Determination of band edges and their influences on photocatalytic reduction of nitrobenzene by bulk and exfoliated g-C3N4

Article abstract of DOI:10.1039/c8cp06855k

Thermally and chemically exfoliated metal-free semiconducting g-C₃N₄ are synthesized from bulk g-C₃N₄. Thorough characterization of the synthesized materials is performed with the help of XRD, FTIR, FE-SEM, PL, surface area analysis and DRS to probe differences in structural, morphological and optical properties between thermally and chemically exfoliated g-C₃N₄. The synthesized materials are exposed to light for photocatalytic reduction of nitrobenzene. The complete reduction reaction mechanism and product selectivity over the synthesized catalysts are studied in this report. The rate of reduction of nitrobenzene is found to be higher with thermally exfoliated g-C₃N₄, and the selectivity of aniline is found to be higher in the case of chemical exfoliated g-C₃N₄. The differences in the reactivity are explained in terms of structure, surface morphologies and band edge positions.

Full text of DOI:10.1039/c8cp06855k

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Determination of Band Edges and Their Influences on Photocatalytic
Reduction of Nitrobenzene by Bulk and Exfoliated g-C N
3
4
Swapna Challagulla, Soumitra Payra, Chanchal Chakraborty, Sounak Roy*
Department of Chemistry, Birla Institute of Technology and Science (BITS) Pilani, Hyderabad Campus,
Jawahar Nagar, Shameerpet Mandal, Hyderabad-500078, India
Abstract:
The thermally and chemically exfoliated metal-free semiconducting g-C N is synthesized from
3
4
the bulk g-C N . Thorough characterization of synthesized materials is performed with the help of
3
4
XRD, FTIR, FE-SEM, PL, Surface area analysis and DRS to probe differences in structural,
morphological and optical properties between thermally and chemically exfoliated g-C N . The
3
4
synthesized materials are exposed for photocatalytic reduction of nitrobenzene. The complete
reduction reaction mechanism and product selectivity over the synthesized catalysts are studied in
this report. The rate of reduction of nitrobenzene is found to be higher with thermally exfoliated
g-C N , and the selectivity of aniline is found to be higher in case of chemical exfoliated g-C N .
3
4
3
4
The differences in the reactivity are explained in terms of structure, surface morphologies and band
edge positions.
Keywords: bluk g-C N , chemical exfoliation, thermal exfoliation, nitrobenzene, photocatalytic
3
4
reduction.
*
Corresponding Author
Email: sounak.roy@hyderabad.bits-pilani.ac.in

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1
. Introduction:
Graphitic carbon nitride, g-C N is one of the most promising semiconducting materials of
3
4
recent choice in the area of photo- and electrocatalysis, as these are environment-friendly, non-
toxic, cheap with unique optical and electronic properties, and good thermal and chemical stability
1
under ambient conditions. g-C N is built with layered tri-s-triazine units linked by planar amino
3
4
groups with a moderate band gap of ~2.7 eV. Single-step polycondensation of carbon and nitrogen
rich organic precursors like melamine, urea, thiourea, cyanamide and dicyanamide yields the bulk
2
-10
g-C N .
Though the bulk material’s band gap is ideal for photo-applications, it faces some
3
4
inherent difficulties due to high electron-hole recombination rate, and less specific surface area
1
1
and large particle size. To improve the catalytic activity of g-C N a wide variety of methods,
3
4
like doping,12-17 incorporating metals,18-23 forming the vacancies24-29 and creating heterojunctions
with other semiconductors have been employed.30-38 The strategies are effective, but requires a
foreign metal composite. Therefore, a simpler, metal free and economic approach to increase the
3
9
1
effectiveness of bulk g-C N are thermal etching, chemical exfoliation or ultrasonic
3
4
exfoliation.40,41 The exfoliated g-C N shows layered structure like graphene. The high specific
3
4
surface area and low recombination rate of charge carriers in the exfoliated materials leads to high
photocatalytic activity. Thermal exfoliation can yield nanosheets of g-C N with a very high
3
4
2
-1 39
specific surface of 306 m g . Variation of temperature in thermal exfoliation of bulk g-C N
3
4
showed different gaseous NO photo-oxidation ability.42 Porous g-C N formed by chemical
3
4
oxidation by conc. H SO of bulk g-C N , which exhibited high water dispersion stability, high
2
4
3
4
4
3,44
activity towards degradation of organic pollutants, and high rate of H evolution.
Chemical
exfoliation of bulk g-C N with conc. HNO is also found to be having ultrasensitive detection
2
3
4
3
+
2 45
limit of Cu . Though there are intensive and substantial amount of research happening on
thermal and chemical exfoliation methods, there is no consensual acceptance on the supremacy of
the one exfoliation method over other. There is hardly any comparative research between the two
4
6
popular methods of exfoliation. The thermal and chemical exfoliation may lead to different
degrees of oxidation and structural and morphological properties, which may give rise to different
band positions of the semiconductors. And in photocatalysis, the precise position of maxima and
minima may lead to different reaction mechanism, rate and product selectivity.
Therefore, in this report, we thermally and chemically exfoliated the bulk g-C N
3
4
synthesized from melamine. We explored these synthesized materials towards photocatalytic

