Diaminotetrazine based mesoporous C3N6 with a well-ordered 3D cubic structure and its excellent photocatalytic performance for hydrogen evolution

Article abstract of DOI:10.1039/c7ta04041e

Novel nitrogen enriched diamino-s-tetrazine based highly ordered 3D mesoporous carbon nitride (MCN-9) hybrid materials with a body centered cubic Ia3d structure having high specific surface areas, large pore volumes, and tunable pore diameters were prepar

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G. P. Mane, D. Park, K. S. Lakhi, K. Ramadass, S. Joseph, W. M. Skinner, U. Ravon, K. Al-Bahily and A. Vinu,
J. Mater. Chem. A, 2017, DOI: 10.1039/C7TA04041E.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Paper
Diaminotetrazine Based Mesoporous C3N6 with Well-Ordered 3D
Cubic Structure and its Excellent Photocatalytic Performance on
Hydrogen Evolution
Received 00th January 20xx,
Accepted 00th January 20xx
Siddulu Naidu Talapaneni,a* Gurudas P. Mane,a,c Dae-Hwan Park,a Kripal S. Lakhi,a Kavitha
Ramadass,a Stalin Joseph,a William M. Skinner,a Ugo Ravon,b Khalid Al-Bahily,b and Ajayan Vinua*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Novel nitrogen enriched diamino-s-tetrazine based highly ordered 3D mesoporous carbon nitride (MCN-9) hybrid materials
with body centered cubic Ia3d structure having high specific surface areas, large pore volumes, and tunable pore
diameters were prepared by employing 3D body centered cubic KIT-6 mesoporous silica having gyroidal porous structure
and various pore diameters as the sacrificial hard templates through
a simple self-condensation followed by
polymerization reaction of aminoguanidine hydrochloride inside the nanochannels of the KIT-6 template. Characterization
results reveal that the prepared materials exhibit 3D porous structure with well-defined mesopores and possess excellent
physical parameters including high surface areas (157–346 m2 g-1), large pore volumes (0.36–0.63 cm3 g-1) and different
pore diameters (5.5–6.0 nm) and a high N/C ratio of 1.87, which is much higher than that of the ideal C3N4 (1.33). The deep
yellow colored MCN-9 with a 3D porous structure also show a good absorption properties with a tunable narrow bandgap
of 2.25-2.5 eV, which is again much lower than that of C3N4 (2.7 eV) and helps to achieve much higher photocatalytic water
splitting activity than non-porous C3N4 and other carbon nitrides under visible light irradiation.
splitting; most of them are metal oxides,3 metal (oxy) sulfides4 and
metal (oxy) nitrides5 which are often based on metal cations with d0
and d10 electronic configuration. However, highly efficient materials
1. Introduction
Hydrogen is an eco-friendly, clean, sustainable and alternative fuel
that can be produced from various renewable and non-renewable
sources including natural gas, solar, water, wind and biomass.
Among these sources, water splitting assisted by electro or
photocatalytic approach for the generation of hydrogen holds great
potential as a clean and sustainable source of energy for the future
generations and further offers near-zero greenhouse gas emissions
and helps to solve energy related issues as it uses renewable solar
energy in an efficient manner.1,2 Therefore, extensive researches
have been carried out by several research groups on the
photocatalytic water splitting for sustainable hydrogen generation.
One of the important requirements of the photocatalytic water
splitting is the semiconducting photocatalytic materials that should
have a suitable band structure in which conduction band edge is
separated from the valence band edge with a minimum band gap of
1.23 eV.
with appropriate bandgap and adequate stability for water splitting
under visible light irradiation has not been found so far and design
and development of such materials is one of the noble missions of
the researchers working in the field of materials science.6 Recently,
carbon nitride based semiconductors have received much attention
in the development of metal free photocatalysts because of their
unique band structure with a visible light response and easy
synthesis process. Among the carbon nitride materials,
semiconducting melem based polymeric carbon nitride i.e., C3N4, is
a
well-known and fascinating material, which is essentially
composed of covalently bonded C and N and shows promising
performance as a metal-free polymeric photocatalyst for water
splitting towards hydrogen production under visible light
irradiation.7 However, lack of porosity in these materials limit their
performance in various applications. Mesoporosity in carbon
nitrides was first realized by Vinu and his co-workers and
discovered world’s first mesoporous carbon nitride (MCN-1) with
ordered pores and excellent textual parameters through a simple
polymerization reaction between carbon tetrachloride and
ethylenediamine by using SBA-15 as a sacrificial hard template.8
Their superior performance also has been demonstrated in various
applications including heterogeneous catalysis, fuel cells,
adsorption and separation of bulky molecules, CO2 capturing, and
energy storage.9 Realizing the importance of mesoporosity in C3N4
materials, Antonietti and co-workers have put significant efforts to
To date, various metal based semiconductors have been
explored for the effective and efficient photocatalytic water
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fabricate C3N4 nanostructures with a ordered mesoporous graphitic
network (ompg-C3N4) and a specific surface area between 86 and
436 m2 g-1 prepared by carbonization of cyanamide or
dicyandiamide using 2D silica templates and improved their
performance towards photocatalytic hydrogen evolution from
water molecule.10 Although ompg-C3N4 samples feature excellent
physical properties and also high nitrogen content, the major
disadvantage associated with this process lies with the precursor
used, cyanamide, which is not only expensive but also highly
explosive and toxic in nature,11 thus restricting its use in large scale
production of ompg-C3N4 materials.
