Conjugated electron donor-acceptor hybrid polymeric carbon nitride as a photocatalyst for CO2 reduction

Article abstract of DOI:10.3390/molecules24091779

This work incorporates a variety of conjugated donor-acceptor (DA) co-monomers such as 2,6-diaminopurine (DP) into the structure of a polymeric carbon nitride (PCN) backbone using a unique nanostructure co-polymerization strategy and examines its photocatalytic activity performance in the field of photocatalytic CO₂ reduction to CO and H₂ under visible light irradiation. The as-synthesized samples were successfully analyzed using different characterization methods to explain their electronic and optical properties, crystal phase, microstructure, and their morphology that influenced the performance due to the interactions between the PCN and the DPco-monomer. Based on the density functional theory (DFT) calculation result, pure PCN and CNU-DP15.0 trimers (interpreted as incorporation of the co-monomer at two different positions) were extensively evaluated and exhibited remarkable structural optimization without the inclusion of any symmetry constraints (the non-modified sample derived from urea, named as CNU), and their optical and electronic properties were also manipulated to control occupation of their respective highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Also, co-polymerization of the donor-acceptor 2,6-diamino-purine co-monomer with PCN influenced the chemical affinities, polarities, and acid-base functions of the PCN, remarkably enhancing the photocatalytic activity for the production of CO and H₂ from CO₂ by 15.02-fold compared than that of the parental CNU, while also improving the selectivity.

Full text of DOI:10.3390/molecules24091779

molecules
Article
Conjugated Electron Donor–Acceptor Hybrid
Polymeric Carbon Nitride as a Photocatalyst for
CO2 Reduction
Asif Hayat 2, Mati Ur Rahman 2, Iltaf Khan 3, Javid Khan 4 , Muhammad Sohail 5,
Humaira Yasmeen 6, Shu-yuan Liu 7,8,*, Kezhen Qi 1,
*
and Wenxiu Lv 1
1
Institute of Catalysis for Energy and Environment, College of Chemistry and Chemical Engineering,
Shenyang Normal University, Shenyang 110034, China; 15164052089@163.com
College of Chemistry, Fuzhou University, Fuzhou 350002, China; asifncp11@yahoo.com (A.H.);
matiurrahman617@yahoo.com (M.U.R.)
Key Laboratory of Functional Inorganic Material Chemistry, School of Chemistry and Materials Science,
Heilongjiang University, Harbin 158308, China; iltafkhanpakistan@gmail.com
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, Key Laboratory of Environment and Energy
Chemistry of Guangdong Higher Education Institutes, School of Chemistry, Sun Yat-Sen University,
Guangzhou 510275, China; javidchemist@yahoo.com
Institute for Advanced Study, Shenzhen University, Shenzhen 518060, China; sohailncp@gmail.com
Key Laboratory of Bio-Based Material Science and Technology, Ministry of Education, Northeast Forestry
University, Harbin 150040, China; humairanefu@yahoo.com
Department of pharmacology, Shenyang Medical College, Shenyang 110034, China
Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Physics and
Electronic Engineering, Harbin Normal University, Harbin 150025, China
2
3
4
5
6
7
8
*
Correspondence: liushuyuan@symc.edu.cn (S.-y.L.); qkzh2003@aliyun.com (K.Q.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Alireza Khataee, Mahdie Safarpour and Sang Woo Joo
Received: 18 April 2019; Accepted: 5 May 2019; Published: 8 May 2019
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: This work incorporates a variety of conjugated donor-acceptor (DA) co-monomers such as
2,6-diaminopurine (DP) into the structure of a polymeric carbon nitride (PCN) backbone using a unique
nanostructure co-polymerization strategy and examines its photocatalytic activity performance in the
field of photocatalytic CO2 reduction to CO and H2 under visible light irradiation. The as-synthesized
samples were successfully analyzed using different characterization methods to explain their electronic
and optical properties, crystal phase, microstructure, and their morphology that influenced the
performance due to the interactions between the PCN and the DPco-monomer. Based on the
density functional theory (DFT) calculation result, pure PCN and CNU-DP15.0 trimers (interpreted
as incorporation of the co-monomer at two different positions) were extensively evaluated and
exhibited remarkable structural optimization without the inclusion of any symmetry constraints
(the non-modified sample derived from urea, named as CNU), and their optical and electronic
properties were also manipulated to control occupation of their respective highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Also, co-polymerization of
the donor–acceptor 2,6-diamino-purine co-monomer with PCN influenced the chemical affinities,
polarities, and acid–base functions of the PCN, remarkably enhancing the photocatalytic activity
for the production of CO and H2 from CO2 by 15.02-fold compared than that of the parental CNU,
while also improving the selectivity.
Keywords: polymeric carbon nitride; donor-acceptor; photocatalysis; CO2 reduction; DFT calculation
Molecules 2019, 24, 1779; doi:10.3390/molecules24091779
www.mdpi.com/journal/molecules

