C3N5: A Low Bandgap Semiconductor Containing an Azo-Linked Carbon Nitride Framework for Photocatalytic, Photovoltaic and Adsorbent Applications

Article abstract of DOI:10.1021/jacs.9b00144

Modification of carbon nitride based polymeric 2D materials for tailoring their optical, electronic and chemical properties for various applications has gained significant interest. The present report demonstrates the synthesis of a novel modified carbon nitride framework with a remarkable 3:5 C:N stoichiometry (C₃N₅) and an electronic bandgap of 1.76 eV, by thermal deammoniation of the melem hydrazine precursor. Characterization revealed that in the C₃N₅ polymer, two s-heptazine units are bridged together with azo linkage, which constitutes an entirely new and different bonding fashion from g-C₃N₄ where three heptazine units are linked together with tertiary nitrogen. Extended conjugation due to overlap of azo nitrogens and increased electron density on heptazine nucleus due to the aromatic π network of heptazine units lead to an upward shift of the valence band maximum resulting in bandgap reduction down to 1.76 eV. XRD, He-ion imaging, HR-TEM, EELS, PL, fluorescence lifetime imaging, Raman, FTIR, TGA, KPFM, XPS, NMR and EPR clearly show that the properties of C₃N₅ are distinct from pristine carbon nitride (g-C₃N₄). When used as an electron transport layer (ETL) in MAPbBr₃ based halide perovskite solar cells, C₃N₅ outperformed g-C₃N₄, in particular generating an open circuit photovoltage as high as 1.3 V, while C₃N₅ blended with MA_xFA_1-xPb(I_0.85Br_0.15)₃ perovskite active layer achieved a photoconversion efficiency (PCE) up to 16.7%. C₃N₅ was also shown to be an effective visible light sensitizer for TiO₂ photoanodes in photoelectrochemical water splitting. Because of its electron-rich character, the C₃N₅ material displayed instantaneous adsorption of methylene blue from aqueous solution reaching complete equilibrium within 10 min, which is significantly faster than pristine g-C₃N₄ and other carbon based materials. C₃N₅ coupled with plasmonic silver nanocubes promotes plasmon-exciton coinduced surface catalytic reactions reaching completion at much low laser intensity (1.0 mW) than g-C₃N₄, which showed sluggish performance even at high laser power (10.0 mW). The relatively narrow bandgap and 2D structure of C₃N₅ make it an interesting air-stable and temperature-resistant semiconductor for optoelectronic applications while its electron-rich character and intrasheet cavity make it an attractive supramolecular adsorbent for environmental applications.

Full text of DOI:10.1021/jacs.9b00144

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Article
C3N5: A Low Bandgap Semiconductor Containing
an Azo-linked Carbon Nitride Framework for
Photocatalytic, Photovoltaic and Adsorbent Applications
Pawan Kumar, Ehsan Vahidzadeh, Ujwal Kumar Thakur, Piyush Kar, Kazi Mohammad Alam,
Ankur Goswami, Najia Mahdi, Kai Cui, Guy M Bernard, Vladimir K. Michaelis, and Karthik Shankar
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00144 • Publication Date (Web): 14 Feb 2019
Downloaded from http://pubs.acs.org on February 14, 2019
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C
3
N
5
: A Low Bandgap Semiconductor Containing an Azo-linked Carbon Nitride
Framework for Photocatalytic, Photovoltaic and Adsorbent Applications
Commented [A1]: We modified the manuscript title slightly in
response to the suggestion from Reviewer#1.
1
Pawan Kumar, * Ehsan Vahidzadeh, Ujwal K. Thakur, Piyush Kar, Kazi M. Alam, Ankur
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1
Goswami, Najia Mahdi, Kai Cui, Guy M. Bernard, Vladimir K. Michaelis and Karthik
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3
3
1
Shankar *
1
Department of Electrical and Computer Engineering, University of Alberta, 9211 - 116 St,
Edmonton, Alberta, Canada T6G 1H9
2
Nanotechnology Research Centre, National Research Council of Canada, Edmonton, Canada
T6G 2M9
3
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada, T6G 2G2
*Email: Pawan Kumar (pawan@ualberta.ca); Karthik Shankar (kshankar@ualberta.ca)
Abstract
Modification of carbon nitride based polymeric 2D materials for tailoring their optical, electronic
and chemical properties for various applications has gained significant interest. The present report
demonstrates the synthesis of a novel modified carbon nitride framework with a remarkable 3:5
C:N stoichiometry (C
melem hydrazine precursor. Characterization revealed that in C
are bridged together with azo linkage, which constitutes an entirely new and different bonding
fashion from g-C where three heptazine units are linked together with tertiary nitrogen.
