Synthesis of Layered Carbonitrides from Biotic Molecules for Photoredox Transformations

Article abstract of DOI:10.1002/anie.201702213

The construction of layered covalent carbon nitride polymers based on tri-s-triazine units has been achieved by using nucleobases (adenine, guanine, cytosine, thymine and uracil) and urea to establish a two-dimensional semiconducting structure that allows band-gap engineering applications. This biomolecule-derived binary carbon nitride polymer enables the generation of energized charge carrier with light-irradiation to induce photoredox reactions for stable hydrogen production and heterogeneous organosynthesis of C?O, C?C, C?N and N?N bonds, which may enrich discussion on chemical reactions in prebiotic conditions by taking account of the photoredox function of conjugated carbonitride semiconductors that have long been considered to be stable HCN-derived organic macromolecules in space.

Full text of DOI:10.1002/anie.201702213

Angewandte
Communications
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International Edition: DOI: 10.1002/anie.201702213
German Edition: DOI: 10.1002/ange.201702213
Photocatalysis
Synthesis of Layered Carbonitrides from Biotic Molecules for
Photoredox Transformations
Can Yang, Bo Wang, Linzhu Zhang, Ling Yin, and Xinchen Wang*
Abstract: The construction of layered covalent carbon nitride
polymers based on tri-s-triazine units has been achieved by
using nucleobases (adenine, guanine, cytosine, thymine and
uracil) and urea to establish a two-dimensional semiconduct-
ing structure that allows band-gap engineering applications.
This biomolecule-derived binary carbon nitride polymer
enables the generation of energized charge carrier with light-
irradiation to induce photoredox reactions for stable hydrogen
pounds such as cyanamide, dicyanamide, triazine and hepta-
zine derivatives. Further modifications on the chemical and
physical properties by doping and extending its electron
delocalization were achieved by using aromatic precursors
containing heteroatoms, including B, S, N and P. For example,
efficient g-CN photocatalysts have been prepared by the
polymerization of dicyandiamide with 2-aminobenzonitrile or
2,4-diaminoquinazoline,[12] and the condensation of urea with
barbituric acid, 2-aminothiophene-3-carbonitrile or diamino-
maleonitrile.[14] Considering the diverse array of organic
precursors with different chemical compositions and elec-
tronic structures, it practically allows the one-pot design of
polymer photocatalysts on a molecular level in a rational
manner, but also enables band-gap engineering, donor/
acceptor design, topological fabrication and modifications of
their physical properties such as p/n characteristics.[24]
À
À
production and heterogeneous organosynthesis of C O, C C,
À
À
C N and N N bonds, which may enrich discussion on
chemical reactions in prebiotic conditions by taking account
of the photoredox function of conjugated carbonitride semi-
conductors that have long been considered to be stable HCN-
derived organic macromolecules in space.
C
onjugated polymers both in their neutral and charged
states have gained great attention due to their promising
applications in electronic devices, such as light-emitting
diodes, lasers, photovoltaic cells, field-effect transistors, and
biology.[1–6] More recently, the applications have experienced
an extension to photoredox catalysis by acting as flexible light
transducers to enable the construction of soft photosynthetic
machinery/device by using easy solution and thermal pro-
cesses.[7–10] Current interest in the emerging field of organic
photocatalysis has focused on the molecular design and
modification of conjugated semiconductors, such as graphitic
carbon nitrides (g-CN) and covalent organic frameworks with
spatially extended p-bonding systems.[11–13] Being a stable
photocatalyst with a two-dimensional conjugated structure
and reduced exciton-binding energy,[14] g-CN has shown great
promise for water splitting,[15] CO2 reduction,[16] and organo-
synthesis,[17,18] which has triggered much research devoted to
organic photocatalysis for energy and environmental appli-
cations.
