Phenyl-grafted carbon nitride semiconductor for photocatalytic CO2-reduction and rapid degradation of organic dyes

Article abstract of DOI:10.1039/c8cy02220h

Molecular engineering of graphitic carbon nitride (g-C₃N₄) is achieved by the copolymerization of π-conjugated phenyl urea, melamine, and urea. Integration of aromatic phenyl rings into the heptazine network of g-C₃N₄ alters its structural, optical and electronic properties. The fusion of the polymeric g-C₃N₄ core with aromatic phenyl groups induces band gap tuning, which greatly improves the separation and lifetime of charge-carriers. As a result, CO₂ photoreduction experiments conducted by using phenyl-grafted g-C₃N₄ afford methane and formic acid in high yields. Furthermore, a selective model organic pollutant rhodamine B dye is rapidly decomposed under visible-light irradiation. This work suggests that pyrolysis of a suitable aromatic π-deficient molecular dopant such as phenyl urea can drastically alter the photo-response of the carbon nitride photocatalyst and may enhance its photocatalytic activity. Hence, the present work is expected to be of significant value in sustainable energy production and environmental remediation.

Full text of DOI:10.1039/c8cy02220h

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R. Manwar, A. Gupta, S. G. Ghugal, R. Boukherroub and S. S. Umare, Catal. Sci. Technol., 2019, DOI:
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Phenyl-grafted Carbon Nitride Semiconductor for Photocatalytic CO -
DOI: 10.1039/C8CY02220H
2
Reduction and Rapid Degradation of Organic Dyes
Devthade Vidyasagar,a Nilesh Manwar,b Akanksha Gupta,a Sachin G Ghugal,c Suresh S Umare,a* and Rabah
Boukherroubd
a Materials and Catalysis Laboratory, Department of Chemistry, Visvesvaraya National Institute of Technology
(VNIT), Nagpur, Maharashtra, 440010, India. Email: ssumare@chm.vnit.ac.in, ssumare1965@gmail.com,
Fax: +91 712 2223230, Tel: +91 712 2801316.
b Chemical and Material Science Division (CMSD), CSIR-Indian Institute of Petroleum (IIP), Mohkampur,
Dehradun, Uttarakhand, 248005, India.
c School of Chemistry, Univ. of Hyderabad, Telangana, 500046, India.
d Univ. Lille, CNRS, Centrale Lille, ISEN, Univ. Valenciennes, UMR 8520 – IEMN, F-59000 Lille, France.
Abstract: Molecular engineering of graphitic carbon nitride (g-C3N4) is achieved by the copolymerization of π-
conjugated phenyl urea, melamine, and urea. Integration of aromatic phenyl rings into the heptazine network
of g-C3N4 alters its structural, optical and electronic properties. The fusion of polymeric g-C3N4 core with
aromatic phenyl groups induces band gap tuning, greatly improves the separation and lifetime of charge-
carriers. As a result, CO2 photoreduction experiments conducted by using phenyl grafted g-C3N4 afford
methane and formic acid in high yields. Furthermore, a selective model organic pollutant rhodamine B dye is
rapidly decomposed under visible light irradiation. This work suggests that pyrolysis of a suitable aromatic π-
deficient molecular dopant such as phenyl urea can drastically alter the photo-response of carbon nitride
photocatalyst and may enhance its photocatalytic activity. Hence, the present work is expected to be of
significant value in sustainable energy production and environmental remediation.