3
N
5
) and an electronic bandgap of 1.76 eV, by thermal deammoniation of
3 5
N
polymer, two s-heptazine units
3 4
N
Extended conjugation due to overlap of azo nitrogens and increased electron density on heptazine
nucleus due to the aromatic π network of heptazine units lead to an upward shift of the valence
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band maximum resulting in bandgap reduction down to 1.76 eV. XRD, He-ion imaging, HR-TEM,
EELS, PL, fluorescence life time imaging, Raman, FTIR, TGA, KPFM etc clearly show that the
properties of C
transport layer (ETL) in MAPbBr
in particular generating an open circuit photovoltage as high as 1.3 V, while C
MA perovskite active layer achieved a photoconversion efficiency (PCE) up
to 16.7 %. C was also shown to be an effective visible light sensitizer for TiO photoanodes in
photoelectrochemical water splitting. Due to its electron- rich character, the C material
displayed instantaneous adsorption of methylene blue from aqueous solution reaching complete
equilibrium within 10 min, which is significantly faster than pristine g-C and other carbon-
based materials. C coupled with plasmonic silver nanocubes promotes plasmon-exciton co-
induced surface catalytic reactions reaching completion at much low laser intensity (1.0 mW) than
g-C which showed sluggish performance even at high laser power (10.0 mW). The relatively
narrow bandgap and 2D structure of C make it an interesting air-stable and temperature-
3
N
5
are distinct from pristine carbon nitride (g-C
3
N
4
). When used as an electron
outperformed g-C
blended with
3
based halide perovskite solar cells, C
3
N
5
3 4
N ,
3 5
N
FA1-xPb(I0.85Br0.15)
x 3
3
N
5
2
3 5
N
3 4
N
3 5
N
3 4
N
3 5
N
resistant semiconductor for optoelectronic applications while its electron- rich character and intra-
sheet cavity make it an attractive supramolecular adsorbent for environmental applications.
KEYWORDS: carbon nitride, melem, s-heptazine, dye adsorption, photocatalysis, water
spilitting, solar cells, plasmonic photocatalysis.
Formatted: Highlight
1
. Introduction
The last few decades have witnessed the rise of semiconducting, all-organic polymers as excellent
metal-free and visible light-active materials for various optoelectronic and energy harvesting
1
applications. Although impressive improvements in performance have been achieved, particularly
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for plastic solar cells, the synthesis procedures for semiconducting polymers are cumbersome and
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difficult to scale up, and the organic semiconductors themselves are unstable under the action of
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heat, light and/or ambient air. Consequently, there are scalability concerns related to
4
semiconducting polymers, and requirement of heavy encapsulation to achieve even modest
durability in the photovoltaic application. The same concerns, related to oxidative stability and
durability, have also ruled out the use of semiconducting polymers in photocatalytic applications.
A very different approach toward forming and exploiting all-organic, polymeric
semiconductors in optoelectronic and energy harvesting applications consists of using doped and
substituted graphenic frameworks as building blocks to achieve two-dimensional (2D)
5
, 5-6
semiconductors with well-defined bandgaps and structural motifs.
The major advantages of
graphenic semiconductors are their chemical robustness, and the simplicity of synthesis. Several
graphenic semiconductors are synthesizable using solvothermal synthesis and/or solid-state
reactions, and graphenic semiconductors are perfectly stable in ambient conditions up to
temperatures of several hundred degrees Ccelsius. As a result of this exceptional stability, almost
no structural or chemical degradation of photocatalytic action is observed even after several re-use
7
cycles.
Among graphenic semiconductors, graphitic carbon nitride (g-C
3 4
N ), composed of tris-s-triazine
(
6 7
s-heptazine, C N ) units bridged together with nitrogen atoms to give a 2D graphitic structure has
gained significant interest due to its astonishing electronic, optical, and physicochemical
8
properties. Continuous repetition of the heptazine motif leads to a bandgap of 2.7 eV with band
edge positions (ECB, –1.1 eV and EVB, +1.6 eV) that render it compatible with sunlight-driven
9
2
water splitting, CO photoreduction and the photooxidation of a number of organic compounds.
3 4
Further, the plentiful presence of electron rich sites and basic nitrogens in the g-C N scaffold
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enables the promotion of various catalytic reactions i.e. alkylation, esterification, oxidation, etc
6a, 10
and pollutant removal (dye adsorption).