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reduction of one of the most harmful industrial effluent nitrobenzene, which affect the respiratory,
central nervous and cardio-vascular systems.47 Photoreduction of nitrobenzene to aniline or
phenylhydroxylamine is a smart way to valorize the harmful pollutants into most useful
intermediate of food additives, pharmaceuticals, agrochemicals, etc.27-29 The detailed mechanism
of photo reduction and product selectivity are being studied. In this photoreduction, the g-C N
3
4
materials absorb light and eject the electrons from valence band to conduction band leaving the
holes in the valance band. The number of electrons and holes, their longevity, position of valance
and conduction band edges, surface porosity and morphology are the key species for the
4
8
photoreduction efficacy. To our knowledge this is the first report on a comparative study of
thermal and chemically exfoliated g-C N towards photocatalytic nitrobenzene reduction and
3
4
elucidating the reaction mechanism with respect to surface and electronic properties.
2
. Experimental:
2
.1 Materials:
Melamine and nitrobenzene were purchased from Sigma Aldrich. Sulfuric acid (H SO )
2
4
was procured from SD fine chemicals (India). Methanol (CH OH) and isopropyl alcohol (IPA)
3
were procured from Fisher Scientific. All chemicals were used as received unless otherwise
mentioned.
2
.2 Methods:
The bulk g-C N was synthesized by calcining the melamine at 550 °C for 4 h with a ramping rate
3
4
of 2.3 °C/min. The obtained g-C N was collected, and milled by using mortar and pestle (g-C N
4
3
4
3
B). For thermal and chemical exfoliation, the popular routes were chosen from literature39,46,49-55
and these can be the representative routes for the two exfoliation methods. The thermally exfoliated
g-C N was obtained by thermal etching of bulk g-C N . About 0.5 g of g-C N B was placed in a
3
4
3
4
3
4
-1
muffle furnace, and was heated up to 500 °C for 2 h with a ramping rate of 5 °C min . A light-
yellow color powder of thermally exfoliated g-C N was obtained finally. The obtained sample is
3
4
tagged here as g-C N TE. For chemical exfoliation, 0.5 g of bulk g-C N was added to 8 mL of
3
4
3
4
conc. H SO and stirred for 3 h. Then 2 mL of distilled water was added slowly. After addition of
2
4
water the temperature of solution was increased due to the exothermic process, and the resulting
suspension was poured into 100 mL of water. The precipitate was filtered and washed several
times with distilled water to obtain pH 7. The white precipitate was re-dispersed into IPA and

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ultra-sonicated for 1 h. The white suspension in IPA was dried at 60 °C for 12 h and collected and
is designated as g-C N CE in the article. The synthesis of the bulk and the thermally and
3
4
chemically exfoliated g-C N can be described as in Scheme 1. A combined method of thermal
3
4
and chemical exfoliation was also tried for comparison. The thermally exfoliated g-C N TE was
3
4
further treated with conc. H SO and stirred for 3 h for chemical exfoliation. This combined
2
4
exfoliated material is labeled as g-C N TCE.
3
4
Scheme 1: The schematic illustration of synthesis of the bulk and the thermally and chemically
exfoliated g-C N .
3
4
2
.3 Characterization:
The crystallinity of the synthesized catalysts was obtained by using Rigaku Ultima IV X-
ray diffractometer (XRD) with Cu K radiation (λ = 1.5418 Å) at a scan rate of 1°/min with step
α
size of 0.01°. The crystallite size of the synthesized materials was measured by using Debye
0
.9휆
Scherer’s formula, =
, where B is the full-width at half maxima, λ is the wavelength of the
퐵퐶표푠휃
radiation and θ is the corresponding angle. The surface morphology of the synthesized g-C N
3
4
materials was studied by using Field Emission Scanning Electron Microscopy (FE-SEM, FEI-
Apreo S). X-ray photoelectron spectra (XPS) of the synthesized catalysts were recorded on PHI
5
000 Versa Prob II (FEI Inc.) spectrometer using Al K radiation (1486.6 eV). Microtrac BEL

Corp mini-II surface area analyzer was used to measure the surface area, pore size distribution,
and N physisorption isotherms of the materials at 77 K. Degassing was carried out in vacuum at
2
1
50°C for 2 h on the synthesized g-C N materials before starting the sorption measurements.
3 4