In contract to ordinary organic precursors, here we move
one step forward by applying biotic compounds to construct
g-CN semiconductors using nucleobases and urea, to answer
the question if g-CN can be produced from lifeꢀs molecules
(Scheme 1).[25] All these compounds are existent in biotic
Many synthetic strategies, including molten-salt
growth[19–21] templating[22] and (solvo)thermal condensation[23]
syntheses, have been applied to obtain g-CN nanostructures
with high performance. In particular, thermal condensation is
a facile and widely adopted approach to drive the construc-
tion and connection of triazine and heptazine tectons to poly-
conjugated systems, mostly using nitrogen-containing com-
Scheme 1. The chemical production of polymeric carbon nitride semi-
conductors from nucleobases and urea.
conditions as lifeꢀs building blocks from simple precursors in
the primordial soup.[26,27] We are therefore interested and
encouraged to enrich the studies on prebiotic (photo)chem-
istry, where HCN polymeric clusters (including triazine-based
super carbonitride tectons) have been considered as the most
readily formed and most stable organic macromolecules in
space.[28] The utilization of these biological precursors for the
chemical synthesis of g-CN-based semiconductors by ther-
mally induced condensation is representative of the extreme
terrestrial and thalassic thermal/hydrothermal conditions of
primordial Earth; for example the temperatures in hydro-
[*] C. Yang, B. Wang, L. Zhang, L. Yin, Prof. X. Wang
State Key Laboratory of Photocatalysis on Energy and Environment,
College of Chemistry, Fuzhou University
Fuzhou 350002 (China)
E-mail: xcwang@fzu.edu.cn
Homepage: http://wanglab.fzu.edu.cn
Supporting information for this article can be found under https://
doi.org/10.1002/anie.201702213.
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thermal vents close to the volcanic edifices have been found
to range from about 608C to 5008C. The potential existence of
photoactive g-CN semiconductors in primordial environ-
ments to induce photoredox transformations of water, CO2
and organics would enrich the discussion on the chemical
evolution of life. Furthermore, the study is also of relevance
for the artificial photosynthesis field.
Nucleobases [adenine (A), guanine (G), cytosine (C),
thymine (T) and uracil (U)] are readily available and stable
precursors. Surprisingly, according to our knowledge, there is
no report on the synthesis of carbon nitride from biomass-
derived building blocks. Here, we propose a facile synthesis of
carbon nitride based materials by thermal condensation of
1) urea with 2) A, G, C, T and U, respectively. We aim to
demonstrate that the incorporation of such building blocks
can lead to the development of carbon nitride for H2
production, and potentially to induce photoredox transfor-
mations under prebiotic conditions.
In a typical synthesis, a certain amount of nucleobase (0, 5,
15, 30, 50, 80 mg) and urea (10 g) were dissolved in water with
stirring, followed by heating at 808C to evaporate water.
Afterwards, the obtained mixture was calcined at 400–5508C
in air. The obtained samples are denoted as CNX, where X
represents the nucleobase. The reference sample from urea is
denoted as g-CN.
The chemical structure and composition of CNX samples
were characterized by X-ray diffraction (XRD) (Figure S2a in
the Supporting Information) and Fourier transform infrared
spectroscopy (FT-IR) (Figures S2b and S3). The XRD
patterns of the samples in Figure S2a show a strong peak at
27.48 related to the (002) interlayer reflection of a layered
crystal, plus a weak reflection at
to urea-derived g-CN under visible light irradiation. More-
over, the photocatalyst synthesized from urea with cytosine
(CNC) exhibits the best activity for producing hydrogen
among all these samples. The superior photocatalytic perfor-
mance of CNC is probably due to the better interaction and
structural matching between cytosine and urea. Since one
cytosine molecule could break into two molecules with the
similar structure to urea under thermal treatment, the
chemical structure of cytosine enables closer combination
with urea and better conjugation into the covalent carbon
nitride frameworks. On the contrary, the polymerization of
urea with other nucleobases with more functional groups is
less straightforward due to steric hindrance. Therefore,
cytosine, which could polymerize with the urea best, was
selected as the monomer to synthesize modified carbon
nitride polymers with high efficiency for photocatalytic
reactions.