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1. Introduction
The excessive use of fossil fuels and the increasing concentrations of the resulting pollutants exert
a negative impact on human life, the environment and energy resources in the form of global warming.
Intensive endeavors are ongoing to meet the challenges of increasing future demand for fuels and the
problems caused by their utilization and need for conservation due to excess pressure on the world
energy infrastructure [1–3]. Enormous efforts have been fueled by scientists to provide sustainable
energy sources, such as solar cells, fuel cells, and biofuels, that are can meet the future demand for
competitive energy. Importantly, the energy supplied by these energy sources is often unpredictable,
and solar cells have been demonstrated to be the most reliable after numerous endeavors [4,5]. The solar
energy coming from the sun has enormous potential and is considered a possibly optimal solution to
the problem of fossil fuel depletion, but solar energy cannot be used directly, and it must be combined
with a carbon-neutral energy source which is a challenge as the system will be solar energy driven,
which is an intermittent process. One technology for solving this global issue is the conversion of solar
energy into chemical energy that can be stored for later use.
In this regard, the use of conjugated polymer semiconductors is of great current interest as it
can economically convert sunlight into chemical energy using different compounds [
other, more common semiconductors [ ]. The dominant factor that optimizes the energy conversion in
solar cells driven by conjugated polymer semiconductors is the electron–hole recombination process,
i.e., exciton decay [ 10]. It is possible to manipulate some multifunctional donor–acceptor based
6,7], contrary to
8
9
,
conjugated materials so they viably provide the necessary charge carrier separation under solar
irradiation [11]. These new intramolecular DA copolymer materials are used to absorb the sunlight
of an appropriate frequency that can cover the whole wavelength spectrum and provide suitable
electron charge mobility [12,13]. In light of this concept, continuous endeavors have been made toward
discovering nanostructured conjugated DA systems that are able to utilize the fascinating photocatalytic
diversity of organic and inorganic semiconductors under visible light illumination. Concomitantly,
several varieties of photocatalyst-based semiconductors are widely utilized in the photocatalytic
process and among them, polymeric carbon nitride (PCN) has been identified as an energized stable
candidate for the corresponding evolution of hydrogen energy from water systems [14
rapidly moving towards practical use due to its easy synthesis, abundance in Nature, low price,
overall flexibility, high yielding manufacturing process, and its stability [16 17]. Numerous negative
,15]. PCN is
,
properties, such as a small surface area, high charge recombination rate, and small light absorption
capability hinder further exploitation of the parental PCN and predict a negative photocatalytic
performance [18
to modifying PCN networks, e.g., heterojunctions [20], noble metal deposition [21], doping [22
and morphological control [24 25], that have significantly improved its photocatalytic activity. Inspired
,19]. Therefore, many scientists have explored a variety of fundamental approaches
,
23],
,
by these developments, co-polymerization has come forth as a new prominent technique to accomplish
this by using new energized organic conjugated co-monomers embedded within the framework of
PCN to enhance its photocatalytic properties. The introduction of these organic motifs within the PCN
structure via the process of co-polymerization optimizes the easy accumulation of the photogenerated
electrons and accelerates their transport, hence fueling improved photocatalytic activity under visible
light irradiation, as compared to the original PCN [26]. This co-polymerization grafts the given
organic motifs within the skeleton of PCN using a facile one-step condensation process, and after the
copolymerization of the PCN, the products demonstrate a high photocatalytic stability and performance.
In light of this concept, Nie et al. created Z-scheme g-C3N4/ZnO microspheres for photocatalytic
CO2 reduction [27
,28]. Hayat et al. investigated use of a conjugated co-monomer, i.e., trimesic acid,
within a PCN framework, which was successfully utilized for the photocatalytic CO2 reduction [29
,
30].
Intramolecular DA co-monomers are of great interest and are recognized as energized candidates that
encroach within the triazine subunit of PCN to display their intramolecular donor–acceptor properties
and enhance the photocatalytic performance of PCN [31,32].

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This work utilized the aromatic electron–donor monomer 2,6-diaminopurine within the PCN
framework to improve the photocatalytic reduction of CO2 into CO and H2 under visible light
illumination. The installation of diamino aromatic moieties on the purine monomer was accomplished
by effecting a nucleophilic substitution in the inner three s-triazine units of PCN thus building a great
intramolecular donor-acceptor polymer for photocatalytic CO2 reduction as depicted in Scheme 1.
More importantly, the monomer selected here forms heterocyclic pyrimidine and imidazole ring
structures that correspondingly act as a backbone modifier [33,34]. The co-polymerization was repeated
using appropriate amounts of different substances belonging to the same class of monomer produces
such as 2,6-dihydroxypurine (xanthine), trimethylxanthine (caffeine) and methylxanthine (theobromine)
in within the PCN skeleton to compare their photocatalytic CO2 reduction with the original samples.
In short, we added different amounts of the electron donor-acceptor co-monomer 2,6-diaminopurine
into the skeleton of urea-containing PCN by a facile one-pot thermo-induced co-condensation method
and studied the products’ versatile application in photocatalytic CO2 reduction for the first time. It has
been discovered that under visible light irradiation the as-synthesized samples exhibit an outstanding
photocatalytic activity for the reduction of CO2 into CO and H2, about 16-fold up higher than that of the
parental PCN. The density functional theory (DFT) calculations, different analytical techniques applied
and the heterogeneous catalysis applications catalyzed by our samples described in this manuscript are
expected to provide fruitful guidance for the fabrication of novel energetic polymeric photocatalysts.
Scheme 1. Possible reaction path for the integration of 2,6-diaminopurine into PCN networks.
2. Results and Discussion
The as-synthesized samples, i.e., CNU and co-polymerized CNU-DP15.0, were analyzed using
DFT calculations to achieve a better visualization of their electronic structures. Along with this
time-dependent DFT (TD-DFT) studies were conducted for the geometrical calculation of excited