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Photoluminescence (PL) spectra were performed on JASCO FP-6300 spectrophotometer. Diffuse
reflectance study (DRS) was carried out on JASCO V-670 UV-visible spectrophotometer.
2
.4 Photocatalytic study:
A cylindrical annular batch reactor was used for the photocatalytic reduction of
nitrobenzene. The reactor was fitted with a medium pressure mercury vapor lamp of 125 W. Water
was circulated through doubled walled borosilicate well, which is present inside the reactor vessel,
to maintain the temperature of the reaction constant. It also helps to shield the IR radiation from
the mercury vapor lamp. For each photoreduction study 50 ppm of nitrobenzene was charged with
5
0 mg of the catalyst in 100 mL (75:25-water:methanol) of solvent. Methanol was used as hole
scavenger. The progress of photoreduction of nitrobenzene was monitored by using gas
chromatograph (Shimadzu GC 2010 plus) fitted with EB-1column and FID detector at 320°C.
Each time, the injection volume was 1 µL of aliquot, and the N carrier gas flow rate was
2
-1
maintained as 50 mL min . The qualitative identification of intermediates and products was
carried out by UV-visible absorption spectra in a JASCO V-650 UV-visible spectrophotometer.
The quantitative product analysis was monitored by using liquid chromatograph (Shimadzu HPLC,
Shimadzu, Japan) equipped with a photo diode array (PDA) UV detector (Model SPDM20A,
Prominence Diode Array Detector, Shimadzu, Japan). C18 column (dimensions: 150 mm x 4.6
mm, 5 mm) was used for separation and column temperature was maintained at 30 °C. A mixture
of methanol and water (60/40 v/v) was used as mobile phase and analyses were carried out under
-1
isocratic conditions at a low rate set at 1 mL min , and the detection wavelength was 254 nm. To
measure the concentration of photogenerated holes, coumarin was converted to fluorescent
umbelliferone in presence of the synthesized catalysts and light, and the fluorescence was
measured at 332 nm. In specific, 15 mg of catalyst was added to 3.5 mL of 0.1 mM aqueous
coumarin solution and was irradiated under UV light till 150 min. After irradiation, 0.5 g of KCl
was added into the suspension and kept in dark for 12 h to get the clear solution for fluorescence
measurement.
2
.5 Electrocatalytic study:
Cyclic voltammetry (CV) was performed by an Autolab potentiostat PGSTAT128N using
a pseudo-airtight electrochemical cell equipped with conventional three electrode system. Glassy

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carbon electrode (GCE) as a working electrode, a Pt-wire as an auxiliary electrode, a saturated
2
Ag/AgCl as a reference electrode. GCE of 3 mm in diameter (0.07 cm surface area) was polished
very gradually with 1, 0.3, and 0.05 mm alumina powder until the mirror finish was obtained. GCE
was washed with water and ethanol mixture and finally with acetone and dried in air at room
temperature. To prepare the working electrodes, 2 mg catalystwas dispersed in 1 mL CH OH with
3
sonication for 30 min to form a homogeneous ink, and then 10 μL of the ink was loaded onto the
GCE. The electrode was dried at room temperature for overnight before the electrochemical tests.
All the electrochemical measurements were performed using 50 ppm of 0.5 M aqueous Na SO
2
4
solution as electrolyte with and without nitrobenzene. The solution was purged with highly
purified argon for 20 min to drive away the dissolved oxygen, and all experiments were carried
-1
out at 25±1°C. The potential was varied from 0.5 V to -1.8 V at a scan rate of 50 mVs to measure
the reduction potential of nitrobenzene by cyclic voltammetry method.
3
. Results and Discussion:
3
.1 Structure and morphology:
The XRD patterns of the synthesized bulk and the exfoliated materials are shown in Figure
. The XRD profile shows the formation of g-C N B after calcination of melamine as it has two
1
3
4
characteristic peaks at 2θ =13.0° and 27.5° corresponding to (100) and (002) planes, respectively.
The (100) plane is attributed to the in-plane structural packing of tri-s-triazine or heptazine units,
and the plane (002) is related to the conjugated aromatic rings of interlayer stacking. The
interplanar distances of d100 and d002 are calculated to be 0.680, and 0.324 nm, respectively in g-
4
6,56,57
C N B. The similar interplanar gaps is also being reported in literature.
The crystallinity of
3
4
g-C N B is very high, and the crystallite size calculated by Scherer’s formula is found to be ~ 5
3
4
nm. In case g-C N TE, and g-C N CE, the position of the highest intense peak from (002) planes
3
4
3
4
is increased by ~0.3° (d002 = 0.320 nm) from the bulk materials indicating denser packing due to
exfoliation. Due to thermal and chemical exfoliation, the nano sheets of g-C N might have
3
4
become more planarized resulting a dense packing.39,58 Further, the lesser intense (002) peaks in
the exfoliated materials compared to the bulk one could be due to the delamination of the planes,
and increase in the disorder of the interlayers indicating disruption of the bulk structure. In case of
g-C N CE, the peak at 2θ = 13° is obscured demonstrating the decreased planar size of the
3
4
heptazine layers. The position of (002) peak in g-C N TCE is significantly shifted to lower angle
3
4