A series of photocatalysts were prepared from mixtures of
urea and different amounts of cytosine. These materials are
denoted as CNCy, where y stands for the amount of cytosine
(y mg). As shown in Figure 1b and Figure S3 the XRD
pattern and FT-IR spectrum of CNCy, respectively, are very
similar to those of urea-derived g-CN, which demonstrates
that the graphitic carbon nitride based structure is maintained
with increasing amount of cytosine. The surface morphology
and texture of the CNC30 sample (after Pt deposition) were
investigated by scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) (Figure 1a). In the
TEM image smooth, flat layers can be seen, and the Pt
particles were distributed uniformly on the surface after
reaction (Figure 1a). There was no obvious difference
138 due to the in-plane repeating
unit of heptazine. The FT-IR spec-
trum in Figure S2b features distinct
peaks from 1200 to 1600 cmÀ1 cor-
responding to the stretch modes of
aromatic CN heterocycles, while
the breathing mode of the triazine
units corresponds to 810 cmÀ1. The
broad peaks between 3500 and
3100 cmÀ1 originate from N H
À
stretches on the surface of the
carbon nitride due to the surface
defective sites as a result of incom-
plete condensation. These results
clearly confirmed that all of the
CNX materials are featuring sim-
ilar crystal and chemical structure
of graphitic carbon nitride.
Then, we investigated the pho-
tocatalytic activities of CNX sam-
ples in water splitting and revealed
the effect of different nucleobase
monomers on the photocatalytic
performances. As shown in Fig-
ure S4, the hydrogen evolution
rates (HER) of CNX materials
increase by 6–8 times as compared
Figure 1. a) TEM of Pt@CNC30 after reaction. Inset: HR-TEM of Pt nanoparticles. b) XRD patterns of
different CNC samples. c) XPS analysis of CNC30 sample. d) Solid-state 13C NMR spectrum of CNC30.
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between the morphologies of the
sample before and after reaction.
During the process of photo-reduc-
tion, the Pt nanoparticles were dis-
persed on the surface of g-C3N4, and
the size distribution of Pt NPs
ranged from 3 to 5 nm (Figure 1a,
inset), the lattice distance of Pt NPs
is 2.28 ꢁ.
The solid-state 13C NMR spec-
tra of CNC30 (Figure 1d) shows two
peaks, the first peak at 164.3 ppm is
ascribed to the C(e) atoms [CN2-
(NHx)], whereas the second one at
155.6 ppm is attributed to the C(i)
atoms of melem (CN3). These sig-
nals confirm the existence of poly(-
tri-s-triazine) structure in CNC30. In
the XPS survey spectrum (Fig-
ure 1c and S5b–d), there are three
elements (C, N and O), similar to
those of g-CN. The O1s peak pres-
ent in CNC30 is due to the surface-
absorbed H2O or CO2, as cross-
checked by the FT-IR analysis
(Figure 1). By increasing the reso-
lution of XPS analysis, we observed
Figure 2. a) UV/Vis DRS spectra. Inset: photograph of the samples. b) Photoluminescence spectra
under 400 nm excitation. c) EPR spectra of CNC30 in the dark (dashed) or with visible light (solid), g-
CN as a reference. d) EIS Nyquist plots. Inset: photocurrent of g-CN and CNC30, on/off photocurrent
two main carbon species in the C1s response at À0.2 V bias potential vs. Ag/AgCl in 0.2m Na2SO4 solution.
spectrum (Figure S5). One carbon
species with a binding energy (BE)
2
À =
of 287.9 eV is identified as sp -bound carbon (C C C), the
other with BE of 284.6 eV was due to carbon impurities. The
N1s XPS spectrum can be deconvoluted into four peaks at
398.4, 399.6, 400.8, and 404.2 eV. The strongest N1s peak at
398.4 eV was assigned to sp2-bound N in N-containing
conjugated system (see Figure 2c dashed dot line), thus
resulting in an enhanced EPR signal in dark condition. When
the samples were irradiated with visible light, the signal could
be enhanced further (Figure 2c, solid lines).
The (photo)electrochemical properties of the CNC30
sample was examined by electrochemical impedance spec-
troscopy (EIS) and photocurrent test. A marked decrease of
Nyquist plots diameter for CNC30 is observed in Figure 2d,
which demonstrated that the electronic resistance of CNC30 is
smaller compared to pristine g-CN. At the same time, the
photocurrent of CNC30 increased by a factor of four compared
to g-CN (Figure 2d inset). This photocurrent enhancement
illustrates that the mobility of the photo-excited charge
carriers is promoted.