3 4
The somewhat wide bandgap of g-C N means that it
can absorb only the ultraviolet and blue fraction of solar spectrum (λ < 450 nm) which limits its
performance in photocatalytic and photovoltaic applications. Doping with various heteroatoms
such as P, F, B, and S has been utilized to improve the visible light absorption profile and
11
3 4
photoefficiency. Like all semiconductors, g-C N suffers the innate drawback of carrier
recombination detrimental to catalytic and photocatalytic processes. Many surface modification
approaches such as increasing the surface area via soft and hard templating, using two or more
precursors, transformation of bulk material into sheets, doping with metals (Ag, Cu, Rh, Pt, Na,
etc) and metal oxides (CoO
x
) for electron and hole capture, coupling with other
semiconductors/metal complexes to form heterojunctions, and blending with graphene have been
12
employed to improve the photocatalytic and catalytic performance of g-C
attention has been paid to chemical structure modification which can lead to the generation of a
more robust, band edge tuned g-C framework with entirely new physicochemical properties
for efficient catalytic/photocatalytic applications. It has been found that addition of extra nitrogen-
rich moieties in the g-C scaffold to increase the N:C ratio from 4:3 ratio in CN can reduce the
3
N
4
.
However, less
3 4
N
3
N
4
bandgap significantly, due to a more extended conjugated network and the participation of the lone
pair on the N atom with the π conjugated system of heptazine motif. Vinu et al. demonstrated the
3 5
synthesis of N-rich carbon nitride (MCN-8) using 3-amino-1,2,4-triazole to afford C N
stoichiometry resulting in a significant decrease in bandgap (2.2 eV) due to extended
13
3 4
conjugation. However, this increase in N:C ratio to 5:3 (from the 4:3 ratio in g-C N ) was due to
the presence of the N-rich 1,2,4-triazole moiety linked to the heptazine motif and not because of
the direct incorporation of the extra N atom in the heptazine nucleus. The same group has also
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reported the synthesis of mono/and di-amino-s-tetrazine based carbon nitride materials (i.e. MCN-
ATN
,
MCN-4, and MCN-9) with
C
3
N
5
to
C
3
N
6
stoichiometry using 3-amino-1,2,4-
14
triazine/aminoguanidine hydrochloride precursor and SBA-15/KIT-6 templating material. The N
rich 1,2,4-triazine or 1,2,4,5-tetrazine moieties were bridged together with tertiary nitrogen in a
similar fashion to triazine- based carbon nitride and a significant decrease in band gap was
observed due to the addition of extra nitrogens. In a recent report, mesoporous triazole and triazine
framework modified carbon nitride materials with C
3
N4.8 empirical formula synthesized by using
5‐amino‐1H‐tetrazole (5‐ATTZ) precursor and their hybrid with graphene displayed excellent
15
performance in the oxygen reduction reaction. Fang et al. reported the synthesis of nitrogen self-
doped graphitic carbon nitride (C 4+x) by heating hydrazine treated melamine in a sealed
ampoule. In C 4+x, the excess N atoms replace terminal C atoms in the heptazine nucleus and the
excess charge on the N atom gets redistributed leading to electron rich heptazine motifs due to
3
N
3
N
which C
3
N
4+x possessed a narrower bandgap (2.65 eV) with concomitant shifts in the conduction
16
and valence band edge positions (ECB, –0.98 eV and EVB, +1.67 eV). In these N rich carbon
nitrides, the N rich triazine or heptazine based unit remains linked together with tertiary nitrogen,
N(C)
heptazine ring system. Similarly, carbon- rich C
and efficient charge separation due to the extended conjugated network. Zhang et al. reported the
hydrothermal synthesis of low bandgap, C rich C materials with extended conjugated networks
using melamine (as heptazine ring source) and glucose (as carbon source) precursors.
However, the use of melamine and other C and N sources can afford only C structures
possessing randomly distributed domains within the C framework due to the uncontrolled
3
and increased stochiometric N:C ratio was due to the replacement of C via N in triazine or
3 4
N
network also facilitates bandgap narrowing
3 4
N
12c, 17
3 4
N
3 4
N
reaction and these regions work as trap centers. Melem (2,5,8-triamino-s-heptazine) considered
Formatted: Font: Italic
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the smallest monomeric unit of g-C
3
N
4
framework, provides the opportunity to manipulate
3 4
chemical structure by incorporating other units in the C N framework in a more controlled
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fashion.