Scheme 1. Schematic representation for the preparation of MCN-9 using
KIT-6.
2. Experimental
2.1 Materials
Tetraethyl orthosilicate (TEOS, 98%), triblock copolymer
In the continued search for more effective and practical
applications of CN materials, various nitrogen precursors such as
guanidine, urea, thiourea, cyanate and thiocyanate have been
thermally converted into the polymeric g-C3N4 material.12-15 It was
found that the change of nitrogen precursors significant alter the
optical and electronic properties of these materials. Recently, Vinu
et al. have reported diamino-s-tetrazine based nitrogen enriched
mesoporous carbon nitride (MCN-4) with 1D structure through the
self-condensation of aminoguanidine hydrochloride followed by
polymerization reaction inside the mesoporous channels of
hexagonal SBA-15 template. The obtained material has a well
ordered mesoporous structure with tunable pore diameters, and
band gaps, and a high nitrogen content (C3N6), and registered
excellent catalytic activity towards Friedel-Crafts hexanoylation of
benzene to yield benzophenone.16 However, the 1D porous
structure in the MCN offers only low specific surface area and pore
volume which limit their potential use in different applications. It is
expected that MCN with 3D porous structure can offer much better
textural parameters than the 1D structure, which provides more
active sites, and offers more resistance to pore blocking. These
features offer better mass transfer of the reactant molecules in the
pore channels and provide easy and direct access for guest
species.17-20
poly(ethylene
glycol)-block-poly(propylene
glycol)-
block-
poly(ethylene glycol) (Pluronic P123, molecular weight 5800 g mol-1,
EO20PO70EO20, 30 Wt.%), aminoguadine hydrochloride (97%),
concentrated hydrochloric acid (Conc HCl, 36%), triethanol amine (≥
99.0%), chloroplatinic acid hexahydratewere (H2PtCl6.6H2O,
≥
37.50% Pt basis), ethanol (99.8%) and hydrogen fluoride (HF, 35%)
were obtained from Sigma Aldrich. Double distilled water was used
throughout the synthesis. All the chemicals were used without
further purification.
2.2 Preparation of mesoporous 3D KIT-6 silica template with
different pore diameters
Preparation of mesoporous 3D KIT-6 silica template with different
pore diameters: KIT-6 with different pore diameters were
synthesized by using a P123 and n-butanol mixture as the structure
directing agent at different synthesis temperatures. In a typical
synthesis, 4.0 g of the amphiphilic triblock copolymer was dispersed
in water (144 g) and HCl solution (7.9 g) and stirred for 4 h. After
that, 4.0 g of 1-butanol and TEOS (8.6 g) were added at once to the
above homogeneous solution under stirring and stirring was
continued at 35°C for 24 h. Subsequently, the reaction mixture was
aged at 100°C for 24 h under static condition. The white solid
product was filtered without washing under hot condition and dried
at 100°C for 24 h in air oven. Finally, the product was calcined at
540°C in air to remove the template. KIT-6 silica template materials
with different pore diameters were synthesized at the synthesis
temperatures of 100, 130 and 150°C. The samples were labeled KIT-
6-X where X denotes the synthesis temperature.
Considering the advantages of 3D mesoporosity in CN materials,
herein we demonstrate for the first time the preparation of a novel
and highly ordered diamino-s-tetrazine based mesoporous carbon
nitride (MCN-9) with 3D body centered cubic porous structure and
tunable textural parameters such as specific surface area, specific
pore volume and pore diameters through the self-condensation
followed by polymerization of nitrogen rich single molecular
precursor aminoguanidine hydrochloride (NH2NHCNHNH2·HCl) by
using 3D mesoporous KIT-6 with tunable pore diameters as
sacrificial hard templates. We also report that the pore diameter,
pore volume and the specific surface area of the MCN-9 increase
with increasing the pore diameter of the KIT-6 template. The
prepared samples are deep yellow in color and exhibit very good
light absorption properties with a band gap of 2.25–2.49 eV, which
is lower than the melon based graphitic C3N4 band gap of 2.7 eV,
2.3 Synthesis of diamino-s-tetrazine based bulk non-porous C3N6
and MCN-9 materials with 3D mesostructure and different pore
diameters
MCN-9 materials with 3D body centered cubic porous structure and
various textural parameters were prepared by using 3D
mesoporous silica KIT-6-X with different pore diameters as
templates. In a typical synthesis, aminoguanidine hydrochloride (4.0
g) dissolved in water (3.0 g) was added to the calcined KIT-6 (1.0 g).
The resultant gel was heated at 100°C for 6.0 h and further heated
at 160°C for another 6.0 h in a programmed oven. A set of the three
samples were prepared using KIT-6 materials with various pore
diameters, which were synthesized at 100, 130 and 150°C
temperatures, and the samples were labeled as MCN-9-X, where X
indicates the synthesis temperature of the mesoporous silica KIT-6
template. The CN/KIT-6 composites were then heated in a nitrogen
flow of 100 mL per minute to 400°C with a heating rate of 3°C min-1
and kept under these conditions for 5 h for polymerization. The
suggesting that MCN-9 absorbs light over
a wider range of
wavelengths than graphitic C3N4 material. The photocatalytic
activity of MCN-9 has been evaluated by photochemical reduction
of water in the presence of an electron donor with visible light (λ >
420 nm). It has been found that MCN-9 registered superior activity
towards H2 evolution over nonporous CN material, 2D mesoporous
MCN and other carbon nitrides under visible light irradiation.