The samples were evaluated in a photocatalytic hydrogen
evolution assay by loading 3 wt.% Pt as co-catalyst and using
triethanolamine (TEOA) as a hole scavenger. Figure 3a
shows that all CNC samples exhibit better catalytic activity
in hydrogen evolution than the pure g-CN. Notably, the
CNC30 sample shows the highest activity for hydrogen
production. With increasing amount (from 5 mg to 30 mg)
of cytosine in the CNC samples, HER becomes gradually
faster. The activity decreased when further increasing the
amount of cytosine to 80 mg (60 mmolhÀ1), but is still higher
than with pure g-CN (37 mmolhÀ1). With 30 mg cytosin an
optimum HER (282 mmolhÀ1) is achieved, which is nearly 8
times faster than with pure g-CN.
À =
aromatic rings (C N C), whereas the weak peak at
399.6 eV is attributed to the tertiary nitrogen N-(C)3 groups.
The peak at 400.7 eV indicated the presence of amino groups
À À
(C N H) and the peak at 404.2 eV was attributed to charging
effects or positive charge localization in heterocycles.
Figure 2a displays the UV/Vis diffuse reflectance spectra
(DRS) of the sample. The optical absorption is red-shifted
from 460 nm for g-CN to 470 nm for CNC30 and finally to
480 nm for CNC80, corresponding to the change of sample
color from pale yellow to deep yellow and to orange. These
changes are attributed to the combination of cytosine
monomer in the CN that effectively broadens the conjugated
p-electron system and thus narrows the semiconductor band
gap, which is also reflected by PL analysis (Figure 2b)
showing gradually red-shift of the emission peaks with
increasing amount of cytosine added. The effect of cytosine
addition on the band structure (valence band and conduction
band level) is shown in Figure S6.
With gradual integration of cytosine into the CN network,
electron paramagnetic resonance (EPR) intensity increases
progressively, due to a well-evolved electronic structure with
extended delocalization of the p-conjugated system (Fig-
ure S7). As expected, the generation of photochemical radical
pairs can be efficiently promoted by this extended p-
The photocatalytic performance of CNC30 is well consis-
tent with its optical absorption (Figure 3b), and the amount of
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(32.3 mmol) and H2 (8.02 mmol), with a selectivity for CO
production of 80.1% (entry 1 in Table S1). CNC30 gives the
highest yield and selectivity for CO production, and there is
no detectable production of CO in the absence of either
CNC30 or light (entries 2 and 3 in Table S1). The evolution of
CO was not observed when CO2 was replaced with Ar gas
(entry 6 in Table S1), which together with the isotopic experi-
ments (Figure S14) confirmed that the source of CO is CO2
and not organic sources present in the system.
Besides H2O and CO2, the process of photochemical
evolution involves a series of organic reactions. We therefore
checked the potential capability of the synthetic g-CN in
photocatalyzing the redox transformation of organic mole-
À
À
cules, including amines, alcohols and the coupling of C C, C
À
À
O, C N and N N bonds (Table 1). As shown in entry 1,
Table 1: Different organic reactions driven by the CNC30 photocatalyst.
Entry
1[a]
Reaction
Conv. [%]
79
Sel. [%]
98
2[b]
19.8
96.5
3[c]
4[d]
5[e]
56
94
98
90
99.3
61.4
Figure 3. Photocatalytic hydrogen evolution activity of the samples.
a) The effect of cytosine amount on HER. b) Wavelength dependence
of the HER with CNC30 loaded with 3 wt.% Pt. Inset: time-dependence
of the HER with CNC30 at different irradiation wavelengths.
Reaction conditions: [a] Substrate (1 mmol), catalyst (50 mg), CH3CN
(10 mL) as solvent, 808C, O2 (1 MPa). [b] Substrate (1 mmol), catalyst
(50 mg), CH3CN (4 mL) as solvent, 1608C, O2 (1 MPa), 24 h. [c] Sub-
strate (0.1 mmol), catalyst (5 mg), trifluorotoluene (1.5 mL) as solvent,
608C, O2 (1 MPa), 3 h under visible light irradiation. [d] Iodobenzene
(0.15 mmol) and benzeneboronic acid (0.6 mmol), catalyst (10 mg with
3 wt.% Pd), 2.5 mL of water and 2.5 mL of ethanol, K2CO3 (1 mmol),
308C, 2 h under visible light irradiation. [e] 30 Vol.% substrate aqueous
solution, catalyst (100 mg), 1 wt.% Pt, 24 h under UV/vis light
illumination. All products were detected by GC-MS.