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side. This could be due to wider packing of (002) due to combined exfoliation. The peak at 2θ =
3° is also obscured over in g-C N TCE. The crystallite sizes of g-C N TE and g-C N CE are
1
3
4
3
4
3
4
derived to be 5.7 and 5.3 nm, respectively. The FTIR spectra in Figure 2 further confirms the
packing structures of the bulk and exfoliated g-C N s. The exfoliated materials show similar
3
4
characteristic IR spectra as of the bulk material. The characteristic peaks in the region of 1200-
-1
39,59
1
650 cm corresponds to N-(C) and C-N=C stretching vibrations.
This confirms that the basic
3
functional units are retained after thermal and chemical exfoliation of the bulk g-C N . The sharp
3
4
-1
60
peak at 810 cm is attributed to heptazine ring vibrations. It is evident from the spectra that the
area under this peak from heptazine ring vibration is reduced in g-C N CE. This corroborates the
3
4
-1
XRD data. The broad band at 3000-3600 cm can be assigned to N-H stretching vibration. IR
spectra g-C N TCE shows similar profile like g-C N CE.
3
4
3
4
27.55
1
000
g-C N B
3
4
27.82
1
000 g-C N TE
3 4
27.83
g-C N CE
1
000
3
4
g-C N B
3
4
26.44
g-C N TCE
3
4
1
000
g-C N TE
3
4
10
15
20
25
30
35
40
2
Figure 1: XRD patterns
bulk and the exfoliated
of the synthesized
g-C N s.
g-C N CE
3
4
3
4
g-C N TCE
3
4
1000
2000
3000
4000
-
1
Wave number (cm )

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Figure 2: The FTIR spectra of the synthesized bulk and exfoliated g-C N s.
3
4
The surface morphology of synthesized materials obtained from FE-SEM is shown in
Figure 3. The g-C N B shows agglomerated lamellar sheets with rough surfaces (Figure 3a-c).
3
4
After thermal exfoliation (g-C N TE) the sheets are found to be in a thin layered fashion (Figure
3
4
3
d-f). From the figure it is clearly evident that the sheets are stacked one by one layer, and the
surface is smooth. The thickness of each sheet is ~ 30-50 nm. Figure 3g-i represents the
morphology and the microstructures of g-C N CE, which are completely different than its
3
4
analogue. There is no acute stacking of the sheets in layered fashion. It should be noted that the
100) peak arising from the heptazine layers were also not found in g-C N CE. The strong
(
3
4
oxidizing power of H SO might have caused the internal stresses, and the exfoliation altered the
2
4
shape of bulk g-C N . The spherical shaped particles have the size of ~ 30 nm. Similar morphology
3
4
was observed in g-C N TCE, due to the effect of acid treatment on the thermally exfoliated g-
3
4
C N . Further information on surface porosity and surface area are obtained from N physisorption
3
4
2
isotherm and pore size distribution plotted in Figure 4(a, b). All the g-C N samples exhibit type
3
4
IV adsorption isotherm representing the synthesized materials are microporous. The specific
surface areas, total pore volume, and mean pore sizes are tabulated in Table 1. The BET surface
area of the g-C N significantly increases after exfoliation. Interestingly, the g-C N CE shows an
3
4
3
4
2
increased surface area of 95.5 m /g, while the g-C N TE sample reaches more than six times
3
4

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2
2
higher specific surface area (up to 216.3 m /g) in comparison with g-C N B (34.9 m /g). Similarly,
3
4
the total pore volume of the exfoliated samples is also increased. Surprisingly, the surface area and
pore volume of g-C N TCE are found to be similar to g-C N B. The higher surface area and the
3
4
3
4
pore volume in the thermally exfoliated g-C N compared to that of the chemically exfoliated one
3
4
could be due to the presence of the thin lamellar sheets in g-C N TE. The obtained high specific
3
4
surface area and pore volume of exfoliated samples enhance the number of active sites and increase
the activity.
a
b
c
5
μm
3 μm
1 μm
d
e
f
5
μm
μm
μm
1 μm
500 nm
g
h
i
5
1 μm
5
00 nm
j
k
l
1
μm
500 nm
5
0
.06
g-C N B
Figure 3: FE-S
(a)
(b)
g-C N B
3
4
EM images of (a-c) g-C N B, (d-f) g-C N TE, (g-i) g-C N CE and (j-l) g-C N
3
4
3
4
3
4
3
4
3
2
2
1
1
00
TCE. g-C N TE
3
4
0.05
3
4
g-C N CE
g-C N TE
3 4
50
00
50
00
3
4
g-C N TCE
0.04
g-C N CE
3 4
3
4
g-C N TCE
3
4
0.03
0.02
0.01
5
0
0
0
0.00
.0
0.2
0.4
0.6
0.8
1.0
0
10
20
30
40
50
60
p/p
r (nm)

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Figure 4: (a) BET adsorption isotherm, and (b) pore size distribution of the synthesized catalysts.
Table 1: BET surface area, total pore volume and mean pore size of the synthesized catalysts.
3
.2 Chemical states and band structure:
The chemical states of g-C N B, g-C N TE, and g-C N CE are analyzed by the
3
4
3
4
3
4
quantitative spectroscopic technique, XPS. The survey spectra show two major peaks corresponds
to C 1s and N 1s shown Figure 5a. The chemically exfoliated g-C N shows a significant O 1s
3
4
3
-1
Materials
BET surface area
Total pore volume (cm g )
Mean pore size (nm)
2
-1
(m g )
g-C N - B
34.9
8.017
49.687
21.94
9.08
13.805
9.263
3
4
g-C N - TE
216.3
95.5
3
4
g-C N - CE
20.653
24.66
3
4
g-C N - TCE
39.5
3
4
peak indicating oxidation of g-C N CE due to acid treatment. The core level spectra of C 1s
3
4
(Figure 5b) show two peaks at 288.2 eV and 284.8 eV. The contaminated carbon is located at 284.8
2
eV, and the peak at 288.2 eV corresponds to sp bonded C to the N (N-C-N) in the g-C N aromatic
3
4
1
1
ring. The C 1s binding energy is found to be slightly increased in g-C N TE, and in g-C N CE
3
4
3
4
compared to the bulk sample. The higher binding energy of C 1s is more pronounced for g-C N
3
4
CE as the acid-oxidation might bring the extra oxygen species (see survey spectra). The
deconvoluted N 1s spectra show (Figure 5c) three N 1s peaks of 401.1, 399.9 and 398.7 eV, from
6
1
C-NH , N-(C) and C-N=C bonding structures, respectively (see scheme 1). Upon exfoliation,
x
3
2
3
the peak area of 398.7 eV from sp N is decreased, and the 401.1, 399.9 eV peaks from sp N are

