
K
T
-
a
I-
T
I-
H
K
N
H
a
P
-
K
N
P
C
I-
-
PHI
a
K
a
N
H
P
-
K-
N
PHI
K-
Na
Na
K
a
C
I-
N
(a)
H
(b)
P
-
K
a
N
melon
NaK-PHI-A
melon
PL AQY=2.03 %
N
N
mpg-C
CND
CNM CNU
mpg-C
CND
CNM
CNU
melon
NaK-PHI-A
Substrates
Substrates
NaK-PHI-A
PL AQY=0.45 %
(d)
(c)
300
400
500
600
700
800
400
500
600
700
λ / nm
λ / nm
-0,4
-0,5
-0,6
-0,7
-0,8
-0,9
150
(c)
(d)
melon
100
melon
50
0
NaK-PHI-A
300
NaK-PHI-A
75
Figure 4. Photocatalytic activities of melon and PHI-baed carbon nitrides
under visible light irradiation for (a) H2 production and (b) CH4 production, (c)
wavelength-dependent H2 evolution of from water and ethanol mixture and (d)
time course H2 and CH4 production catalyzed by optimized NaK-PHI-A.
50
100
125
0
100
200
400
Re(Z) / KOhm
Time / s
Figure 3. (a) UV-Vis absorption spectra, (b) room temperature steady state
Photoluminescence spectra, (c) transient photocurrent under visible light
irradiation and (d) electrochemical impedance spectroscopy (EIS) of melon
and NaK-PHI-A.
The photocatalytic performance of the as-prepared melon
and PHI-based samples was then firstly evaluated by H2
production from water in the presence of Pt as cocatalysts and
triethanolamine (TEOA) as a sacrificial agent for oxidation. As
shown in Figure S11, all the PHI-based carbon nitrides exhibit
much better performances (178-342 µmol h-1) than the melon-
based counterparts, even much better than the mpg-CN (94
µmol h-1) with large surface area, indicating overall better
photochemistry for the complete reaction chain. As specific BET
The optimized D-A heterostructured carbon nitrides also
show significantly improved optical and electronic properties,
when compared to the melon-based counterparts. UV-Vis
spectra of both melon and PHI-based carbon nitrides in Figure
3a and Figure S10, but also sheer colour reflect the obvious
difference in optical absorption. The new system has a stronger
transition moment as a semiconductor, but also an extra CT
band partly covering the visible spectrum. The optical band gaps
of the reference and the new material are calculated to be 2.65
and 2.51 eV, respectively. This absorption, when attributed to a
sigma- Pi star transition, is forbidden for perfectly symmetric and
planar units,[32-34] but might be activated by the creation of the
new packing mode as described in HR-TEM. Another option is
the direct optical excitation of a D-A transition. Figure 3b depicts
the room-temperature photoluminescence (PL) spectra of the
materials. It is easy to observe that NaK-PHI-A possesses much
surface area of NaK-PHI-based carbon nitrides is only ~11 m2 g-
1
,
much lower than that of mpg-CN (125 m2 g-1), this
improvement is even more remarkable for a heterogeneous
photocatalyst.
Besides, the PHI-based carbon nitrides also show excellent
performance for photocatalytic H2 production by reforming of
another, more critical sacrificial agent, i.e. ethanol. From Figure
4a we can see that all melon-based carbon nitrides are inactive
for photocatalytic oxidation of ethanol, as evidenced by slow H2
production (lower than 5 µmol h-1). In contrast, all PHI-based
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10.1002/anie.201908322
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COMMUNICATION
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G. Z. thanks the Alexander von Humboldt Foundation for a
postdoctoral fellowship. This work is financially supported by the
Max Planck Institute of Colloids and Interfaces and the National
Natural Science Foundation of China (21761132002 and
21425309), the National Key R&D Program of China
(2018YFA0209301), and the 111 Project (D16008).
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Keywords: packing geometry • electron deficient monomers •
carbon nitride • water splitting • photocatalysis
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Copolymerization with
electron deficient monomers
in salt melt optimizes the
nucleation, local packing
mode, band structure and
optical properties of carbon
nitride polymers.
G. Zhang,* M. Liu, T. Heil, S.
Zafeiratos, A. Savateev, M.
Antonietti and X. Wang*
Page No. – Page No.
Electron Deficient Monomers
Optimizes Nucleation and
Enhances Photocatalytic
Redox Activity of Carbon
Nitrides
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