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and ground states at the level of the Beck three-parameter hybrid functional (B3LYP)/6-31G (d, p)
as implemented in Gaussian 09. As shown in Figure 1, the electronic distributions in the lowest
unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy state
of CNU and CNU-DP15.0 were elucidated (Figure S1). A high kinetic stability is indicated by the
high HOMO-LUMO energy gap (∆Eg = 3.65 eV). Table S1 depicts the electronic excitation transition
parameters of the CNU and CNU-DP15.0 samples and therefore the CNU-DP15.0 molecules possess
high oscillator strength and dipole moment which emphasize their strong intermolecular electron
push and full effect. The electronic excitation transition parameter ∆EST (S1-T1 = 0.39) change of
the CNU-DP15.0 was also predicted using the B3LYP set and the results are summarized in Table S2.
Table S3 lists the calculated values of Mulliken Atomic Charges (a.u.) for CNU-DP15.0 and it can be
seen that the original sample attains a smaller band gap after the introduction of the monomer.
Figure 1. Electronic structure of polymeric trimer models including the optimized HOMO and LUMO
for CNU trimer (a) and DP-doped CNU trimer (b), corresponding HOMO–LUMO gap (c).
The as-synthesized samples i.e., pristine CNU and co-polymerized CNU-DPx samples were
next subjected to X-ray diffraction pattern determination as depicted in Figure 2a. The final XRD
characterization for all of the samples reflected no noticeable alterations and displayed the two main
characteristic peaks at 27.5◦ and 13.1◦ assigned to the (002) and (100) planes of the crystal structure.
The peak at 27.5◦ of the crystal structure face is a strong, broad and intense peak corresponding to the
inter-planar stacking of aromatic systems [35
,
36]. All samples show these peaks and the aforementioned
peak 27.5◦ peak due to disturbance of graphitic motifs resulting from the co-polymerization [37
,38].
The peak displayed by all samples at 13.1◦ is a weak peak indexed to the (100) crystal plane
which demonstrates the in-plane structural packing motif and the inter-planar stacking of aromatic
systems [39,40]. It is noticeable that there is a small distortion in the XRD peak intensities in the range
of ~15 to 25◦ that gradually increases with the increasing concentration of DP co-monomer in the
CNU structure.

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Figure 2. (a) XRD patterns and (b) FTIR spectra of CNU and CNU-DPx samples.
FT-IR spectra were recorded to verify the entire core chemical skeleton of PCN before and after
the co-polymerization of DP co-monomer with the PCN. Multiple peaks were identified by analyzing
the spectra illustrated in Figure 2b. The peaks located in the range between 2900 to 3300 cm−1 are
broad peaks ascribed to the adsorbed H2O molecules and N-H vibrations originated from the partial
condensation [41]. The peaks in the range between 1100–1800 cm−1 correspond to the typical stretching
vibration modes of C-N and C=N heterocycles [42,
43]. The peak appearing at 810 cm−1 originates from
the significant bending vibration of the s-triazine motifs [44]. The characteristic FTIR peaks for the
co-polymerized samples display a slight shift of the 810 cm−1 peak and therefore suggest a red shift
due to the extra carbon content. The FTIR results reveal that all the samples display almost similar
peaks and positions before and after the co-polymerization of DP co-monomer in the framework of
PCN and hence no major alteration was revealed, hence confirming the successful deconvolution of
the co-polymerization process.
The pristine CNU and co-monomer polymerized CNU-DPx samples were evaluated by N2
adsorption-desorption isotherms in order to detect their specific surface area, pore volume and
aggregate porosity (Figure 3a). All the samples were subjected to sorption analysis using nitrogen gas
as a sorbate and intrinsically a classic type IV isotherm was obtained, hence confirming the existence
of mesoporous properties within all samples. After calculation, the pristine CNU sample gives a
specific surface area of 39.89 m2g−1 and this surface area increases abruptly after the addtion of DCQ
co-monomer in the CNU skeleton. Thereafter, the integration of various amounts of DP within the
CNU sample result in an excellent surface area and thus the samples, i.e., CNU-DP5.0, CNU-DP10.0
CNU-DP15.0 and CNU-DP20.0 give surface areas of 131.98, 163.64, 214.53 and 202.2 m2g−1, respectively,
as listed in Table S4.
,
Figure 3.
(a) N2 adsorption-desorption isotherms (77 K), (inset) pore size distribution graph, and (b)
CO2 adsorption isotherms (273 K) for CNU and CNU-DP15.0 samples.

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More excitingly, the inset of Figure 3a presents the corresponding BJH pore size distribution of the
samples and highlights a notable difference between the pure CNU and co-polymerized CNU-DPx
samples. The N2 adsorption-desorption isotherms and their corresponding BJH pore size distribution,
pore size and volume of other co-polymerized samples are illustrated in Figure S2 and Table S4.
The isotherms of simulated carbon dioxide adsorption of the parental CNU vs. CNU-DP15.0 were
analyzed at 298 K and 1 bar atm. pressure, respectively. Here the simulated isotherms demonstrate
a small overestimation due to some unremoved solvents. Interesting our CO2 adsorption isotherm
indicates a type V behavior calculated at 273 K, as depicted in Figure 3b. Similarly the deducted
isotherm showed a variable shape that was convoluted due to the pore size of samples and also
manifested the electrostatic interactions of CO2 within the large-sized pores of the samples. The figure
shows the CO2 adsorption isotherms as a comparison for the validation of the adsorption capabilities
of CNU and CNU-DP15.0. In brief, the best CNU-DP15.0 sample displayed an extreme adsorption
plot as compared of CNU and therefore manifests improved phenomena in its molecular sites which
optimize the charge carrier probability during CO2 photofixation reactions.
XPS analysis was conducted to investigate the chemical status and elemental composition of the
samples and also mirrored the effect before and after the co-polymerization of 2,6-diaminopurine within
CNU. The XPS survey spectra demonstrate that C, N, O are present in all of the samples while a small
amount of S was only detected in the co-polymerized CNU-DP15.0 sample as shown in Figure 4a,b.
The high resolution XPS spectra of O assigned at 531 eV are presumed due to the presence of moisture
in the atmosphere are depicted in Figure S3. The high resolution XPS spectra of S are interpreted as
due to the incorporation of 2,6-diaminopurine co-monomer within the CNU as illustrated in Figure S4.
Moreover the high resolution XPS spectra of C1 s for both CNU and CNU-DP15.0 show two distinct
peaks and the apparent chemical shift in the binding energies (C, N) for both samples is depicted in
Figure 4c,d. The peaks assigned at 284.6
the aromatic carbon atoms and the sp2 hybridized adventitious carbon (N-C=N and C-NH2) in the
skeleton of PCN [45 46]. The high resolution N 1s XPS spectra for parental CNU and co-polymerized
±
0.1 and 287.8
±
0.1 eV for both samples are associated to
,
CNU-DP15.0 summarized in Figure 4e,f, demonstrated four peaks at different positions, i.e., at 398.5,
399.5, 401.2 and 404.09 eV, attributed to sp2-hybridized nitrogen from C-N=C bonds, the nitrogen within
the tri s-triazine rings e.g., N-(C)3 group, terminal uncondensed C-N-H amino groups with implication
of the charge diffused in the heterocyclic cage [47]. The atomic percentages of the elements derived
from the XPS analysis are listed in Table S5. Meanwhile the elemental analysis of the samples was
performed in order to confirm the carbon and nitrogen composition before and after the grafting of the
aromatic 2,6-diaminopurine rings within the pristine CNU. The parental CNU manifested the calculated
C/N molar ratio of 0.63 and this value successively increased after the addition of 2,6-diaminopurine
co-monomer to the framework of PCN (Table S4). All of the co-polymerized samples display boosted
C/N molar ratios and the best sample, i.e., CNU-DP15.0, capable of undergoing a more energized
photocatalytic process, possesses a prodigious C/N molar ratio of 0.68.
Solid-state 13C-NMR spectra were used to investigate the extra carbon added to the structure of
CNU upon the co-polymerization of DP co-monomer. The solid-state 13C-NMR spectra manifested the
expected differences between the spectra of the blank CNU sample versus co-polymerized CNU-DP15.0
as seen in Figure 5a. By optimizing the final resolution of the spectra, both of samples demonstrate
similar obvious characteristic peaks at position 157.5 and 165.8 ppm associated to the 3-s-triazine-based
PCN. A new weak peak appeared in the spectrum of CNU-DP15.0 photocatalyst as compared of
CNU, demonstrating the incorporation of extra new carbon due to the copolymerization of DP
co-monomer in the CNU framework. The electron paramagnetic resonance (EPR) spectra were
recorded for all as-synthesized samples in order to mirror their calculated band structure as illustrated
in Figure 5b. The result confirms that each catalyst has a single Lorentzian line tuning at position
3515G which increases during the co-polymerization process. The g-value of this single Lorentzian
line is calculated as 2.0034 which is a feasible configuration due to delocalization of the π-conjugated
system. Actually, the process of incorporation of co-monomer DP into the cage of CNU abruptly