H2 increased linearly with time under different wavelengths
(Figure 3b, inset), suggesting that the main driving force of
the photocatalytic reaction is the harvested visible photons.
The stability of CNC30 was examined by operating the
experiments under the same reaction conditions for several
runs (Figure S8). Except for the first run, the reaction was
treated without light for one hour to ensure there was no H2
gas in the reaction system. A slight deactivation was noticed
in the first four runs. When an appropriate amount of TEOA
was added to the reaction solution, the activity of H2
evolution improved in the fifth run. This indicates that the
decrease in activity after the first run is mainly due to the
decreased concentration of TEOA. It is noted that no obvious
structural changes were observed (Figure S9–S10).
Next, the apparent quantum yield (AQY) of the best
sample was examined by studying catalytic kinetics using
different amount of photocalalysts. Figure S11 shows that
AQY initially increases with increasing amounts of CNC30,
then reaches a plateau at a maximum value of 7.1% before
decreasing slightly upon further increasing the amount of
catalyst. We therefore applied 75–100 mg sample to check the
intrinsic AQY where the reaction is limited by charge
separation at the interface to rule out mass diffusion effects.
We further studied the photocatalytic activity of CNC30 in
CO2 reduction (Figure S12–S13), using Co(bpy)3Cl2 as redox
mediator and TEOA as electron donor under visible light (l >
420 nm). CNC30 photocatalyzed the generation of CO
amines can be photooxidized into imines by g-CN under
oxygen atmosphere, which are regarded as important electro-
philic intermediates in organic synthesis. The conversion is
3
À
79% and the selectivity 98%. Activation of sp C H bonds
can also be achieved, for instance, in entry 2, the CH2 group
=
À
was oxygenated to C O selectively. The C O bond in benzyl
=
alcohol could be oxidized to C O (entry 3). In addition, as
shown in entries 4 and 5, the synthesized g-CN is also an
active photocatalyst for the coupling of aromatic halides and
the cross-linking of ketone and alcohol. Of particular note,
the reactions in entries 4 and 5 are both anaerobic reactions
under light-irradiation, suggesting a link to the evolution of
algae for photosynthesis. All reactions mentioned above
proceed with excellent selectivity and yield, which proves that
heterogeneous photocatalysis offers a promising route to
realize green organic syntheses under solar irradiation in
ambient conditions.
In conclusion, we have demonstrated a biotic precursor
approach to manipulate the texture, surface and optical
properties of g-CNX polymers synthesized from urea and
nucleobases. By optimizing the synthetic recipe, hydrogen
evolution reaction under visible light (l > 420 nm) reached
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282 mmolhÀ1 with an apparent quantum yield of 7.1%, which
is 8-fold higher than with g-CN produced from urea alone.
This polymeric catalysts are promising for water splitting, by
coupling with other semiconductors and deposition of an
appropriate co-catalyst. The diversity of biomass molecules
and the flexibility of bottom-up synthesis will enable the
rational development of efficient polymeric light-harvesting
transducers, which could potentially have emerged in nature
by thermal assembly of lifeꢀs building blocks from simple
compounds. This work may enrich the discussion on chemical
reactions under prebiotic conditions by considering a role of
naturally occurring carbonitride semiconductors. This study
could also extend g-CN photocatalysis to redox transforma-
tions of organic molecules including amines, alcohols and the
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Acknowledgements
This work was financially supported by the National Basic
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Natural Science Foundation of China (21425309 and
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Conflict of interest
The authors declare no conflict of interest.
Keywords: carbon nitride semiconductors · chemical evolution ·
nucleobases · photocatalysis · urea
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Manuscript received: March 1, 2017
Revised: March 27, 2017
Final Article published: && &&, &&&&
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Communications
Photocatalysis
A prebiotic photocatalyst? Graphitic
carbon nitrides synthesized from nucleo-
bases and urea enable the photogenera-
tion of charge carriers to induce photo-
redox reactions for H2 production and
C. Yang, B. Wang, L. Zhang, L. Yin,
X. Wang*
&&&& — &&&&
À
Synthesis of Layered Carbonitrides from
Biotic Molecules for Photoredox
Transformations
heterogeneous organosynthesis of C O,
À
À
À
C C, C N and N N bonds. The findings
enrich the concept of photochemical
reactions in the primordial soup.
6
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