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MCN-9 materials were recovered after dissolution of the silica constant, h is the Planck constant, c is the speed of light, S is the
framework in 5 wt% hydrofluoric acid by filtration, washed several irradiation area, P is the intensity of irradiation light, t is the
times with ethanol, and dried at 100°C. Nonporous CN was photoreaction time and λ is the wavelength of the monochromatic
prepared using the above conditions except for the addition of the light.
mesoporous silica template in the synthesis mixture. Bulk non-
porous C3N6 was prepared without any external template using 2.5 Characterization
aminoguanidine hydrochloride precursor and carbonized it at The powder X-ray diffraction (XRD) patterns of MCN-9 materials
400 °C with a heating rate of 3 °C min-1 and kept under these were collected on a Rigaku diffractometer using Cu Kα (λ = 0.154
conditions for 5 h.
nm) radiation. The diffractograms were recorded in a 2ϴ range
from 0.6 to 10° with a 2ϴ step size of 0.0017 and a step time of 1 s.
Nitrogen adsorption and desorption isotherms were measured at -
2.4 Electrochemical measurements
Electrochemical measurements were performed in a conventional 196 °C on a Quantachrome Autosorb 6 sorption analyzer. All
three electrode cell, using a Pt plate and a Ag/AgCl electrode (3 M samples were outgassed at 250 °C for 6 h prior to the nitrogen-
KCl) as counter electrode and reference electrode, respectively. 0.2 adsorption measurements. The specific surface area was calculated
M Na2SO4 aqueous solution was used as an electrolyte without any using the Brunauer–Emmett–Teller (BET) method. The pore size
additive and was purged with nitrogen gas for 2 h prior to the was obtained from the adsorption branch of the nitrogen isotherms
measurements. The working electrode was prepared on the glassy using the Barrett–Joyner–Halenda (BJH) method. The morphology
carbon electron. In the typical electrode preparation, 5 mg of the of the MCN-9 materials prepared under different synthesis
MCN-9-150 has been dissolved in 0.8 ml of absolute ethanol, then conditions was observed on a Hitachi S-4800 field emission
10 µl of nafion was added to the above mixture and then kept this scanning electron microscope using an accelerating voltage of 5.0
sample under sonication for 30 min to get slurry. 1 µl of the slurry kV. The HRTEM images were obtained using a JEOL-3000F and a
has been deposited on the glassy carbon electrode and dried it at JEOL-3100FEF field emission high-resolution transmission electron
100°C prior to analysis.
microscope equipped with a Gatan-766 electron energy-loss
spectrometer. The preparation of the samples for HRTEM analysis
involved sonication in ethanol for 2 to 5 min and deposition on a
2.5 Photocatalytic reaction conditions
Photocatalytic reactions were carried out in a Pyrex top irradiation copper grid. The accelerating voltage of the electron beam was 200
reaction vessel connected to a glass closed gas circulation system. kV. Elementary analysis was done using a Yanaco MT-5 CHN
H2 production was performed by dispersing 0.1 g well ground analyzer. The FT-IR spectra of the MCN-6 materials were recorded
catalyst powder in an aqueous solution (100 mL) containing on a Nicolet Nexus 670 instrument by averaging 200 scans with a
triethanolamine (10 vol.%) as sacrificial electron donor. Pt was resolution of 2 cm-1, measuring in transmission mode using the KBr
photodeposited on the catalysts using H2PtCl6 dissolved in the supported pellet technique. Electrochemical measurements such as
reactant solution. The reactant solution was evacuated several Mott-Schottky plots were performed on a CHI 760C electrochemical
times to remove air completely prior to irradiation under a 300 W workstation.
Xe lamp and a water cooling filter. The wavelength of the incident
light was controlled by using an appropriate long pass cut-off filter.
3. Results and discussion
The temperature of the reactant solution was maintained at room
temperature by a flow of cooling water during the reaction. The
The optimized hard template synthesis process for the preparation
evolved gases were analyzed by gas chromatography equipped with
of MCN-9 through the self-condensation followed by
a thermal conductive detector.
polymerization reaction of aminoguanidine hydrochloride using KIT-
The apparent quantum efficiency (AQE) for H2 evolution was
6 template is shown in Scheme 1. It should be mentioned that a
performed in the same experiment set-up but with a 405 nm
constant amount of aminoguanidine hydrochloride was introduced
semiconductor diode laser as incident light source. The irradiation
in the mesopore channels of the KIT-6 templates irrespective of
area was 7.92 cm2. The total intensity of irradiation was determined
their pore diameter. The structural analysis of the prepared
by averaging 40 points of the irradiation area. The average intensity
materials was examined by powder X-ray diffraction (XRD) analysis.
was estimated to be 4.24 mW cm-2 (ILT 950 spectroradiometer).