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changes this
the photogenerated electron and hole-producing chain. This aforementioned process involves the
association of unpaired electrons with the -bonded nano-sized clusters on the CNU surface due to
carbon atoms [48 49]. Figure 5c shows the EPR signals in both the dark and visible region of pristine
π-conjugated system which results in an abundance of carbon that improves and regulate
π
,
CNU and the superior photocatalyst CNU-DP15.0. The final result leads to the conclusion that under
visible light irradiation the signal intensity increases greatly, producing more photogenerated carriers.
Figure 4.
(a,b) XPS survey spectrum, (c,d) high-resolution XPS spectra of C 1s, and (e,f) N 1s for CNU
and CNU-DP15.0
.
Figure 5.
(
a
) Solid-state 13C-NMR spectra, (
b
) EPR spectra of CNU and different amount of CNU-DPx
samples, and (
c) EPR spectra of CNU and CNU-DP15.0 in the dark and under visible light irradiation
(>420 nm).

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The intra structure integrity of the as-synthesized samples was further studied by conducting
typical SEM and TEM experiments as displayed in Figure 6. The morphology of pure CNU shows
a distorted irregular lamellar scale which implies crystallinity (Figure 6a) while this morphology is
converted into stacked agglomerated nanosheets of a few nanometers in size after the incorporation of
DP co-monomer in the skeleton of CNU samples, as manifested in Figure 6b. The TEM image of pure
CNU demonstrates a redundant isolated sheet which results in a diffused contrast level as displayed in
Figure 6c. The TEM morphology of the co-polymerized CNU-DP15.0 sample reflects a dense sheet in
the form of embedded belts which clearly show a strong degree of stacking as shown in Figure 6d.
Figure 6. (a,b) the SEM images and (c,d) TEM images of CNU and CNU-DPx samples.
Approaches such as UV–Vis DRS and photoluminescence (PL) spectroscopy were implemented
for the exploration of the physicochemical properties of pure CNU and the effect of 2,6-diamino-
purine co-monomer addition the urea skeleton in CNU. As displayed in Figure 7a, UV–Vis DRS
shows the absorption edge of pure CNU at ca. 450 nm confirming the intrinsic band structure
configuration. The absorption edge of the corresponding co-polymerized product, i.e., CNU-DPx
shows an enhanced net wavelength phenomenon as compared of pristine CNU. The UV–Vis DRS
spectra are also reflected by macroscopic color changes from pale yellow toward a deeper brown
hence confirming the enhancement of the visible light harvesting tendency of the inter-triazine
oligomers of CNU by the successful process of co-polymerization, as seen in the inset of Figure 7a.
More excitingly, the absorption optical edge of CNU was successfully red shifted after the addition of
2,6-diaminopurine co-monomer.
This process of co-polymerization of DP monomer within CNU therefore optimizes the consequent
transition of photogenerated electrons that move from the HOMO toward the LUMO driven by the
addition of carbon species. The average band gap of co-polymerized samples tends to decrease
compared with the parent CNU as a function of their optical absorption, as manifested by DRS and also
deduced from the DFT calculations. Figure 7b shows the PL spectra of pure CNU over co-polymerized
CNU-DPx samples in order to depict the excitation and separation of photo-generated electrons
and holes. The samples demonstrate fluorescence emission decay around 445 nm and therefore a
noticeable broad change in the PL emission intensity occurs in the co-polymerized CNU-DPx samples.
The ascribed alteration changes the PL emission toward longer wavelengths which justifies the existence
of a charge carrier recombination process that is enhanced due to addition of DP co-monomer within the
CNU framework by the supramolecular aggregation process. The enhancement in optical absorption