Shiraishi et al. reported the synthesis of modified CN-polydiimide framework (g-
C
3
N
4
/PDI
x
) by solid-state reaction between melem and electron deficient pyromellitic dianhydride
/PDI could be tuned by limiting
the number of PDI units in the framework. Heterostructured (Cring-C ) embodiments of
conductive, in-plane, π conjugated carbon rings incorporated in the C matrix were prepared by
thermal dehydrogenation reaction between glucose and melem, and the obtained Cring-C
heterostructure achieved fast spatial charge transfer from g-C to Cring motif facilitating efficient
The replacement of amino functionalities on melem/melamine by nitrogen-
(
PMDA), and demonstrated that the band edge positions of g-C
3
N
4
x
1
9
3 4
N
3 4
N
3 4
N
3 4
N
18d, 20
water splitting.
rich functionalities i.e. azide (–N
triazido-s-heptazine, (C )(N
3
) expedited the synthesis of N-rich carbon nitride i.e. 2,5,8-
2
0-21
6
N
7
3
)
3
which after thermal heating, afforded N-rich carbon nitride.
Likewise, triazine containing N-rich CN was also synthesized by thermal annealing of 2,4,6-
20, 22
triazido-1,3,5-triazine [cyanuric triazide, (C
3
N
3
)(N
3
)
3
].
However, the synthesis procedure
involved sodium azide and concomitant shock sensitive explosion hazards; furthermore, azide
intermediates are highly undesirable.
Herein, we demonstrated the synthesis of novel modified carbon nitride framework with a
C
3
N
5
stoichiometry by thermal deammoniation of 2,5,8-trihydrazino-s-heptazine, also known as
melem hydrazine (MH), as a safe and environmentally begnign precursor (Figure 1). The obtained
carbon nitride modified framework was denoted as C due to its 3:5 C:N stoichiometric ratio.
Characterization studies revealed that the C framework contains heptazine moieties bridged
Formatted: Highlight
3 5
N
3 5
N
together by azo linkage (–N=N–). The presence of azo linkage extends the π conjugated network
due to overlap between the p orbitals on N atoms constituting the azo bond and π system of
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heptazine motif which resulted in the reduction of the electronic bandgap to 1.76 eV. C
displayed improved photosensitization properties at longer wavelengths for solar water splitting.
Further, due to the increased electron charge density on the ring nitrogen, C exhibited
instantaneous adsorption of methylene blue from aqueous solution. Solar cell devices fabricated
using low band gap C , as an electron transporting layer (ETL) in MAPbBr based perovskite
solar cells demonstrated improved power conversion efficiency (PCE), open circuit voltage (Voc
etc. compared to solar cells made from g-C based ETL due to tuned band alignment. Blending
a small amount of C (4.0 wt%) with MA
3 5
N
3 5
N
3
N
5
3
)
3
N
4
3
N
5
x
FA1-xPb(I0.85Br0.15
)
3
perovskite active layer led to an
2
increase in PCE up to 16.68% with Voc of 1.065V and Jsc of 22.87 mA/cm higher than
3 4 3 4 3 5
conventional and g-C N blended solar cell architectures. Compared to g-C N , C N exhibited a
remarkably enhanced performance in the plasmon-exciton co-driven photoreduction of 4-
nitrobenezenethiol to 4,4'-dimercaptoazobenzene.
Figure 1. Chemical structure of g-C
3
N
4
3 5
and carbon nitride modified C N framework.
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2. Results and discussion
Melem (2,5,8-triamino-s-heptazine) served as the precursor monomeric unit for the synthesis of
polymer. Melem was synthesized by heating melamine at 425 ºC overnight followed by
purification in boiling water. The obtained melem was treated with hydrazine hydrate
(NH NH .H O, 55% in water) in an autoclave at 140 ºC for 24 h. The treatment of melem with
2
hydrazine hydrate transformed amino (–NH ) functionalities into hydrazino (–NH–NH )
3 5
C N
2
2
2
2
2
functionalities which afforded melem hydrazine, MH (2,5,8-trihydrazino-s-heptazine). The
3
obtained white melem hydrazine was subjected to programmed heating at 450 ºC for 2 h to obtain
3 5
orange colored C N polymer (Figure 2) (See supporting information for experimental details).
Melem hydrazine has a highly hydrogen bonded structure which facilitates the formation of an
azo-bridged heptazine framework by thermal condensation. Previously, Gillan also reported the
formation of similar azo-bridged functionalities by heating nitrogen- rich 2,4,6- cyanuric triazide
or triazido-1,3,5-triazine (C
3
N
3
)(N
In the same report Gillan suggested that transformation of cyanuric triazide into azo-
bridged triazine carbon nitride framework proceeded through the nitrene intermediate (C (N N:)
and that the formation of C from melem hydrazine might proceed via a similar intermediate
3 3
) to form differential composition triazine- based carbon
22, 24
nitride.