Figure 1 displays the lower and wide angle powder XRD patterns of
The AQE was calculated by using the following equation:
the MCN-9-X materials where X denotes the synthesis temperature
of the KIT-6 template. As can be seen in Figure 1, all the samples
exhibit a peak at lower angle 1° and a few higher angle reflections
Ne
ꢀꢁ
2 × ꢂ × ꢀꢃ × ℎ × ꢄ
ꢅ × ꢆ × ꢇ × ꢈ
between 1 to 3° indicating the presence of highly ordered 3D
porous structure similar to the parent KIT-6 template. The high
intense peak at around 1° can be indexed as (211) Bragg reflection
of the body centered cubic mesostructure with Ia3d space group.
AQE =
× 100% =
× 100%
Where, Np is the total incident photons, Ne is the total reactive
electrons, M is the amount of H2 molecules, NA is Avogadro
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Table 1. Textural parameters and C, H, N, S analysis of 3D body centered cubic MCN-9 materials.
d(211) Spacing
(nm)
From CHNS (Wt%)
H
Unit cell
a0 (nm)
ABET
(m2.g-1)
Vp
dp, BJH
(nm)
(cm3.g-1)
Material
C
N
N/C
1.87
2.02
2.03
2.08
MCN-9-100
MCN-9-130
MCN-9-150
Bulk C3N6
8.0
9.0
9.2
-
19.6
22.04
22.53
-
157
268
346
10
0.36
0.55
0.63
0.02
3.02
4.6
4.7
-
29.9
27.8
28.6
29.4
55.24
56.4
57.8
61.2
2.6
2.6
2.75
2.72
It should be noted that the 3D mesostructure and structural region, which provides evidence of an increase of the d-spacing and
order of the materials is significantly affected as the pore diameter the unit-cell parameter (Table 1). The unit-cell constants for MCN-9-
of the KIT-6 template is increased. For example, a broad (211) 150, MCN-9-130 and MCN-9-100 are calculated to be 22.53, 22.04,
reflection with a significant reduction in the intensity is observed and 19.6 nm, respectively. To check the crystalline structure and
for MCN-9-100, revealing a reduction in a structural order in the the graphitic character of the MCN-9, the materials were
MCN-9-100. The presence of disordered mesoporous structure in characterized by wide angle powder XRD measurement. Figure 1
MCN-9-100 sample may be attributed to the smaller pore diameter (inset) shows the wide angle powder XRD patterns of MCN-9
together with small interconnected pores in the KIT-6-100 template materials. All the samples exhibit a relatively sharp reflection at the
which does not provide enough space for the formation of a high 2θ value of 26.94° corresponding to the interlayer distance of 0.33
molecular weight polymeric network based on the diamino-s- nm, which can be attributed to the interlayer distance of the carbon
tetrazine moieties. In addition, these low molecular weight nitride sheets (resembling the (002) reflection of graphite) due to
polymers formed inside the pores of the KIT-6-100 may be the intrinsic atomic arrangement of the C and N in the CN wall
disintegrated at high temperature annealing process, leading to a structure. However, the peak is much broader than that observed
slight destruction in the structural order of the final MCN-9-100. On for the pure nonporous carbon nitride nanostructure which can be
the other hand, the intensity of the (211) reflection for MCN-9-130 attributed to either a reduced correlation length due to the primary
and MCN-9-150 is significantly higher than that of MCN-9-100, mesoporous channels or the intra-layer periodicity of the CN layers.
indicating a significant improvement in the mesoporous structural
Mesoporosity and further information on the nature and the
order of these materials. Thus, it can be concluded that pore size of pores in MCN-9 materials were also obtained from the
diameter of the template plays a critical role in controlling the nitrogen adsorption-desorption measurements. Figure 2A shows
mesostructure of the MCN materials and the template materials the N2 adsorption–desorption isotherms of MCN-9 prepared by
having the largest pore diameter favors the formation of well- employing the KIT-6 template with different pore diameters. The
ordered MCN mesostructured. Similar effect has also been textural parameters such as the specific surface area, specific pore
observed for the MCN reported previously.16,18,20
volume and the pore diameter of the MCN-9 samples are also
summarized in Table 1. Except for the MCN-9-100 sample, all the
other samples show type IV adsorption isotherm, according to the
IUPAC classification and featurecapillary condensation with a small
hysteresis in the mesopores, which indicates the presence of well-
ordered mesopores in the samples. The position of the capillary
condensation step shifted from lower relative pressure to higher
relative pressure with increasing the pore diameter of the template.
This reveals that the pore size of the MCN-9 is increased with
increasing the pore size of the KIT-6 template, which is quite
consistent with the unit cell expansion observed from the lower
angle powder XRD patterns in Figure 1 and the corresponding
increase in the d-spacing (Table 1).
The amount of nitrogen adsorbed on the MCN-9-150 is more
than that of the MCN-9-130 and MCN-9-100. The specific surface
area and specific pore volume increased from 157 to 346 m2 g-1 and
0.36 to 0.63 cm3 g-1, respectively. Noteworthy, the capillary
condensation step in the isotherm of the MCN-9-100 sample is
neither sharp nor clear, indicating the presence of disordered
mesoporous structure in the sample, although KIT-6-100 (Figure S1
and Table S1 in the supporting information), exhibits a well-ordered
mesostructure and excellent textural parameters. These results
confirm that MCN-9-100 material possesses disordered
mesoporous structure and the replication process is not successful
when KIT-6-100 template (8.0 nm) is used. On the other hand, KIT-
6-130 and KIT-6-150 templates with 9.9 and 11.3 nm (Table S1)
Fig. 1 Lower angle and wide angle powder XRD (inset) patterns of (a) MCN-9
-100 (b) MCN-9-130 and (c) MCN-9-150 prepared by using different pore
diameters of KIT-6 silica templates.