Keywords: Carbon nitride; Phenyl grafted g-C3N4; CO2 photoreduction; dye degradation; photocatalysis
Introduction
Rational design of visible active photo-systems to generate value-added chemicals from carbon dioxide (CO2)
holds great potential in solving energy calamity. Solar photocatalysis is a strategic resource to mitigate the
negative effects of environment and energy crisis. So far, a wide variety of materials have been examined as
visible active photocatalysts, including metal oxides,[1] metal nitrides,[2] chalcogenides,[3] phosphides,[4] metal-
organic frameworks,[5] layer-double hydroxides,[6] mixed oxides,[7] hybrid composites[8] and conjugated
polymers.[9] In particular, conjugated polymers are of special interest due to their low-cost, easily processable,
and environmentally benign properties. Recently, two-dimensional graphitic carbon nitride (g-C3N4) emerged
as a metal-free visible active photocatalyst.[10] Polymeric g-C3N4 materials are new additions to the carbon
family and are highly praised as multifunctional wonder materials, rich playgrounds for solar photocatalysis.[11]
Indeed, g-C3N4 holds enormous promise as a potential photocatalyst for environmental remediation and
energy technologies.[12] The immense practical importance of g-C3N4 based materials is due to their unique
thermal, physicochemical, optical properties (band gap of 2.7 eV), and their metal-free semiconducting
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nature.[13] Although g-C3N4 has several exciting properties, the quantum efficiency of bulk g-C3N4 is still
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DOI: 10.1039/C8CY02220H
restricted by rapid charge recombination, low surface area, and limited visible light absorption.[14] This led to
an upsurge in the modification of g-C3N4 synthetic strategies. In this context, several methods have been
adopted to retard the high charge recombination rate and extend the visible light absorption of pristine g-
C3N4.[15]
The formation of heterostructures,[16] metal-doping,[17] molecular engineering,[18] supramolecular
preorganisation,[19] use of hard or soft templates,[20] and copolymerization[21] strategies have been
implemented to enhance the photoactivity of bare g-C3N4. Direct co-polymerisation of g-C3N4 precursors with
π-electron conjugated systems can extend the optical absorption towards longer wavelength. For instance,
Zhang et al. exemplified that co-polymerisation of dicyandiamide with barbituric acid as the molecular dopant
modified the triazine network of g-C3N4 and extended the optical absorption further into the visible region with
superior photocatalytic properties.[22] Modification with a series of aromatic π-electron conjugated systems
such as 4-amino-2,6-dihydroxypyrimidine,[23] 2,6-diaminopyridine,[24] 3-aminothiophene-2-carbonitrile,[25] 2,4-
diamino-6-phenyl-1,3,5-triazine,[26] acetoguanamine,[27] thymine,[28] barbituric acid[29] and 2,4,6-
triaminopyrimidine[30] was performed to significantly red shift the optical absorption edge of bulk g-C3N4 and
improve its photoactivity. These reports recommend that incorporation of co-monomer building blocks into
polymeric network of g-C3N4 can change the physicochemical properties and result in high photoactivity of
modified g-C3N4.[31]
Motivated by the latest outcomes, we endeavoured to integrate phenyl urea as an aromatic π-electron
conjugated motif into the g-C3N4 framework and inspect the impact of phenyl-grafting on photocatalytic CO2
reduction and mineralisation of organic dyes. In this viewpoint, phenyl urea was prepared from urea and
copolymerised with melamine under suitable experimental conditions.
Results and Discussion
The stepwise preparation process for phenyl grafted g-C3N4 is depicted in Figure S1. As illustrated in Figure
S2, during thermal polymerisation the amino units of melamine and phenyl urea are involved in the
polymerisation process to form phenyl grafted g-C3N4. The optimized molecular functionalization of g-C3N4
with π-electron rich phenyl urea can modulate electronic energy levels and accelerate the reaction rate.
However, an excess amount of phenyl urea would deteriorate the visible light absorption, deform tri-s-triazine
network and can create new recombination sites for the charge carriers. Therefore, for the optimized phenyl
grafted g-C3N4, the amount of monomer was set to 20 mg per 10 g of urea/melamine content and the
structural, morphological and electronic properties of the resulting catalyst are thoroughly investigated.
Powder X-ray diffraction (PXRD) analysis was used to ascertain the crystalline phase and structural
information of as-synthesized materials. The PXRD pattern of polymeric g-C3N4 and Ph20-g-C3N4 displayed
unique diffraction peaks at 2θ value of 27.50° and 27.14°, respectively, which correspond to the hkl (002)
reflection arising from the interlayer stacking of aromatic tri-s-triazine rings (Figure 1a). However, the (002)
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reflection peak of Ph20-g-C3N4 sample is comparatively broader than the peak of g-C3N4. This slightly wider
DOI: 10.1039/C8CY02220H
(002) reflection of Ph20-g-C3N4 is likely due to the incorporation of guest phenyl molecules. Compared to g-
C3N4, the (002) reflection in the Ph20-g-C3N4 shifted to lower diffraction angle and displayed a larger d-spacing
value of 0.332 nm, which denotes increased interlayer distance. The addition of phenyl urea induced
pronounced intensity of in-plane (100) reflection in the Ph20-g-C3N4 sample. Furthermore, the low-intense and
broad peaks of g-C3N4 in Ph20-g-C3N4 suggest low-crystalline nature of the material. These crystalline
rearrangements in Ph20-g-C3N4 strongly indicate the successful integration of phenyl urea into the polymeric
tri-s-triazine framework.