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increased. This effect is more prominent over chemically exfoliated sample. According to the
surface concentration of C and N determined from XPS, it is found that the atomic ratio of C to N
decreases in the exfoliated samples. The C to N is 0.99 for g-C N B, whereas, the same is found
3
4
9
3
to be 0.79 and 0.83 for g-C N TE and g-C N CE, respectively. The elemental chemical states
3
4
3
4
regulate the band gap of the materials. The band gap of the bulk and the exfoliated samples are
obtained from the DRS by using the Kubelka–Munk method, and represented in Figure 6a. g-C N
3
4
is an indirect band gap material with allowed transition. Kubelka-Munk factor (K) is calculated by
2
using the formula, K = (1-R) /2R, where R represents the % reflectance and E stands for energy
of the incident radiation. The band gap can be determined by extrapolating lines and taken their
intersection point. The band gap of g-C N B, g-C N TE, g-C N CE and g-C N TCE is found to
3
4
3
4
3
4
3
4
be 2.78 eV, 2.99 eV, 3.04 eV and 3.03 eV, respectively. The ~0.2 eV wider band gap of the
3
9
exfoliated samples compared to the bulk are in agreement with literature. The slightly higher
band gap of the chemically exfoliated sample compared to the thermally exfoliated one could be
due to higher degrees of oxidation and oxygen content as observed from XPS. The combined
exfoliated material showed similar band gap as chemically exfoliated one. The PL spectra of the
synthesized catalysts are shown in Figure 6b. The intensity of thermally and chemically exfoliated
samples is higher than bulk material. Interestingly, g-C N TCE shows lower PL intensity than all
3
4
other catalysts. The wavelength of fluorescence emission spectrum shows a significant blue shift
of 30 nm from bulk to exfoliated g-C N . This clearly confirms the shift in the band gap, and this
3
4
is attributed to shift in the edges of both conduction and valance band by quantum confinement.

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N1s
g-C N B
3 4
(
b)
(c)
(a)
g-C N B
g-C N B
3
4
3
4
C1s
N1s
O1s
C1s
g-C N TE
g-C N TE
3 4
3
4
g-C N TE
3
4
g-C N CE
3
4
g-C N CE
3
4
g-C N CE
3
4
0
200
400
600
800 1000 1200 282
284
286
288
290
292
394
396
398
400
402
404
Binding Energy (eV)
Binding Energy (eV)
Binding Energy (eV)
Figure 5: (a) The survey spectra, and the core level spectra of (b) C1s, and (c) N1s of the
synthesized catalysts.
(
a)
(
b)
2
.0
.5
.0
.5
.0
g- C N B
3
4
600
400
200
0
g- C N B
3 4
g- C N TE
3
4
g- C N TE
3
4
g- C N CE
3
4
1
1
0
0
g- C N CE
3 4
g- C N TCE
3
4
g- C N TCE
3 4
6
5
4
3
2
400
450
500
550
600
Energy (eV)
Wavelength (nm)
Figure 6: (a) Kubelka–Munk plot from DRS, and (b) the PL spectra of the synthesized g-C N s.
3
4

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DOI: 10.1039/C8CP06855K
3
.3 Photocatalytic reduction:
The photocatalytic activity of synthesized bulk and exfoliated materials was evaluated by
studying the reduction of nitrobenzene under UV light. Prior to light irradiation, the adsorption-
desorption equilibrium between catalyst and nitrobenzene was allowed to attain by stirring the
catalyst charged reactant solution for 30 min duration in dark. The progress of the conversion of
nitrobenzene was monitored by GC, and the photocatalytic reduction profile of nitrobenzene over
the synthesized catalysts is plotted in Figure 7. It is interesting to find out that though the surface
area of g-C N B is significantly lower than the exfoliated samples, still the adsorption in dark over
3
4
the bulk materials is quite high. The exfoliated materials in spite of having higher surface area, did
not adsorb much of nitrobenzene. However, after the onset of the light, the nitrobenzene reduction
over g-C N TE is remarkably faster than the other samples. The high surface area of g-C N TE
3
4
3
4
after light exposure might have played a major role in separating the electrons and holes, which
expedited the reduction reaction. The nitrobenzene is completely removed within 60 min over g-
C N TE, while over g-C N CE, g-C N TCE and g -C N B it takes 120, 150 and 150 min,
3
4
3
4
3
4
3
4
-6
-6
-7
respectively. The initial rate of reduction is found to be 2.12×10 , 10.0×10 , 1.63×10 and
-7
-1
-1
8
.05×10 mol L min for g-C N B, g-C N TE, g-C N TCE and g-C N CE, respectively.
3 4 3 4 3 4 3 4
Though the initial rate of reduction is slower over g-C N CE, eventually the chemically exfoliated
3
4
material outperforms the g-C N B. The reduction kinetics is fitted best with first order reaction,
3
4
-1
and the rate constants are derived to be 0.023, 0.058, 0.018 and 0.019 min for g-C N B, g-C N
4
3
4
3
TE, g-C N TCE and g-C N CE, respectively (Supporting Information, Figure S1). The higher
3
4
3
4
rate of reduction of nitrobenzene over the thermally exfoliated material can directly be correlated
with its high surface area and porosity. To probe the hypothesis of higher electron-hole separation
over higher surface area materials, the generation of hole was determined in presence of light. The
Figure S2 in Supporting Information indeed shows the higher amount of hole generation over g-
C N TE corroborating the hypothesis of lesser electron-hole recombination over the high surface
3
4
area material.