It can be also observed that an increase in the pore diameter of the
templates causes a shift of the (211) peak towards the lower-angle
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pore diameters yielded a faithfully replicated structure of MCN-9- structure of the MCN-9 is changed from ordered to disordered as
130 and MCN-9-150 with an excellent mesoporous structural order. the pore diameter of the template is decreased. MCN-9-100 (Figure
This is quite consistent with the data obtained from XRD analysis. 3(a-b)) sample shows disordered mesoporous structure with a lot of
From these results, one can conclude that the 3D template with the defect sites and cracks along the wall structure which is consistent
large pore diameter is critical to obtain well-ordered mesoporous with the lower angle powder XRD and nitrogen sorption
MCN-9 materials through the nanotemplating process.
measurments showing the decrease in the long range
mesostructural order. On the other hand, the TEM images of MCN-
9-130 and MCN-9-150 materials clearly display highly ordered
mesoporous structure with a linear array of mesopores, which are
arranged in regular intervals. These well ordered mesopores are
originated from the mesoporous silica wall structure that would
convert into the mesopores of the resultant carbon nitride after the
HF treatment. The cross sectional TEM images of MCN-9-130 and
MCN-9-150 samples clearly show the presence of well ordered 3D
body centered cubic mesostructure, which is almost same as that
reported for the 3D mesoporous silica template, KIT-6 (Figure 3c
and Figure 3e). This clearly demonstrates that 3D mesoporous
carbon nitride with body centered cubic Ia3d structure type has
been truly replicated from the KIT-6 mesoporous silica template. It
should also be noted that the lattice fringes of crystalline CN
network were not observed for any of the samples, revealing that
the samples are partially amorphous in nature. It is also confirmed
by the TEM images that the pore diameter and the unit cell
constant of MCN-9-150 are larger than those of MCN-9-100 which is
again supported by the data obtained from porosity and lower
angle powder XRD measurements. The morphology of the MCN-9
materials having different textural parameters was also observed by
high resolution scanning electron microscopy (HRSEM) and the
images are shown in Figure S2. As can be seen in Figure S2, all the
samples show the morphology similar to that of the KIT-6 template
further confirming the successful replication process.
Fig. 2 (A) Nitrogen adsorption–desorption isotherms and (B) BJH pore-size
distributions of MCN-9 materials with different textural parameters
synthesized by employing 3D KIT-6 having various pore diameters (open
symbols: desorption, closed symbols: adsorption); MCN-9-100 (circles),
MCN-9-130 (diamonds) and MCN-9-150 (squares).
Figure 2B shows the BJH adsorption pore size distribution of MCN-
9-100, MCN-9-130 and MCN-9-150. The change in the pore
diameter of the MCN-9 materials upon increasing the pore
diameter of the template is clearly evident from the pore-size
analysis. All of the samples show a main peak that mainly comes
from the mesopores formed after dissolution of the silica matrix
from the template. It should also be mentioned that the full-width
half-maximum of the BJH adsorption pore-size distribution of MCN-
9-150 is larger than that of MCN-9-100 and MCN-9-130. Contrary to
the MCN-4 materials, the expansion of pore diameter of the MCN-9
materials was limited up to 4.7 nm. This could be mainly due to the
perfect pore filling of the CN polymer matrix in the 3D ultra large
mesopores of KIT-6 materials than the unidimensional hexagonal
mesopores of SBA-15 materials.
The surface topology of the MCN-9 materials with different
structural order and pore diameters can be observed from TEM.
The representative TEM images of MCN-9 materials are shown in
Figure 3. These images clearly indicate that the mesoporous
Fig. 3 HRTEM images of (a-b) MCN-9-100, (c-d) MCN-9-130 and (e-f)
MCN-9-150.
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Figure 4 shows the electron energy loss (EEL) spectra of the materials calculated from the EDX analysis is found to be ca. 1.9,
MCN-9 materials having various textural parameters. The nitrogen which is much closer to the values of the N/C ratio obtained from
to carbon atomic ratio calculated from the EELS analysis for all the CHNS analysis (Table 1) and from the EELS measurement (N/C =
samples is found to be 1.9±0.1, which is very close to the N/C 1.9±0.1), confirming a high nitrogen content in the samples similar
atomic ratio observed for previously reported diamino-s-tetrazine to the MCN-4 materials. Elemental mapping of C and N atoms in the
based MCN-4 samples with 2D hexagonal porous structure. These MCN-9 samples is shown in Figure S3. The data reveal that the
results are in good agreement with the data obtained from the carbon (C) and nitrogen (N) are uniformly distributed throughout
CHNS analysis (Table 2). The shape of the EEL spectra of all the samples which suggest that the walls are composed of several
samples is almost similar and features a sharp C and N K-edge peaks. layers of periodic CN network with the diamino-s-tetrazine moieties,
The C K-edges of the spectrum are located at 292.2 and 301.0 eV as observed by the wide angle powder XRD measurements. No
which can be attributed to the 1s-π* and 1s-σ* electronic transition other element was found in the elemental mapping, indicating that
of sp2 hybridized carbon trigonally bonded to the nitrogen.20,21 The the material is mainly composed of C and N.