The optical properties of polymeric g-C3N4 were assessed by UV–Vis diffuse reflectance absorption
spectroscopy (UV-Vis DRS). The UV-Vis spectrum of g-C3N4 shows an absorption band with an edge at 456
nm corresponding to the blue region, which is attributed to the intrinsic n-π* or π-π* transition of aromatic tri-s-
triazine repeating units.[32] The absorbance spectrum of Ph20-g-C3N4 exhibits a red shift of the absorption band
edge from ca. 456 nm to 466 nm (Figure 1b), which is likely due to π-conjugation of phenyl group as well as
the excitation from band tail/localized states caused by the incomplete long-range ordering of tri-s-triazine
structure.[33] It suggests that the introduction of phenyl urea alters the structural configuration of the tri-s-
triazine ring and enhances the visible light harvesting capacity towards longer wavelength. Moreover, the
energy band gap of g-C3N4 and Ph20-g-C3N4 calculated from the Kubelka-Munk (K-M) plots were 2.72 and
2.66 eV, respectively (Figure 1c). The slight red shift in energy band gap of the Ph20-g-C3N4 is a result of
additional aromatic phenyl moieties on the g-C3N4 network, which effectively relocate π-conjugated electrons
and tailored highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
energy levels.[34] The valence band (VB) and conduction band (CB) edge potential of Ph20-g-C3N4 estimated
by Mulliken electronegativity equations were +1.65 and -1.01 eV, respectively. As compared to unmodified g-
C3N4, Ph20-g-C3N4 showed about 0.06 eV decrease in the band gap energy, which means that phenyl grafting
increased the sp2 character in g-C3N4 by graphitization which leads to excess aromaticity. Further, UV-visible
absorption spectrum of Ph-g-C3N4 displayed a broad absorption tail in the visible region, which is likely due to
the n→π transition of N-C=C of newly formed aromatic phenyl ring. The estimated second optical band gap of
absorption tail is found to be 1.96 eV. Thus, the Ph-g-C3N4 exhibits extended light absorption from UV-visible
to visible region, and lower the photo-excitation energy.
Fourier transform infrared (FT-IR) analysis of synthesized materials revealed formation of graphitic
carbon network. The corresponding FT-IR spectrum of Ph20-g-C3N4 is comparable to the reported g-C3N4 data
and shows strong stretching bands at 3200-2900 cm-1 (-NH/-NH2), 1100-1576 cm-1 (-CN,-C=C), and 809 cm-1
corresponding to condensed aromatic –CN rings (Figure 1d).[35] A close look in the 1600-1400 cm-1 region
reveals a band at 1504 cm-1 corresponding to aromatic C=C stretch (Figure 1e).[36] The supramolecular
assembly of phenyl urea into the tri-s-triazine framework decreased the intensity of 809 cm-1 signal in Ph20-g-
C3N4 sample. These characteristic alterations in graphitic carbon network are further revealed by 13C solid-
state cross-polarization magic angle spinning (CP-MAS) nuclear magnetic resonance (NMR) spectroscopy.
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Carbon environment of g-C3N4 analysed by solid-state 13C MAS-NMR depicts twoDsOiIg: 1n0a.1l0s3V9ai/eCwt8A1Crt6Yic04le2.O262nl0inHe
and 156.3 ppm, which are assigned to sp2 bonded carbon atoms of inter-linked tri-s-triazine rings and C(i)N3
groups of the cyameluric group (Figure 1f).[37] However, sp2 carbon atoms in the Ph20-g-C3N4 are shifted to
higher values of 157.9 and 165.9 ppm (Figure 1f). Moreover, both signals are broadened and exhibit an
increase in the resolution of the signal to noise ratio (Figure f inset). This indicates the modification or
disorder in the carbon environment of g-C3N4 network, which is ascribed to the additional effect of aromatic
phenyl carbon atoms. Furthermore, indistinct broad humps in 13C NMR spectrum of Ph20-g-C3N4 at 130-140
ppm reflects C=C phenyl type environment.[38] Conversely, no clear peak of aromatic C=C was observed due
to the low amount of phenyl urea in the condensation process.
Figure 1. Characterisation details of g-C3N4 and Ph20-g-C3N4: (a) powder XRD patterns, (b) UV-Vis
absorbance spectra, (c) Kubelka-Munk plots, (d) FT-IR spectra, (e) enlarged portion of FT-IR spectra in the
1400-1600 cm-1 region, and (f) Solid state 13C MAS NMR spectra.