Physical Chemistry Chemical Physics
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g-C N B
3
4
1.0
0.8
0.6
0.4
0.2
0.0
g-C N TE
3
4
g-C N CE
3
4
g-C N TCE
3
4
Dark
-30
0
30
60
90
120
150
Time (min)
Figure 7: The photocatalytic reduction profile of nitrobenzene over bulk and exfoliated g-C N s.
3
4
The products and intermediates of this photoreduction reaction were monitored by UV-
Visible spectrophotometer and HPLC. The Figure 8a shows the aniline formation over the bulk
and the exfoliated materials. To our surprise, the aniline formation does not at all follow the
nitrobenzene reduction sequence. There is least aniline formation over g-C N B, while g-C N
3
4
3
4
CE shows highest aniline formation followed by g-C N TCE. Nitrobenzene photoreduction was
3
4
continued to 3 h, and the highest selectivity of aniline is observed over g-C N CE and g-C N
4
3
4
3
TCE is 50 ppm and 46 ppm, respectively, which is ~100% conversion of nitrobenzene to aniline.
The selectivity of aniline formation was moderate (56 %) over g-C N TE. This suggests formation
3
4
of other product(s) or intermediate(s). The time dependent UV Visible spectra of the reaction
aliquots were collected and a representative data with g-C N B is plotted in Figure 8b (Detailed
3
4
study in Supporting Information, S3). The spectra show the formation of intermediate phenyl
hydroxyl amine (at 306 nm), which is not fully converted to aniline (at 225 nm) with time. This is
corroborated by the HPLC results in Figure 8a, which shows only ~ 8% aniline formation from
nitrobenzene. The UV-Visible profiles reveal that over g-C N CE there is lowest formation of
3
4
phenyl hydroxyl amine compared to the other two catalysts (Figure S3). From the experimental

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Physical Chemistry Chemical Physics
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DOI: 10.1039/C8CP06855K
observation, the probable mechanism of photocatalytic reduction of nitrobenzene over g-C N can
3
4
be written as below:
―
+
Step: 1
Step: 2
푔 ― 퐶 푁 +ℎ푣 → 푒 + ℎ
3 4 퐶퐵 푉퐵
퐶퐻 푂퐻 + ℎ + →퐻퐶퐻푂 + 퐻 +
3
푉퐵
NO2
NHOH
NH2
4
e-+4H+
2e +2H
-
+
Step: 3
The holes in the valance band generated upon illumination of light oxidize the methanol and
+
+
produce H ion. The electrons in the conduction band and H ions in combination can reduce
nitrobenzene in a stepwise fashion; first to phenylhydroxylamine and then eventually to aniline. It
can be concluded from this section that among the four catalysts, the thermally exfoliated g-C N
3
4
shows the fastest nitrobenzene reduction with more selectivity towards phenylhydroxylamine,
whereas, the chemically exfoliated g-C N CE shows moderate rate of nitrobenzene reduction with
3
4
more selectivity towards aniline. The chemical treatment of thermally exfoliated g-C N also
3
4
shows high selectivity to aniline. This indicates that the doubly exfoliated sample behaves similar
to that of chemically treated one.

Physical Chemistry Chemical Physics
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3
.5
.0
.5
.0
.5
.0
(
b)
g-C N B
(a)