N K edges are seen at ca 410.8 and, 422.0 eV which may be
attributed to the nitrogen atoms sp2-hybridized with carbon and
nitrogen, respectively.16,20,21 These results further confirm the
presence of sp2 hybridized carbon and nitrogen throughout the
sample. This was possible only because of the fact that the
formation of diamino-s-tetrazine based aromatic moieties in the
mesoporous wall structure. Interestingly, the shape of the C and N K
edges in the spectrum is quite similar, suggesting a presence of
strong bonds between C and N atoms in all of the MCN-9 samples.
Moreover, the intensity and the shape of the π* and σ* peaks are
quite consistent in all of the MCN-9 samples irrespective of the pore
diameter, indicating the presence of tetrazine moieties in all of the
products.
Fig. 5 Energy dispersive X-ray spectroscopy (EDX) analysis of (a)
MCN-9-100, (b) MCN-9-130, and (c) MCN-9-150.
In order to check the information concerning the nature of C and N
bonding and the amount of carbon and nitrogen content, the MCN-
9 materials were characterized by XPS and FT-IR spectroscopy. The
XPS survey spectrum and the separated C1s and N1s spectra of
MCN-9-150 are shown in Figure 6. The description of C1s and N1s
peaks is also summarized and tabulated in the Table 2. The XPS
survey spectrum (Figure 6A) shows the presence of C, N, and O. The
presence of a little amount of oxygen can be attributed to the
moisture, ethanol, atmospheric O2, or CO2 adsorbed on the surface
of MCN-9-150. The overall atomic nitrogen to carbon ratio of MCN-
9-150 determined by XPS is found to be 1.9, which is in close
agreement with the results obtained from EELS, EDX and CHN
Fig. 4 Electron energy loss spectra (EELS) of diamino-s-tetrazine based 3D
cubic mesoporous carbon nitride (MCN-9) materials with various textural
remarkable considering the poor thermodynamic stability of N in
parameters prepared from KIT-6-X templates: (a) MCN-9-100, (b) MCN-9-
analysis showing a high content of nitrogen. This is quite
the carbon matrix at a high temperature. This is mainly due to that
130, and (c) MCN-9-150.
fact that the CN framework composed of diamino-s-tetrazine
Chemical composition and purity of the MCN-9 materials were
moieties is quite stable at high temperature, providing a high
further confirmed by energy dispersive X-ray (EDX) analysis done at
thermal stability. The XPS C1s spectrum of MCN-9 shows a major
different locations of the samples and elemental mapping. Figure 5
peak centered at 287.6 eV and two minor peaks at about 284.6 eV
shows the energy-dispersive X-ray (EDX) pattern of MCN-9 samples
and 293.1 eV. The main peak at 287.6 eV is very sharp with a high
with different pore diameters and their corresponding elemental
intensity and is assigned to sp2 carbon atoms bonded to three
mapping. Peaks for the elements C and N are clearly seen in the
neighboring nitrogen atoms.9,22 A similar peak was also assigned to
EDX spectrum. The peak corresponding to O was not observed in
the diamino-s-tetrazine based 2D MCN-4 material. The minute peak
the EDX spectrum, indicating that the sample is highly pure and free
with lowest energy contribution at 284.6 eV is attributed to sp2
from any contamination from either the atmospheric oxygen or
carbon atoms in the sample. On the other hand, the highest energy
oxynitrides or amorphous silica template. The N/C ratio of MCN-9
contribution at 293.1 eV corresponds to sp2 hybridized carbon in
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the aromatic ring attached to terminal uncondensed NH2 groups.
The N 1s spectrum of MCN-9-150 was mainly deconvoluted into
three main peaks centered at binding energies of 398.3 eV, 400.2
eV and 403.5 eV. The main peak centered at 398.3 eV is attributed
to nitrogen atoms bonded to sp2 carbon atoms in the diamino-s-
tetrazine based CN network. The peaks at higher binding energies
at about 400.2 and 403.5 eV are ascribed to the nitrogen trigonally
bonded to the carbon atoms and the N=N bonding configuration,
respectively, which is due to the presence of diamino-s-tetrazine
based nitrogen atom arrangement in the MCN-9 wall structure.23
These results are quite consistent with the results obtained for the
C1s spectra.
Fig. 7 FTIR spectra from (a) MCN-9-100, (b) MCN-9-130, and (c) MCN-9-150
samples.
The surface chemical bonding state of the MCN-9 materials was
characterized by FT-IR spectroscopy. Figure 7 shows the FT-IR
spectra of MCN-9 samples having different textural parameters. All
the samples show similar IR bands suggesting that the nature and
the coordination of the MCN-9 materials are not affected by the
pore diameter of the template used in the preparation process. The
IR bands in the region of 740-790 cm-1 are attributed to the syn and
antiphase vibrations of N=N of the tetrazine ring. The peaks at 1321
and 1421 cm-1 are the characteristic of C=N and N=N stretching
modes of tetrazine moieties present in the MCN-9.15 Another band
at 1574 cm-1 is assigned to the graphite-like C-N band, mainly
originated due to the aromatic ring modes of diamino-s-tetrazine
molecule. Higher order absorptions at 3192 and 3327 cm-1 are
ascribed to the terminal N-H stretching of uncondensed amino
groups attached to the sp2 hybridized carbon and the O-H
stretching of the H2O molecules adsorbed on the surface of the
sample. A very weak absorption at 2168 cm-1 is also present in the
spectrum, which may be attributed to N=C=O stretching due to the
contaminated oxygen either from atmospheric O2 or CO2. Thermal
stability of the MCN-9 samples in the presence of air was studied by
TG-DTA and the result is shown in Figure S4. As shown in the TG
curve, the MCN-9 sample is stable upto 450°C in the presence of air
but decomposes slowly upon increasing the temperature above
450°C.