The elemental composition and chemical states of Ph-g-C3N4 is characterized by X-ray photoelectron
spectroscopy (XPS). The XPS survey spectra of g-C3N4 and Ph-g-C3N4 comprise two sharp peaks at 288.0
and 398.0 eV, which are related to the C1s and N1s elements (Figure S3), respectively.[39] It is worth to note
that the XPS data in Table S1 shows an increase in carbon amount for Ph-g-C3N4 as compared to that of g-
C3N4; this extra carbon contribution is likely from aromatic phenyl groups in Ph-g-C3N4. The high resolution
C1s spectrum of g-C3N4 is deconvoluted into two peaks. The primary peak at binding energy of 285.2 eV is
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ascribed to adventitious carbon (C-C), while the peak at 288.4 eV is originated from C-N=C coordinaVtiienwgAruticnleitOsnline
DOI: 10.1039/C8CY02220H
of tri-s-triazine ring.[40] The high resolution C1s XPS spectrum of Ph-g-C3N4 is fitted with three different carbon
species. The two peaks centered at 284.8 and 288.2 eV are due to sp2 bonded C-C atom and heterocyclic C-
N=C species, respectively.[41] Notably, the sp2 C-C bonding mode signal at 284.8 eV is profound in Ph-g-C3N4
C1s XPS spectrum, which corroborates the increased aromatic carbon environment in the material (Figure
S3). The small contribution peak found at 286.2 eV is assigned to C-O-C type linkage in Ph-g-C3N4.[29a] The
N1s XPS spectra of g-C3N4 and Ph-g-C3N4 were nearly identical with peaks at 398.0, 400.0, and 404.0 eV
corresponding to the contribution of C=N, N-(C)3, and N-H bonding modes, respectively (Figure S3).[27] Based
on the XPS results, we assume that the increase in contribution of sp2 -C=C- species can be due to the phenyl
grafting of heptazine ring.
Morphological and microstructure analysis
The field emission scanning electron microscopy (FE-SEM) images of g-C3N4 and Ph20-g-C3N4
displayed broken sheet-texture (Figure 2a, d). Energy-dispersive X-ray spectroscopy (EDS)
analysis confirmed that the materials consist mainly of carbon and nitrogen with a trace amount of oxygen
contamination (Figure 2b, e). The C/N molar ratio increased from 0.56 for g-C3N4 to 0.70 for Ph20-g-C3N4.
Besides, a slight increment in the carbon amount was observed in the Ph20-g-C3N4 sample ((Figure 2c, f)).
This increase in carbon content is likely due to the incorporation of aromatic phenyl urea within the g-C3N4
framework, which further evidences the successful engineering of g-C3N4 tri-s-triazine network.
Figure 2. (a, d) FE-SEM images, (b, e) energy-dispersive X-ray spectra and quantitative elemental
composition of (c) g-C3N4 and (f) Ph20-g-C3N4.
The microstructure of g-C3N4 significantly changed with the incorporation of aromatic phenyl urea.
The TEM image of g-C3N4 reflects transparent thin sheet structure, whereas Ph20-g-C3N4 displays compact
self-rolling appearance of g-C3N4 layers due to the synergistic effect of intramolecular hydrogen bonding and
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van der Waals interaction (Figure 3a, d). Similar TEM observations were previously obtained for melamine
DOI: 10.1039/C8CY02220H
and 2,4,6-triaminopyrimidine copolymerised samples.[30] The integration of π-aromatic phenyl groups into tri-s-
triazine network increased the 2D interplanar packing distance of heptazine units from 0.321 to 0.334 nm
(Figure 3b, e). The obtained interplanar (d-spacing plot profile, Figure S4) distance results are consistent
with the values obtained from PXRD analysis. The selected area diffraction (SAD) pattern of g-C3N4 exhibits
concentric rings, whereas the SAD pattern of Ph20-g-C3N4 displays amorphous characteristics with diffuse ring
pattern (Figure 3c, f). These microstructural alterations in heptazine units may contribute to the enhancement
of surface area and subsequently the photoactivity. The specific surface area and pore size of the developed
materials were analyzed by nitrogen adsorption-desorption isotherms. The N2 isotherms of both g-C3N4 and
Ph20-g-C3N4 reflected type-IV isotherms. The calculated Brunauer-Emmett-Teller (BET) specific surface area
of Ph20-g-C3N4 was determined to be 94.74 m2 g-1, which was 10-fold higher than that of unmodified g-C3N4
(BET surface area of g-C3N4: 9.55 m2 g-1) (Figure S5a). The larger specific surface area of Ph20-g-C3N4 could
increase the area of photon incidence and expose more catalytic sites, which would be more advantageous in
the photocatalytic process.
Figure 3. TEM images, HR-TEM images and SAD patterns of g-C3N4 (a-c) and Ph20-g-C3N4 (d-e),
respectively.