60
50
40
30
20
10
0
3
4
Initial
min
0 min
20 min
0 min
5 min
0 min
0 min
120 min
50min
80 min
3
0
1
g-C N TE
3
4

g-C N CE
3
4
2
3
4
6
9
g-C N TCE
3
4
2

1
1
1
1
0
.5   Aniline


 Nitrobenzene
 Phenylhydroxylamine
0.0
0
30
60
90
120
150
180
200
250
300
350
400
Time (min)
Wavelength (nm)
Figure 8: (a) The formation of aniline over the bulk and the exfoliated catalysts, and (b) the time
dependent UV Visible spectra for the photoreduction of nitrobenzene over g-C N B.
3
4
3
.4 Correlation of structural, morphological and electronic properties with catalysis:
The structural and morphological studies revealed that in the chemically exfoliated sample
the layered structure perpendicular to the [100] direction due to the heptazine ring is ruptured,
whereas the same is retained in the thermally exfoliated material. The retention of the nano lamellar
structure in g-C N TE might have helped the material to attain very high surface area, which
3
4
resulted in higher rate of photoreduction of nitrobenzene. On the contrary, g-C N CE due to acid
3
4
3
oxidation had more oxygen species and higher sp N, however, it is ambiguous to comment
whether this has led to more aniline selectivity during photoreduction. To have a better insight the
different photocatalytic behavior and the reaction mechanism of the two exfoliated samples,
further electrochemical studies were conducted to probe the reduction potentials of the species in
comparison to the band maxima and minima. To measure the reduction potentials of the
intermediates and the products, the cyclic voltammetry measurement of nitrobenzene over bulk
and exfoliated g-C N were performed. The voltammograms for the reduction of 50 ppm
3
4
nitrobenzene on g-C N B, g-C N TE, g-C N CE, and g-C N TCE are shown in Figure 9a-d.
3
4
3
4
3
4
3
4
The reduction potential of g-C N B (Figure 9a) in the absence of nitrobenzene shows one
3
4
irreversible peak, which is attributed to the reduction of g-C N . Upon addition of nitrobenzene
3
4
the reduction started at -0.50 V, and then two cathodic peaks are obtained at -0.72 V peak (A) and
1.13 V peak (B). An anodic peak at -0.15 V peak (C) is observed in the return cycle. The peak
-

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(A) in the first cycle is generated by the four-electron reduction of nitrobenzene to
phenylhydroxylamine, and then phenylhydroxylamine continued to be reduced to aniline through
6
2
a two-electron process at peak (B). The anodic peak at -0.15 V (C) is obtained in reverse cycle
is possible due to the formation of nitrobenzene by the oxidation of intermediate nitrosobenzene.
However, in the second cycle, along with the existing redox signals, one new cathodic peak is
generated at -0.33 V (D) owing to the two-electron reduction of nitrobenzene to nitrosobenzene.
Similar patterns are also observed in case of g-C N TE, g-C N CE and g-C N TCE (Figure 9b-
3
4
3
4
3
4
d) showing similar redox phenomena in exfoliated g-C N . Staring from the nitrobenzene, the
3
4
electrochemical reduction generates the aniline via nitrosobenzene and phenylhydroxylamine
6
2
intermediates. The representative chronoamperometric study taking g-C N B modified electrode
3
4
at -1.2 V reveals the aniline formation, which is detected by HPLC measurement shown in
Supporting Information Figure S4. However, from the Figure 9a-d, it is evidenced that the cathodic
peak current of aniline formation in exfoliated g-C N s is higher than bulk g-C N to imply the
3
4
3
4
better aniline formation in exfoliated g-C N s.
3
4

Physical Chemistry Chemical Physics
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0
.5
.0
0
.5
.0
(a)
(b)
C
C
D
0
0
D
B
A
A
B
-0.5
-1.0
-1.5
-0.5
-1.0
-1.5
g-C N B Na SO
4
3
4
2
g-C N TE Na SO
4
3
4
2
st
g-C N B Na SO +Nitrobenzene (1 Cycle)
st
3
4
2
4
g-C N TE Na SO +Nitrobenzene (1 Cycle)
3
4
2
4
nd
g-C N B Na SO +Nitrobenzene (2 Cycle)
nd
3
4
2
4
g-C N TE Na SO +Nitrobenzene (2 Cycle)
3
4
2
4
-
1.75 -1.40 -1.05 -0.70 -0.35 0.00
0.35
-
1.75 -1.40 -1.05 -0.70 -0.35 0.00
0.35
Potential (V) vs Ag/AgCl
Potential (V) vs Ag/AgCl
0
.2
.0
(d)
(c)
C
0.5
0
C
D
0.0
-
0.2
0.4
0.6
0.8
1.0
D
B
A
A
B
-
-
-
-
-0.5
-1.0
-1.5
-2.0
g-C N TCE Na SO
4
g-C N CE Na SO
4
3
4
2
3
4
2
st
st
g-C N TCE Na SO +Nitrobenzene (1 Cycle)
g-C N CE Na SO +Nitrobenzene (1 Cycle)
3
4
2
4
3
4
2
4
nd
nd
g-C N CE Na SO +Nitrobenzene (2 Cycle)
g-C N TCE Na SO +Nitrobenzene (2 Cycle)
3
4
2
4
3 4 2 4
-
1.75 -1.40 -1.05 -0.70 -0.35 0.00
0.35
-1.75 -1.40 -1.05 -0.70 -0.35 0.00
Potential (V) vs Ag/AgCl
0.35
Potential (V) vs Ag/AgCl
Figure 9: The cyclic voltammograms for the reduction of nitrobenzene over (a) g-C N B, (b) g-
3
4
C N TE, (c) g-C N CE, and (d) g-C N TCE.
3
4
3
4
3
4
In an attempt to understand the product selectivity in photochemical reduction of
nitrobenzene, we elucidated the energy level of three g-C N s and compare them with
3
4
corresponding reduction potential of phenylhydroxylamine to aniline. The position of conduction
band minima (CBM) is calculated by using the following equation:
퐸퐶퐵푀 = ― 퐸표푛푠푒푡 푟푒푑 ― 0.49 + 4.8 푒푉
[ ]
Where 퐸표푛푠푒푡 푟푒푑 are onset oxidation potential, and reduction potentials, respectively. The
valence band maxima (VBM) is calculated from EVBM and the band gap obtained in DRS study.
The calculated values for EVBM, ECBM, onset reduction potentials for the conversion of