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hence lowers the charge recombination intensity and finally improves the electron photogeneration
rate and tends to increase the transfer process from the valence band toward the conduction band
rendering CNU-DPx a useful material for photocatalytic reactions.
Figure 7. (a) UV–Vis DRS spectra of the CNU and CNU-DPx samples and (b) the photoluminescence
spectrum under 400 nm excitation.
2.1. Photocatalytic CO2 Reduction
The as-prepared samples i.e., parental CNU and co-polymerized CNU-DPx were utilized for CO2
reduction reactions under visible light irradiation. The reaction system was composed of a mixture of
distilled water and the organic solvent acetonitrile (MeCN), Co(bpy)32+ co-catalyst and TEOA which act
a sacrificial electron donor. The organic MeCN is used as main solvent due to its great solubilization of
the whole reaction system [50]. Figure 8 shows the evolution if the two main products, i.e., CO and H2,
after the photocatalytic CO2 reduction reaction. The reaction system was subjected to photocatalysis
during a four-hour reaction cyclic, resulting in a highest yield of 79.32 µmol of CO and 16.01 µmol
of H2, therefore demonstrating a remarkable capability of CNU-DP15.0 as a superior photocatalyst as
compared of the parental CNU (Figure 8a). The above results depict a boosted photocatalytic CO2
reduction rate with respect to the co-catalyst Co2+and confirms that this enhancement is the result
of the introduction of 2,6-diaminopurine co-monomer within the skeleton of CNU. As discussed
in the experimental section, varied amounts of co-monomer were added within the CNU samples
in order to examine the effect of monomer amount on photocatalytic activity. Initially the parental
CNU was applied to photocatalytic CO2 reduction therefore demonstrating the baseline activity as
presented in Figure 8b. This photocatalytic activity increases abruptly after the co-polymerization of
DP co-monomer within the CNU framework and therefore CNU-DP15.0 shows a remarkable catalytic
activity with a CO evolution rate of 31.1 µ
molh−1, almost 15.02 times higher than the parental CNU.
This enhancement in the catalytic activity of CNU-DP15.0 depends on the co-polymerization process of
light harvesting phenomena which is an important factor for accelerating the photoredox performance
of PCN. It also demonstrates that this activity tends to decline when the amount of co-monomer
increases beyond 15 mg. By integrating a large amount of co-monomer the conjugated shape of PCN is
destroyed. The spinal photocatalyst CNU-DP15.0 was investigated predominantly for the evolution
of CO/H2 products in term of changes of its optical absorption phenomena as depicted in Figure 8c.
Before these wavelength experiments were optimized, a Xe lamp was employed by changing its filter
cut-off in the visible range region and irradiating the concerned samples. The results indicate an
outstanding evolution of CO and H2 products by modulating the intensity of the visible light and
even display an energetically favorable photocatalytic performance at 510 nm as compared of pristine
CNU. The above result verifies that the association of sunlight harvested photons efficiently accelerates
the photochemical reaction of CO2 reduction into its main products. The superior CNU-DP15.0
sample was subjected to prolonged repetitive experiments in order to determine its stability for CO2
photo-reduction under visible light. The sample after use was collected, washed, filtered and added
into a fresh reaction and investigated for five cycles. As seen from Figure 8d, the product is eminently

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stable and therefore no any change was observed in the evolution of CO/H2 products. The vitality
of the stability protocol depends on the activation of the co-catalyst cobalt species and hence there a
rejuvenated optimized production of CO/H2 showing a TON of ca. 170, which is a notable achievement
in photocatalytic diversity. After prolonging the experiments, the sample was recollected by washing
and utilized for structural characterizations such as XRD, FTIR, DRS and XPS for comparison with
fresh a sample as depicted in Figures S5 and S6. As suggested from the above results that was no
noticeable change in the crystalline, chemical, and surface structure of the used samples, therefore
indicating a highly stable photocatalyst for the photoreduction of CO2.
Figure 8.
a catalyst under visible light irradiation. (
into the CNU framework for CO2 photoreduction as compared to blank CNU. (
(
a
) Time-production plot of CO and H2 by photoreduction of CO2, using CNU-DP15.0 as
) The effect of integration of different amounts of DPx
) The dependence
) Stability of the CNU-DP15.0 as
b
c
of CO2 photo-reduction on different incident light wavelengths. (
d
a photocatalyst.
To confirm that the photocatalytic CO2 reduction reaction was really dependent on the effect of
the reaction solvent, several control experiments were performed utilizing different protic solvents
(e.g., MeCN, DMSO, THF, DMF and H2O) as illustrated in Figure 9a. The results demonstrate that when
pure H2O was used in the reaction system, no detection of CO or H2 products occurred. When organic
solvents were utilized individually in each reaction system, i.e., MeCN, DMSO, THF, or DMF a
number of potential products were extracted. Among the aforementioned solvents, acetonitrile (MeCN)
displayed exclusive photocatalytic CO2 reduction activity under visible light irradiation. The above
result manifests no interaction between the CO2 molecules and desired solvent and explains the
optimized CO2 reduction process [33]. Several protocols have been optimized by utilizing CNU-DP15.0
photocatalyst for the photoreduction reaction of different CO2 sources and therefore several reactions
were addressed in different temperature regions (10–50 ◦C) in order to establish the dependence of the
evolution of CO on the reaction temperature. As depicted in Figure 9b, the evolution of CO showed an
enhanced yield of 45.37
of Figure 9b. Below and above this temperature, the CO2-to-CO conversion rate slightly declined,
µ
mol/h at 30 oC with an excellent selectivity of 81.58 % as seen in the inset