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Visible light-driven CO2 photoreduction
The CO2 photoreduction performance of g-C3N4 and Ph20-g-C3N4 catalyst was studied in a solvent
mixture of N,N-dimethylformamide/water/triethanolamine (DMF/H2O/TEA = 30/10/10 mL) using 100 mg of
photocatalyst. After 24 h of visible light irradiation, 1 L of sample was withdrawn and corresponding products
in the reaction mixture were monitored by injecting into GC–TCD and GC–FID. All Phx-g-C3N4 samples
emanated high amount of methane (CH4) upon light irradiation. The concentration of CH4 after 24 h of visible
light irradiation using the Ph100-g-C3N4 catalyst was 0.907 moles g-1, which is about 4-fold higher than that
produced using pure g-C3N4 (Figure 4a). The high amount of CH4 produced by Ph100-g-C3N4 is likely due to
the availability of excess active sites on the surface with rich surface area of 78.69 m2/g. The total amount of
methanol (CH3OH) produced was nearly 4-fold higher for Ph20-g-C3N4 catalyst than the bulk g-C3N4 (Figure
4b) The quantum efficiency for methanol obtained from the CO2 photoreduction system was found to be
0.96%. The details of quantum yield estimation is given in supporting information. To delight, the formic acid
(HCOOH) concentration determined by HPLC analysis revealed interesting results. The HCOOH production
rate after 24 h light irradiation using the Ph20-g-C3N4 photocatalyst was 5.2 V/V% after 10-times dilution, which
[42]
is significantly higher than the values reported for several photocatalysts.
The CO2 photoreduction activity
of metal-free Ph20-g-C3N4 catalyst is relatively higher than or comparable to the many reported metal based
photocatalysts (Table S2). Furthermore, the Ph20-g-C3N4 catalyst delivers much higher yields of methanol and
formic acid, while maintaining good stability and recyclability. To circumvent the possibility of alternative
carbon source in the experimental system, blank test was conducted under identical reaction conditions by
replacing CO2 with N2 gas, which inferred no hydrocarbon products formation. This suggests that no carbon
products were emanated from the photocatalyst or any other carbon contamination in the photocatalytic
system.
Figure 4. (a) Amount of methane formation and (b) yield of formic acid and methanol obtained in CO2-
photoreduction using g-C3N4 and Ph20-g-C3N4 catalysts. Reaction conditions: 100 mg of photocatalyst; 10 mL
water; 40 mL 3:1 (v/v) DMF/TEA; light source: 20W white cold LED; reaction time: 24 h.
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DOI: 10.1039/C8CY02220H
Till date, very limited studies postulated the mechanistic aspect of CO2 photoreduction.[43]
Nevertheless, the precise acceptable CO2 photoreduction pathway in heterogeneous catalysis is still a subject
of high scrutiny. Herein, we propose a plausible reaction pathway for the formation of CH4, CH3OH, and
HCOOH on the surface of the solid Phx-g-C3N4 catalyst. Polymeric Phx-g-C3N4 redox potentials are suitable to
drive CO2 photoreduction to hydrocarbons. At first, tri-s-triazine ring of Phx-g-C3N4 is activated by visible light
irradiation, which results in the generation of electrons (e-) in the conduction band (CB) and holes (h+) in the
valence band (VB). As illustrated in step (i) of Figure 5, surface adsorbed CO2 molecules strongly interact
with Pz orbitals or lone pairs of aromatic nitrogens in Phx-g-C3N4 via π-π stacking mode and initiate electron
CO⦁2―
transfer from CB of Phx-g-C3N4 to surface-bound CO2 forming partially charged
radical anion.[44]
CO⦁ ―
However, the complete formation of
radical anion is thermodynamically hindered reaction on the Phx-g-
2
CO⦁
-
C3N4 catalyst, owing to its lower reduction potential than CO2/
(-1.90 V).[45] The partial transfer of an
2
electron to (C-O) π* orbital destabilizes the linear symmetry of CO2 and its molecular geometry is distorted to
a bent structure.[46] As a consequence, repulsions on electrophilic carbon increase, contributing to the
lowering of the LUMO of bent CO2.[47] Thus, lower potential barrier further enables multistep electron addition
into the LUMO of bent CO2. The electrons in CB of Phx-g-C3N4 have reduction potential of -1.01 eV, which is
more negative than the reduction potentials of CO2/HCOOH (-0.61 V), CO2/CO (-0.53 V), CO2/HCHO (-0.48
V) CO2/CH3OH (-0.38 V), CO2/CH4 (-0.24 V vs. NHE).[48] In parallel, the oxidation potential of Phx-g-C3N4 VB
holes (h+) (+1.65 eV vs. NHE) is sufficiently positive than the oxidation potential of H2O/O2 (+0.82 eV vs.
NHE). As a result, multiple electrons and protons transfer may lead to the cleavage of C-O and formation of
C-H bonds associated with corresponding photoreduction products (Figure S6).