Page 19 of 25
Physical Chemistry Chemical Physics
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nitrobenzene to phenyl hydroxyl amine, and phenyl hydroxyl amine to aniline are presented in the
Table 2. A schematic energy level diagram is constructed in Figure 10 to compare the different
aniline formation in three g-C N s. Here the energetics of the valance and conduction bands are
3
4
represented in vacuum scale coupled with the normal hydrogen electrode (NHE). It is reported in
the literature that the reduction of nitrobenzene to phenylhydroxylamine is faster, and the reduction
to aniline from phenylhydroxylamine is slower, concluding the second step to be rate determining
step.63-65 Therefore, in the overall nitrobenzene photoreduction, the product selectivity towards
aniline would crucially depend on the favorable gap between ECBM and phenylhydroxylamine to
aniline reduction potential. From the Figure 10a it is evident that the onset potentials of
phenylhydroxylamine to aniline are positioned below the ECBM of all three synthesized catalysts.
The energy gap between CBM to onset potential of phenylhydroxylamine to aniline is found to be
high in g-C N CE (2.11 eV) compared to g-C N CE (2.05 eV), g-C N TE (2.0 eV) and g-C N
3
4
3
4
3
4
3
4
B (1.82 eV) (Figure 10b). Therefore, electron transfer from the CBM of g-C N CE to the energy
3
4
level of phenylhydroxylamine to aniline reduction would be most feasible than others. This clearly
shows that the selectivity for the photoreduction of nitrobenzene to aniline is more in g-C N CE,
3
4
followed by g-C N TCE, g-C N TE and g-C N B. As the reduction reactions are basically
3
4
3
4
3
4
acceptance of electrons by the reactant, the electron movement should be facile if the conduction
band edge is placed energetically above the reduction potential of the reactant to product. This
suitable energetic position of the donor and acceptor should enhance the product selectivity47, 66
Thus, the faster reduction of nitrobenzene is observed in g-C N TE when compared with g-C N
4
.
3
4
3
CE, g-C N B and g-C N TCE and the selectivity of product aniline is found to be higher in g-
3
4
3
4
C N CE.
3
4

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Table 2: The calculated values for EVBM, ECBM, onset reduction potentials for the conversion of
nitrobenzene to phenyl hydroxyl amine, and phenyl hydroxyl amine to aniline.
EVBM* (eV) ECBM (eV) PHA/AN (V) in ΔE (PHA to AN)
#
Material
NHE
-1.11
-0.98
-1.06
-1.05
(eV)
1.82
2.00
2.11
2.05
g-C N - B
-5.71
-5.97
-6.21
-6.13
-2.93
-2.98
-3.17
-3.10
3
4
g-C N - TE
3
4
g-C N - CE
3
4
g-C N - TCE
3
4
*
Calculated by EVBM = ECBM - Band gap from DRS
#
ΔE = Energy gap between the CBM and onset reduction potential in eV
Figure 10: The energetics of the valance and conduction bands of the synthesized catalysts are
represented in vacuum scale coupled with the reduction potentials of reactant, and intermediate
with respect to normal hydrogen electrode (NHE).

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4
. Conclusion:
The bulk and thermally and chemically exfoliated g-C N were synthesized, and
3
4
thoroughly characterized before employing as a photocatalyst for nitrobenzene reduction. The key
findings are following:
1
. The (100) planes arising from the heptazine unit are ruptured in the chemically exfoliated g-
C N due to higher degrees of oxidation, whereas the same is retained in the thermally exfoliated
3
4
material. The surface area and porosity of the thermally exfoliated g-C N is much higher than g-
3
4
C N CE.
3
4
2
. The surface area and porosity of the bulk and exfoliated g-C N did not affect the adsorption of
3 4
nitrobenzene in dark. However, the higher surface area helped separating the electrons-holes upon
exposure of light, which significantly controlled the rate of rate of photoreduction.
3
. The exfoliated materials show higher rate of nitrobenzene reduction. The thermally etched
exfoliated g-C N outperformed the rest of its analogues in reduction of nitrobenzene. The
3
4
chemically exfoliated g-C N showed highest selectivity towards aniline formation, whereas the
3
4
thermally exfoliated material showed higher selectivity towards phenyl hydroxyl amine.
. The H SO treatment of thermally exfoliated material shows improved selectivity to aniline
4
2
4
when compared with g-C N TE.
3
4
5
. The conduction band minima with respect to the reduction potentials of phenyl hydroxyl amine
to aniline were found to be determining the product selectivity.
Conflicts of interest
There are no conflicts to declare.

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DOI: 10.1039/C8CP06855K
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