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o
therefore confirming 30 C as the optimal reaction temperature for this chemical system. At high
temperature an abundant amount of CO2 molecules is discharged from the reaction system which has
a negative influence on the economical catalytic performance of CO2 reduction.
Figure 9. (a) Various control experiments of CNU-DP15.0 photocatalyst using different organic solvents
for the generation of CO and H2 under visible light. (b) Evolution of CO and H2 at various reaction
temperatures (inset shows the selectivity).
Isotopic experiments have been performed using 13CO2 gas under the same reaction conditions
in order to investigate the source of the evolved CO product. The CO photocatalytically evolved
after two hours of irradiation was analyzed by gas chromatography-mass spectrometry (GC-MS).
The net result of the GC chromatogram was a peak at 3.87 min having m/z = 29 therefore attributed
to 13CO as presented in Figure 10a,b. The aforementioned results prove that during the reaction, the
evolved CO is indeed produced due to a reductive deoxygenation of the CO2 reactant under visible
light under mild conditions. The net photocatalytic performance of CO2 reduction was intrinsically
moderate, which was intimately affected by the consistent stability of CO2 molecules with their
alternating configuration [51,52]. The photocatalysis of CO2 reduction combined with induced photons
are more favored than direct CO2 reduction. For this, we replaced the water solvent with MeCN
to optimize the reaction medium, and further accomplished the tuning between the photocatalytic
activity and the volumetric H2O gradient. As depicted in Figure S7a, when water was introduced
into the reaction system along with acetonitrile then the photocatalytic activity of the CO2-to-CO
conversion was intensively accelerated when the best CNU-DP15.0 sample was utilized. In control
experiments, we studied different water/acetonitrile ratios (0/6, 1/5, 2/4, 3/3, 4/2) and ultimately the
1/5 volumetric ratio showed a remarkable catalytic activity under the applied conditions, which is
about five time higher than that of the non-water-added system. This promotion in the photocatalytic
activity is attributed to the water content in the reaction system which lowers the thermodynamic
barrier and accelerates the reaction kinetics. The addition of water also improved the reaction with a
Ru-photosensitizer which generates the active species for the photocatalytic reaction [53]. When the
content of water was increased more an inhibitory effect occurred, which caused a decline in the
photocatalytic CO2 reduction reaction. Figure S7b demonstrates the CO2 photo-reduction by utilizing
monomers similar of 2,6-diaminopurine but different group attachments and compares its evolution
products with the best sample, also illustrated in Table S6. Several control experiments were conducted
in order to determine various parameters that impact the reaction system are illustrated in Table S7.
Initially, the reaction was conducted without the inclusion of either CNU or light, resulting in no
evolution of CO and H2 (Table S7). The result reflected that the cobalt ion co-catalyst part without the
inclusion of the main catalyst cannot drive the CO2 reduction reaction. The presence of semi-conductor
photocatalyst along with cobalt ions absorbs a high amount of light to produce photogenerated
electrons and facilitate their transport from the valence band toward the conduction band resulting in
the conversion of the CO2 source. Some control experiments were performed without the induction of

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sacrificial agent and co-catalyst therefore demonstrating a negligible amount of CO products, as shown
in Table S7. The CO2 gas was replaced instead with nitrogen and argon gas in the reaction system and
hence no evolution of CO products occurred, therefore demonstrating the degradation influence of the
main catalyst, i.e., PCN, along with other organic compounds such as TEOA and bpy. Some reference
reactions depicted that the photocatalytic CO2 reduction terminated without the inclusion of cobalt
ions or ligands or both (Table S7). The basic role these ligands is to act as electron mediators during the
photoreductive process, further analyzed by the PL spectra of the system as discussed before.
Figure 10. (a) Mass spectra (m/z = 29), and (b) gas chromatogram of the carbon source of the evolved
CO in the photocatalytic reduction of 13CO2.
2.2. Photoelectrochemical Measurements
As mentioned before the photocatalytic activity of co-polymerized CNU-DP15.0 displayed an
optimized efficiency in terms of CO2 reduction as compared to blank CNU samples, which can be
mainly associated to the constitutional charge mobility of photogenerated electrons and holes in the
co-polymerized samples. Photoelectrochemical measurements have been performed for both the
parental CNU and co-polymerized CNU-DP15.0 samples in order to establish the apparent tendency
of charge carriers and their transport process within these samples. Figure 11a shows the transient
photocurrent density curves of blank CNU and co-polymerized CNU-DP15.0 samples and therefore
manifests a drastic cathodic photocurrent spike for the specific on/off light interval under solar
irradiation (λ = 420 nm). The photocurrent density course of the as-polymerized CNU-DP15.0 sample
is extremely enhanced compared to that of the parental CNU, suggesting that the conjugated electron
donor-acceptor monomer interaction within the framework of CNU hence ameliorates the efficiency of
the charge transfer process. Meanwhile, electrochemical impedance spectroscopy (EIS) measurements
were evaluated for both of samples in the dark region in order to verify the above conclusion,
as depicted in Figure 11b. The result demonstrates the charge transfer resistances of pure CNU
and co-polymerized CNU-DP15.0 are 324.9, and 277.1 ´Ω, respectively. The decrease in semicircular
Nyquist plots (resistance capacity) of the co-polymerized CNU-DP15.0 as compared to CNU suggests
a superior reaction rate on the surface of the CNU-DP15.0 electrode. This promotion in the reaction
mechanism of the co-polymerized sample manifests as an energized transport and separation of
photogenerated charge carriers at the surface. It is of great importance that the integration of a high
amount of purine within the CNU structure hence decreases the LUMO energy state of the acceptor
which intimately results in a decline in the photocatalytic activity of CO2 reduction. This incorporation
of a high amount hence effects a destruction of the structure of PCN and therefore an extensive
amount of photogenerated electrons will be emitted in the form of vibrational/thermal energy, etc.
which is also a key factor to lower the photocatalytic activity of CO2 reduction. The above conclusion
postulates the assumption that the integration of conjugated donor-acceptor DP monomer within the
matrix of CNU significantly enhances the photogenerated electrons and holes and also accelerates its