Figure 5. Schematic illustration of steps involved in CO2 photoreduction and standard reduction potentials of
corresponding CO2 reduction products at pH 7 with reference to normal hydrogen electrode (NHE), 25°C and
1 atm gas pressure. Basic steps involved in CO2 reduction: (i) sorption of CO2 on the catalyst surface, (ii)
photo-generation of charge carriers, (iii) photoreduction of CO2 into suitable products, (iv) oxidation of water
into O2 and (v) recombination of electron-hole pairs.
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Photocatalytic rhodamine B degradation
The improvement in photoactive performance of Phx-g-C3N4 catalysts was further assessed by
investigating the degradation of RhB dye under visible-light irradiation. All phenyl grafted Phx-g-C3N4 catalysts
demonstrated higher photocatalytic activity and RhB dye was fully degraded within 25 min of visible-light
illumination (Figure 6a). Conversely, bulk g-C3N4 exhibited less than 40% degradation of RhB over 30 min of
light illumination. The RhB degradation can be fitted with pseudo second-order kinetics and calculated rate
constant (k) of Ph5-g-C3N4 and Ph100-g-C3N4 are 0.252 and 0.216 min-1, respectively (Figure S7a). Again
Ph20-g-C3N4 photocatalyst is the most efficient for RhB degradation with a rate constant (k) of 0.307 min-1. The
variation of the degradation rate of Phx-g-C3N4 samples is likely due to the difference in electronic properties
among g-C3N4 matrix. To note, increasing the content of phenyl urea behind the optimum level may induce
deformation of tri-s-triazine core and results in poor performance. Thus, higher RhB degradation activity of
Ph20-g-C3N4 sample is ascribed to the optimum content of phenyl urea. Further, the control experiment
performed in the absence of photocatalyst displayed negligible degradation of RhB, suggesting that the
degradation is catalytic in nature. It is well known that RhB degradation occurs via two pathways.[49] In first,
direct cleavage of conjugated chromophore with timely decay in absorbance and the second path involves the
formation of N-de-ethylated intermediates, which induces the blue or hypsochromic shift of characteristic
absorbance maxima.[50] As shown in Figure S7b, the UV–visible absorption spectrum of RhB shows timely
decay in absorbance at 553 nm, which corroborates the direct cleavage of C=C and C=N bonds in the
conjugated chromophore.[51] It is well known that RhB degradation kinetic mainly depends on surface area of
the catalyst, optical absorption and lifetime of charge carriers.[52] In the present system, integration of electron-
rich phenyl urea into g-C3N4 framework strongly influenced the surface area and lifetime of photo-excitons,
which drastically enhanced RhB degradation.
Table 1. Specific surface area, energy band gap and RhB degradation parameters.
BET surface area
(m2 g-1)
RhB degradation rate
constant (k), (min-1)
0.038
Composition
Energy Band Gap (eV)
g-C3N4
9.55
2.72
2.66
Ph20-g-C3N4
94.74
0.307
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Figure 6. (a) Change in relative concentration of RhB as a function of irradiation time and (b) Effect of
charge-carrier quenchers on RhB degradation over Ph20-g-C3N4 catalyst. The bars indicate percentage of RhB
dye degradation after 25 min light irradiation.
To identify the role of reactive oxidative species (ROS) involved in rapid photodegradation of RhB,
charge-trapping experiments were performed. Use of p-benzoquinone as a scavenger effectively suppressed
O⦁
)
is the main ROS in the system and plays a
-
the RhB degradation rate, implying superoxide radical anion (
2
major role in fast mineralization of RhB. Although, other scavengers showed moderate effect on the rate of
O2⦁
-
RhB degradation, the
quenching was pronounced (Figure 6b). Under visible light irradiation, photo-
O⦁
-
[53]
induced electron transfer from CB of Ph-g-C3N4 to the dissolved oxygen results in the formation
.
This
2
reaction is thermodynamically feasible in the present system due to the fact that the CB of Ph-g-C3N4 (CB: -
O⦁
-
1.01 eV vs. NHE) is more negative than the standard reduction potential of O2/
(-0.046 vs. NHE).[54] In
2
contrast, the VB (+1.65 V vs. NHE) reduction potential of Ph-g-C3N4 is lower than the reduction potential of
OH-/OH⦁ (+2.7 V vs. NHE), and thus cannot produce OH⦁ upon reaction of H2O with VB holes.[55] Based on the
scavenger experiments, the possible repetitive reactions occurring on the surface of catalyst are proposed.