One of the most interesting features of the diamino-s-tetrazine
based MCN-9 materials is their narrow band gap, which has been
estimated by UV-Vis absorption spectroscopy (Figure 8). Shown in
Figure 8, intrinsic absortion edge of MCN-9 covers longer
wavelengths of visible region, indicating small band gap energy. All
of the MCN-9 samples show typical semiconducting absorption with
an estimated band gap in the range of 2.2–2.5 eV, which is less than
that of the melon based graphitic C3N4 band gap (2.7 eV).
Interestingly, the UV-Vis absorption is shifted to higher wavelengths
as the pore diameter of MCN-9 decreases. Among the samples
studied, MCN-9-100 shows the lowest band gap of 2.25 eV whereas
MCN-9-150 registered the largest band gap of 2.49 eV. The
difference in the band gap of MCN-9 with different pore diameters
Fig. 6 (A) XPS survey spectra, (B) C1s XPS survey spectra, and (C) N1s
XPS survey spectra of MCN-9-150.
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could be attributed the number of CN layers in the wall structure of can have a significant effect on the electronic structure of MCN-9
samples. It is expected that MCN-9-150 has thicker walls as such as band gap and band position, and their performance in
compared to MCN-9-130 and MCN-9-100, causing a significant various applications including photocatalysis. To determine the
difference in the final band gap of the materials.
band gap structure of MCN-9 samples, we have carried out the
electrochemical Mott-Schottky measurements and XPS valence
band spectra (ESI, Fig. S6), which revealed typical n-type
characteristics for the MCN-9-150 semiconductor. From the Mott-
Schottky plots, we have observed the flat band potential of -0.93 eV
for MCN-9-150 sample, which is less than that of graphitic C3N4
whose conduction band edge is of -1.42 eV.31 As shown in Figure S6,
the valence band (VB) values of the MCN-9-150 and 3D mesoporous
C3N4 were approximately +1.56 and +2.08 eV respectively.
Fig. 8 Band gap data and UV-Vis spectra (inset) of (a) MCN-9-100, (b) MCN-
9-130 and (c) MCN-9-150.
The chemical structure of MCN-9 was further confirmed by the
synchrotron-based near-edge X-ray absorption fine structure
(NEXAFS). The NEXAFS data and the proposed molecular structure
of CN walls in MCN-9 are shown in Figure 9. The results are also
compared with the NEXAFS spectra of non-porous g-C3N4 prepared
from dicyandiamide precursor. As shown in the C K-edge spectra
(Figure 9A), the main characteristic peaks of carbon nitride
observed in the MCN-9 and g-C3N4 materials are as follows: π*C–N–C
(C1) at 288.9 eV and σ*C–C (C2) at 294-300 eV. As previously
reported,24-26 the heptazine based g-C3N4 structure shows three
main characteristic resonances predominantly at 399.7, 401.7, and
402.7 eV in the N K-edge spectra (Figure 9B), which are attributed
to π*C–N–C (N1: one tri-s-triazine hetero ring structure), π*N–3C (N3:
graphitic 3-fold N configuration), and π*N–3C (N4: sp3 bridging N
among three tri-s-triazine moieties, respectively (Figure 9C).
Fig. 9 (A) C K-edge and (B) N K-edge NEXAFS spectra of (a) MCN-
C3N6 and (b) g-C3N4. Schematic molecular structure of (C) g-C3N4
and (D) MCN-9 with stoichiometry of C3N6.