Molecules 2019, 24, 1779
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transportation strikingly. The detailed photocatalytic mechanism is as shown in Figure 12. Due to
this co-polymerization, the resultant surface phenomena and optical absorbance rate of PCN was
remarkably increased, which results in a lowered rate of electron-hole recombination making it a
superior material for heterogeneous photocatalytic CO2 reductions.
Figure 11. (a) Periodic on/off photocurrent (b) the EIS Nyquist plots b/w CNU & CNU-DP15.0 at Ag/AgCl
in a 0.2M Na2SO4 aqueous solution (pH 6.8) response under visible light irradiation (λ=420 nm).
Figure 12. Schematic proposed mechanism of photocatalytic CO2 reduction.
3. Experimental Section
3.1. Materials
All chemicals used in synthesis and photocatalytic reaction were utilized directly without any
further purification. In all the experiments the distilled water supplied by an appropriate internal
laboratory apparatus is mandatory. Urea (>99%) was supplied by Sinopharm Chemical Reagent Co. Ltd.
(Fuzhou, China) and 2,6-diaminopurine, trimethylxanthine (caffeine) and methylxanthine (theobromine)
2,2-bipyridine (bpy), CoCl2, H2PtCl6 were obtained from Sigma Aldrich (Fuzhou, China). The organic
solvents such as triethanolamine (TEOA), trichloromethane (TCM), N,N-dimethylformamide (DMF),
acetonitrile (MeCN) and tetrahydrofuran (THF) were bought from Sinopharm Chemical Reagent Co.,
Ltd.. The main gases such as carbon dioxide (CO2) and 13CO2 were provided from Fuzhou Lianzhong
Industrial Gases and Hess Chemical gas Center (Beijing, China).

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3.2. Synthetic Procedures
In a typical procedure, PCN was synthesized by taking 10 g of urea in 15 mL of distilled water
and heating at 80 ◦C for a day and a night in an oil bath system under continuous stirring in order to
evaporate the water completely. After, the resultant products were transferred into an alumina crucible
with a cover and heated to 600 ◦C for 2 h at a heating rate of 5 ◦C/min. After performing various
polycondensation processes at the appropriate temperature, the as-synthesized samples were collected,
ground and named CNU. The co-polymerized samples were prepared by taking 10 g of urea with
different amount of co-monomer, i.e., 2,6-diaminopurine in 15 mL of distilled water and keeping the
mixture in an oil bath at 80 ◦C for a day and a night until a solid residue was left. The solid products
were transferred to an alumina crucible with a cover and heated to 600 ◦C for 2 h at a heating rate of
5 ◦C/min and named CNU-DPx where x is the different amount of given monomer and is equal to
5, 10, 15, 20 and 25 mg. For comparison, the same class of monomer but with different groups such
as 2,6-dihydroxypurine (xanthine), trimethylxanthine (caffeine) and methylxanthine (theobromine)
were also introduced within the PCN framework giving species such as CNU-XT15.0, CNU-CF15.0 and
CNU-TB15.0 using the above procedure. The amount of co-monomer embedded in other precursors of
PCN was kept as 15 mg because this monomer amount in the CNU showed superior activity.
3.3. Characterization
The as-prepared samples were examined by X-ray diffraction (XRD) measurements on a Bruker
D8 Advance diffractometer (Bruker, Germany). A Nicolet Magna 670 Fourier transform infrared
spectrometer (Thermo Electron Corp., Madison, WI, USA) was used to record the Fourier transform
infrared (FT-IR) spectra of all samples. The X-ray photoelectron spectroscopy (XPS) measurements
were performed on an ESCALAB250 instrument (Thermo, Thermo, VG Microtech, UK) equipped with
a monochromatized Al Ka line source (200 W). The samples were subjected to solid-state 13C-NMR
experiments that carried out on a Bruker Advance III 500 spectrometer. The electron paramagnetic
resonance (EPR) measurements were conducted on a Bruker model A300 spectrometer. The emission
scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) experiments were
performed a New Generation SU8010 (Hitachi, Japan) and a model JEM 2010 EX (JEOL, Tokyo, Japan)
instrument, respectively. The UV–Vis diffuse reflectance spectra (DRS) of as-synthesized samples
were run on a Cary 500 Scan UV–Vis spectrophotometer (Varian, Darmstadt, Germany) with barium
sulfate as the reference. A FI/FSTCSPC 920 spectrophotometer (Edinburgh, UK) was used for the
measurement of the photoluminescence (PL) spectra of all samples. The N2 adsorption-desorption
and CO2 adsorption isotherms were characterized on an ASAP2020 system (Micromeritics, Boston,
MA, USA). The photocatalytic produced gases were analyzed by an 7820A gas chromatograph
(Agilent, Boston, MA, USA) equipped with a thermal conductivity detector (TCD) and a TD-01 packed
column. The 13CO2 isotopic experiment for the detection of 13CO gas was performed on a HP 5973 gas
chromatography-mass spectrometry (GC–MS) system.
3.4. Photocatalytic CO2 Reduction Test
The photocatalytic CO2 reduction experiments were performed under visible light irradiation by
utilizing a 300 W Xe lamp with a 420 nm cut-off filter. The reaction was done in an 80 mL Schlenk
flask reactor that contained 30 mg of photocatalyst, 24 mg CoCl2 6H2O, 15 mg 2,2-bipyridine, 1 mL
·
triethanolamine (TEOA) dissolved in 6 mL of solvent, i.e., water/acetonitrile (H2O/MeCN = 1 mL/5 mL).
Before irradiation, the reaction system was degassed and backfilled under vigorous stirring with pure
CO2 gas by a connected air pump and finally pure CO2 (1 atm) was added to the reaction system.
The temperature of the reaction system was controlled at 30 ◦C with a water cooling system and set up
for irradiation under visible light and stirring. After irradiation, the produced gases were detected by
the Agilent 7820A gas chromatograph. 13CO2 gas was utilized as reactant for conducting the isotopic