The formation of these highly active ROS resulted in the rapid decomposition of RhB. Henceforth, the
O2⦁
effective species responsible for RhB degradation was
- , OH⦁ and holes have a minor effect on the
degradation rate.[56] Alternatively, a possible photosensitized electron transfer mechanism occurring on the
surface of Ph20-g-C3N4 catalyst is likely to occur (Figure 7).
Ph
g
C N + h푣 푒[-CB] + ℎ[+VB]
- →
3 4
-
(i)
O2 + 푒[-CB] O⦁
-
→
-0.2 V
(ii)
2
O⦁2 + 2 H2O + 2푒[-CB] 3OH - + OH⦁
-
→
+0.2 V (iii)
+1.3 V
OH - + ℎ[+VB] OH⦁
→
(iv)
(v)
OH⦁ + RhB CO2 + H2O
→
ℎ[+VB] + RhB CO2 + H2O
→
(vi)
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DOI: 10.1039/C8CY02220H
Figure 7. The schematic mechanism of photosensitized electron injection and formation of hydroxyl radicals
by the excitation of Ph-g-C3N4 under visible light irradiation.
Recyclability test of catalyst
Recyclability study unravels the stability of catalyst for practical application. Repetitive RhB
photodegradation experiments were performed using the same recycled catalyst. After each cycle, the
photocatalyst was separated by centrifugation and repeatedly washed with deionized water, acetone and
dried in air oven for 24 h. Interestingly, Ph20-g-C3N4 catalyst was robust and retained high photoactivity for four
cycles (Figure S8a). The robustness of catalyst was also assessed by cyclic CO2 reduction experiments using
the same catalyst under identical conditions. As shown in Figure S8b, the methanol generation after five
cycles was almost the same as of fresh catalyst. Furthermore, FT-IR analysis of the samples recorded after
each cycle shows strong bonding modes of tri-s-triazine ring (Figure S8c), suggesting high structural stability
of catalyst for long-term practical applications. To determine the structural changes in Ph-g-C3N4 catalyst after
five cycles, PXRD and SEM image of recycled catalyst were recorded (Figure S8d, e), which demonstrated
characteristic features of pristine g-C3N4 and 2D morphology of catalyst. These results reflect that the
developed catalyst is highly stable and efficient towards CO2 to methanol photoconversion. The zeta potential
of g-C3N4 dispersed in Milli-Q water was -21.4 mV whereas that of Ph-g-C3N4 was -11.2 mV. The higher zeta
potential value of Ph-g-C3N4 further indicates its colloidal stability in the water medium at pH 7. In parallel, the
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water dispersion study of Ph20-g-C3N4 shows high colloidal stability for five days (Figure S9), suggesting the
DOI: 10.1039/C8CY02220H
easy access to active catalytic sites to promote the photoactivity.
Mechanistic aspects of photoactivity enhancement
To unravel the charge-dynamic kinetics of photo-excitons, room temperature photoluminescence (PL)
analysis was performed at an excitation wavelength of λ=360 nm. The PL spectrum of g-C3N4 displayed a
peak at 470 nm, which is due to band-to-band recombination of charge-carriers. The emission spectrum of
Ph20-g-C3N4 exhibited a blue–shifted peak at 460 nm, which indicates an up-shifting of the conduction band
caused by the depletion of charge carriers.[57] The wider energy band gap of the g-C3N4 was further confirmed
by the red-shift of the fluorescence emission spectrum by 10 nm due to quantum size effect. Further, PL
quenching suggests that recombination of radiative electron-hole pair in Ph20-g-C3N4 was greatly suppressed
due to the extra planar π-aromatic rings (Figure 8a). The charge carrier lifetimes of g-C3N4 and Ph20-g-C3N4
were measured by time-resolved PL (TRPL) via time-correlated single photon counting (TCSPC) at the
wavelength of the maximum emission peak. In the TRPL spectra, Ph20-g-C3N4 showed slower decay kinetics
compared to g-C3N4, which may be attributed to the improved electron transport and/or change in electronic
band structure induced by the π-conjugated phenyl groups (Figure 8b). The average carrier lifetime of the
bulk g-C3N4 and Ph20-g-C3N4 were 1.51 ns and 2.20 ns, respectively. The improvement in average lifetime of
charge carriers in the Ph20-g-C3N4 is due to the retarded recombination rate which indicates that extended
band tail is involved in electron-hole recombination of Ph20-g-C3N4. The band tails can act as electronic
trapping states of electron-hole pair and consequently increase the lifetime.[58] The fluorescence lifetime
imaging microscopy (FLIM) produced distinctive contrast images corresponding to the exponential decay rate
of fluorescence lifetime in g-C3N4 and Ph20-g-C3N4 samples (Figure 8i-ii). Here, the fluorescence lifetime is
the time taken by the fluorophore to remain in the excited state before returning back to the ground state.[59]
The FLIM creates contrast images based on the fluorescence lifetime of an individual component and the
average lifetime is colour coded. The high contribution from long lived fluorophore components of the Ph20-g-
C3N4 is detected in the green region. The green FL distribution can be clearly distinguished in both g-C3N4 and
Ph20-g-C3N4 FLIM images.