The photocatalytic activity of the MCN-9 samples was evaluated
for H2 production from an aqueous proton solution under visible
light irradiation (λ > 420 nm) and the results are also compared with
the photocatalytic activity of nonporous CN prepared without using
any hard template. Chloroplatinic acid and triethanolamine were
used as precursors to create H2 reduction sites (Pt nanoparticles)
and electron donors, respectively. The photocatalytic experiments
were carried out with the water-insoluble MCN-9 and non-porous
CN powders modified by 3 wt% platinum as co-catalyst. Remarkably,
MCN-9 sample shortly reached steady H2 production from water
containing 10 vol% triethanolamine as an electron donor, whereas
nonporous CN showed very poor catalytic performance. Figure 10
shows the time course of H2 evolution obtained over the MCN-9-
150 sample under visible light (λ > 420 nm) irradiation. On the
contrary, no reaction occurred in the dark even in the presence of
the MCN-9 material. MCN-9-150 shows the best H2 evolution
activity with catalytic performance of 31.5 µmol h-1 (0.0910 µmol h-
1/unit surface area), whereas non-porous C3N6 registered with poor
catalytic performance of 5.5 µmol h-1 (0.55 µmol h-1/unit surface
area). The H2 evolution on the MCN-9 was about 5 times higher
However, MCN-9 with C and N stoichiometry of C3N6 shows a
completely different C and N bonding environment in the wall
structure as presented in the N K-edge spectra and proposed
molecular structure (Figure 9D). The graphitic C-N-C bonds (N3) are
not observed in the wall structure of MCN-9. The N4 peak position
of MCN-9 is also slightly shifted to lower energy state compared to
that of g-C3N4, indicating that the sp3 bridging N of MCN-9 is
affected by electron migration from N-rich three C2N4 moieties
around the bridging N atom. It is also interesting to note that the
N2 peak (terminal C-N-H bonds) of MCN-9 shows much higher
intensity than that of g-C3N4, indicating that a high nitrogen
contents including –NH and –NH2 groups are created in the surface
edges of MCN-9 framework. These results confirm that the
molecular structure of CN in the walls of MCN-9 is completely
different from that of MCN with C3N4 stoichiometry. It is expected
that the structure with tetrazine moieties, density of these inbuilt
amine functional groups and the presence of structural defect sites
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than that of the non-porous C3N6 sample. The apparent quantum
efficiency of MCN-9-150 was calculated to be 0.21% at 405 nm
(Table S2) by using equation (1).
4. Conclusions
In summary, highly ordered three dimensional diamino-s-
tetrazine based mesoporous carbon nitride materials (MCN-9)
with a high specific surface area, a large pore volume, tunable
pore diameters and C3N6 stoichiometry were prepared
through the self-condensation followed by polymerization of
The production of H2 increases steadily with prolonged time of
light irradiation. After four consecutive runs (12 h), a total of 0.5
mmol H2 gas (11.2 mL) was produced, and no deactivation of the
photocatalysts was found, suggesting the excellent stability of
MCN-9-150 as an organic photocatalyst for solar H2 generation. This
is a very important finding as it provides the pathway for the
continuous production of hydrogen from aqueous solution and
further makes the ways for the commercialization of this
technology. It should be mentioned that no nitrogen gas was
observed, indicating the strong binding of N within the covalent
structure of MCN-9. No reaction occurred when the system was
illuminated in the absence of MCN-9-150 catalyst, which confirms
that the reaction is indeed driven by light absorption on the catalyst.
The hydrogen evolution rate of MCN-9 under visible light irradiation
is higher than that of the reported g-C3N4 materials.26-31 It is also
interesting to note that the activity of the MCN-9 is much higher
than that of mesoporous C3N6 with 2D structure, mesoporous C3N4
prepared from the KIT-6-150 template and non-porous C3N4 (Figure
S5). The excellent photocatalytic performance of MCN-9 on the
hydrogen evolution could be attributed to the combination of three
dimensional pore structure with a high specific surface area and
large pore volume, and unique tetrazine molecule structure
together with the low energy band gap which allow larger
absorption of light under visible region and more interaction with
the water molecules. We surmise that these MCN nanostructures
with unique molecular structure could also be used in various
applications ranging from organic solar-cells, organic light emitting
diodes to organic components for molecular electronics. We also
strongly believe that the photocatalytic activity of diaminotetrazine
based MCN materials could be enhanced by altering their electronic
structure with the doping of either sulphur or phosphorous or other
inorganic metal or metal oxide nanoparticles.
nitrogen
enriched
aminoguanidine
hydrochloride
(NH2NNHCNHNH2.HCl) as a single molecular precursor using
3D KIT-6 with different pore diameters as templates. High N
content and a well ordered 3D mesoporous structure in MCN-9
materials were confirmed by EDX, EELS, XPS, CHNS, lower
angle powder XRD, N2 adsorption/desorption and HRTEM
analysis. The textural parameters of the MCN-9 samples can be
finely controlled with a simple adjustment of the pore
diameter of the KIT-6 silica template. XPS, EELS and FT-IR
spectroscopy results reveal that MCN-9 was mainly composed
of diamino-s-tetrazine moieties with a N/C ratio of 1.9. The
band gap of these materials was estimated by UV-Vis spectra
and found to be in the range of 2.25–2.5 eV. The MCN-9 shows
excellent activity for photochemical reduction of water with
visible light in the presence of Pt as a co-catalyst and electron
donors. MCN-9 also registered much higher performance than
the non-porous C3N4 and C3N6, 2D mesoporous C3N4 and 2D
mesoporous C3N6. The combination of unique structure with
tetrazine moieties in the wall structure, low band gap (2.25-2.5
eV) and 3D mesoporous structure favors for the excellent
photocatalytic activity of MCN-9 for hydrogen evolution. We
believe that MCN-9 materials with exceptional textural
parameters, lower band gap and high nitrogen content could
pave the way for using these materials for various other
applications including solar cells, organic light-emitting diodes
to organic components for molecular electronics, energy
storage, capacitors, adsorption and separation of small and
large organic toxic and biomolecules, vitamins and amino
acids, and chemosensors.
Acknowledgements
A. Vinu is grateful to Australian Research Council for the Future
Fellowship Award, Bessel Humboldt Award, and SABIC for the
research grants. The authors also acknowledge the support of
Dr. Anton Tadich from Australian Synchrotron for the soft X-
ray beam time, and Dr. Wang for the photoelectrochemical
measurements.
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