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experiments under the same photocatalysis conditions and after irradiation the products were analyzed
using gas chromatography-mass spectrometry (GC–MS).
3.5. Electrochemical Analysis
The electrochemical measurements used working electrodes which were pretreated by the
doctor-blade method. The paste was coated on a fluorine-doped tin oxide (FTO) substrate cleaned
and washed several times with ethanol, acetone and distilled water, respectively. The as-synthesized
samples i.e., parental CNU and CNU-DP15.0 (5 mg) were used to make fine slurries by dissolving them
in 2 mL of dimethylformamide (DMF) under ultrasonication. The as-prepared slurries were pasted
onto the FTO electrodes after drying in air for a day and a night to obtain CNU and CNU-DP15.0
photocatalysts. The electrochemical analyses were conducted by using a CHI electrochemical
workstation (BioLogic VSP-300, Paris, France) that is composed of three conventional electrodes,
i.e., working, reference and counter electrode. An electrolyte of 0.2 M sodium sulfate electrolyte
solution (Na2SO4) aqueous solution were used in which the working electrode i.e., CNU and
CNU-DP15.0, counter electrode Pt sheet and the reference electrode Ag/AgCl were immersed. A Xe
lamp was used as irradiation source that illuminate the working electrode from the back side
(FTO substrate/semiconductor interface) in order to decrease the effect of semi-conductor thickness.
All parts of the working electrode were covered, and only 0.2 cm2 areas are exposed for light illumination.
The photocurrent measurement such as periodic on/off response of CNU and CNU-DP15.0 electrode is
indexed at 0.4 V bias vs. Ag/AgCl and the perturbation signal was applied at 10 mV with a frequency
of 1 kHz. During the electrochemical impedance spectroscopy (EIS) experiments, the perturbation
signal was similarly 10 mV, obtained by alternating the frequency range from 200 kHz to 10 mHz.
4. Conclusions
Numerous proposed intramolecular conjugated monomers are quenched by the co-polymerization
process or are unusable due to their complex synthesis or high cost. Conjugated donor-acceptor
co-polymers are considered an efficient rejuvenated candidate due to their superior performance,
low cost, abundance and a stabilized organic conjugated co-monomer was introduced into the main
skeleton of PCN to enhance to photocatalytic activity. The optimization of the conjugated donor-acceptor
2,6-diaminopurine co-monomer copolymerized with the host PCN has been postulated. This conjugated
donor-acceptor nitrogen-rich co-monomer DP is essentially composed of imidazole rings grafted by a
pyrimidine (electron-donor/electron acceptor) that are considered a superior donor-acceptor system
and energetically stable. This integration of DP co-monomer with PCN semi-conductor accelerates a
nucleophilic reaction at the position of the adjacent amino group due drive supramolecular aggregation
(co-polymerization) and form a long polymer having high surface area and better photocatalytic activity
than the parent compound. The as-fabricated samples were successfully analyzed using different
characterization techniques to explain its electronic, optical properties, crystal phase, microstructure and
morphology that were influenced by the interaction with the DP co-monomer. DFT calculations on pure
PCN and the CNU-DP15.0 material trimers (interpreted as the result of incorporation of the co-monomer
at two different positions) were comprehensively evaluated and they exhibited a remarkable structural
optimization without the inclusion of any symmetry constraints and also allows manipulation of their
optical and electronic properties. The incorporation of the donor-acceptor (D-A) 2,6-diaminopurine
co-monomer within PCN influences the chemical affinities, polarities and acid–base functions of
PCN and enhances the photocatalytic activity for CO2 reduction to CO and H2 and also improves
its selectivity. The present work presents an interesting concept and validates the experimental and
theoretical information for developing a catalyst with enhanced photocatalytic performance.
Supplementary Materials: The supplementary materials are available online. Figure S1: Calculated spatial
electron distributions of HOMO and LUMO for CNU and co-polymerized CNU-DP15.0
. Figure S2: N2
adsorption-desorption isotherms (77 K), (inset) pore size distribution graph, for (a) CNU and CNU-DP10.0
and (b) CNU and CNU-DP20.0samples, respectively. Figure S3: XPS results of high-resolution spectra of O 1s for

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CNU and CNU-DP15.0. Figure S4: XPS results of high-resolution spectra of S 1s for CNU and CNU-DP15.0. Figure
S5: XRD (a) and FTIR (b) for fresh sample and used sample of CNU-DP15.0. Figure S6: C 1s (a) and N 1s (b) UV-Vis
DRS (c) for fresh sample and used sample of CNU-DP15.0. Figure S7: (a) Utiilizing CNU-DP15.0 photocatalyst by
using different volumetric ratio of water and acetonitrile (b) same class of monomer having different groups for
the generation of CO and H2 under visible light. Table S1: Electronic excitation transition parameters of CNU
and Co-polymerized CNU-DP15.0. Table S2: Electronic excitation transition parameters of CNU-DP15.0. Table
S3: Calculated values of Mulliken Atomic Charges (a. u.) for CNU-DP15.0, DFT/ B3LYP/6-31G method. Table S4:
Physicochemical properties of COand H2 evolution during CO2 reduction of as-prepared samples. Table S5: The
element derived by XPS results. Table S6: Comparison of as-prepared samples with same class of co-monomers
having different groups. Table S7: Study of various conditions on controlled experiments.
Author Contributions: A.H. and M.U.R. have performed the experiments, data analysis and article writing. The
other coauthors revised manuscript and help in technical correction. The corresponding author revised the paper,
deal and corresponds to journal.
Funding: This research was funded by National Natural Science Foundation of China (51602207), Doctoral
Scientific Research Foundation of Liaoning Province (20170520011), Program for Liaoning Excellent Talents in
University (LR2017074), and Project of Education Office of Liaoning Province (LQN201712).
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are not available from the authors.
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