Figure 8. (a) Photoluminescence (PL) emission spectra and (b) Time-resolved PL lifetime decay spectra of g-
C3N4 and Ph20-g-C3N4. PL-decay plot is obtained with the excitation wavelength of 405 nm from a picosecond
pulsed diode laser at room temperature and corresponding FLIM images of (i) g-C3N4 and (ii) Ph20-g-C3N4.
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DOI: 10.1039/C8CY02220H
Conclusions
We emphasize that suitable electron rich π-organic moiety such as phenyl group can significantly alter the
physicochemical properties of g-C3N4 and can enhance its photocatalytic property. The photocatalytic activity
of Ph-g-C3N4 samples towards CO2 reduction and RhB degradation was significantly enhanced compared to
unmodified g-C3N4 photocatalyst. The copolymerisation of amino/cyano precursors with nucleophile
containing aromatic compounds facilitates incorporation of electron deficient π-extended aromatic groups into
the g-C3N4 network. The bottom-up strategy of molecular doping offers the tuning of g-C3N4 electronic
properties, increases the lifetime of charge carriers and promotes photoactivity. However, the amount of
molecular dopant is extremely critical to maintain the tri-s-triazine core structure; excess content of molecular
dopant can collapse the g-C3N4 network, resulting in poor photoactivity. In this regard, identifying relevant
functional groups to extend optical absorption property of g-C3N4 can pave new directions in metal-free
semiconductor photocatalysis. Further, high temperature of pyrolysis is a great challenge to preserve the π-
conjugation of molecular dopant. Therefore, research developments on π-extended g-C3N4 materials have
long scope and thus the present study can be of vital interest. We believe that in near future modified π-
extended g-C3N4 photocatalysts can find potential applications in solar cells and water-splitting electrodes.
Experimental Section
Synthesis of Phenyl-Urea
Phenyl urea is synthesized by adopting previously reported procedure with certain laboratory modification.[60]
In detail, about 20 g of urea and 12 g of aniline were dissolved in 100 mL of deionized water and transferred
into 250 mL of round bottom flask fitted with reflux condenser. To this reaction mixture about 5 mL of HCl (4
M) and 2 mL glacial acetic acid were instantly added and the resulting mixture was refluxed at 140°C for 2 h.
Then, the warm flask containing undissolved precipitate of diphenyl urea and hot liquid phase phenyl urea
were separated by filtration. Finally, the hot phenyl urea solution was thoroughly crystallized in ice-cold bath.
Synthesis of Phenyl grafted g-C3N4
A mixture of urea (10 g) and phenyl urea (10, 20, 50, 100 and 200 mg) were ground together for at least 30
min using mortar pestle and transferred into silica crucible. The corresponding mixture was covered with a thin
layer of melamine (1.5 g), capped and calcined in air furnace at 550°C for 2h. A schematic stepwise synthetic
procedure is given in Figure S2. The advantage of thin layer of melamine is to prevent the rapid evaporation
of contents and improve the reaction yield. Indeed, condensation performed without melamine protection gave
no product. A plausible mechanism for the formation of Ph20-g-C3N4 network is illustrated in Figure S3. The
final solid samples were grounded into fine powders and denoted as Phx-g-C3N4, where ‘x’ corresponds to the
amount of phenyl-urea content in milligrams. For comparison, pure g-C3N4 was synthesized by a direct
polymerisation of melamine under similar calcination temperature.
Acknowledgments
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D. Vidyasagar acknowledges director VNIT, Nagpur for a research fellowship. NM likesDOtoI: 1a0.c10k3n9o/Cw8lCeYd0g22e20H
HRDG-CSIR, New Delhi for the award of CSIR-Nehru Science Postdoctoral Research Fellow with their fiscal
assistance. Sachin G. Ghugal would like to thank DST-SERB National Postdoctoral Fellowship scheme, Govt.
of India for research funding (PDF/2017/002951). SSU is thankful to DST-SERB for financial assistance
through project number SB/EMEQ-052/2014SERB. Authors also thank IISc, Bangalore NMR Facility, SAIF-IIT
Madras, SAIF-Shillong, and SAIF-Chandigarh for characterization support.
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