3
3 2
)
3
N
5
due to the thermolabile nature of hydrazine functionalities. The structures of melem, melem
hydrazine and hydrogen bonded melem hydrazine are given in supporting information (Figure S1).
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Figure 2. Synthesis schematic of C
3
N
5
from melem via melem hydrazine (Atom color: N - blue, C - gray and H
- white). The vials show the distinct color of the reaction product contrasted with that of the precursor.
3 5 3 5
Figure 3. (a) He-ion image of C N , and HR-TEM images of C N (b) at 50 nm, (c) at 10 nm and (d) at 5 nm
scale bar; left and right insets showing SAED diffraction pattern and interplanar d spacing, respectively.
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The surface morphology of the C
3
N
5
polymer was investigated using a He-ion microscope
equipped with an electron flood gun to facilitate positive charge neutralization accumulated from
the He-ion beam (Figure 3a). The He-ion images of MHP show a rough, crumpled graphenic
scaffold with some erupted morphologies which indicate that the high temperature treatment of
MH monomeric unit facilitated polymerization into an irregular sheet-like structure. The fine
structure of C
HR-TEM) (Figure 3b-d). The carbon nitride like layered sheet architecture is clearly evident in
the TEM image of C at 50 nm scale bar (Figure 3b). Under long duration exposure of the
electron beam, C starts to degrade and shrink which likely resulted due to high energy electrons
3 5
N material was determined using high resolution transmission electron microscopy
(
3 5
N
3 5
N
breaking the –N=N– linkage. HR-TEM images at 10 nm and 5 nm scale bar show crystallite fringes
of nanoporous multilayered sheets with an interplanar d-spacing of 0.32 nm, corresponding to the
002 plane of the graphitic structure (Figure 3c, 3d, and inset). The observed d-spacing in C
identical to g-C from which we infer that during the thermal polymerization step, the stacking
pattern of sheets in C remains similar to that in bulk g-C . The broad, less intense ring in the
3 5
N was
3 4
N
3
N
5
3 4
N
selected area electron diffraction (SAED) pattern was attributed due to diffraction of electrons by
the 002 plane; however, the low intensity of the ring suggests amorphous nature of the material
Formatted: Highlight
(
inset of Figure 3d).
The surface chemical composition of the synthesized material was investigated using X-
ray photoelectron spectroscopy, XPS (Figure 4). The XPS elemental survey scan of C shows
3 5
N
peaks corresponding to C1s, N1s, Na1s, Cl2p, and O1s (Figure S2a). The presence of Na1s and
+
Cl2p is due to intercalated Na ions in the supramolecular cavity of the polymeric motif (Figure 1)
and the residual NaCl formed during the purification step of MH. After excluding Na1s, Cl2p and
O1s peaks, the at% of C and N in the C
3
N
5
were found to be 36.76 63.24 % and 63.24 36.76 %
Formatted: Highlight
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respectively, which represent an empirical formula of C
obtained composition matched well with theoretical C
stoichiometric carbon nitride materials. The high resolution XPS spectrum of C
was deconvolved into two peak components at binding energies of 284.8 eV and 287.9 eV
3
N
5.16 for the C
3
N
5
polymer (Table 1). The
(N-62.50 at% and C-37.50 at%)
in C1s region
3 5
N
3 5
N
3
corresponding to the presence of sp and sp hybridized carbons, respectively (Figure 4a). The sp
2
3
carbon peak originated from adventitious carbons, edge group carbons and turbostratic carbons
2
polymer while the relatively stronger sp peak appeared due to
present in the scaffold of C
3
N
5
2
5
N=C–N type aromatic carbons which constitute the carbon nitride like framework of C
core level HR-XPS in N1s after deconvolution gave two peak components located at 398.7 eV and
00.2 eV. The peak at a binding energy of 398.7 eV was assigned to tertiary N–(C) and secondary
C=N–C nitrogens present in the aromatic ring structure while another peak at 400.2 eV was due
3
N
5
.
The
4
3
2
5-26
to the presence of primary residual –NH
2
and bridging C–N=N–C type nitrogens (Figure 4b).
From the N1s XPS spectrum, the at % of N present in aromatic ring (Nring) and bridging (Nbridging
)
were found to 60.47 % and 39.53 % respectively and the at% ratio obtained was 3:2, which strongly
supports the proposed structure in which two heptazine units are interconnected with the azo (–
N=N–) motif and is also consistent with the theoretical C
3
N
5
azo linked structure (Table 1).
+
Furthermore, HR-XPS in Na1s region gave a peak at 1071.9 eV due to the presence of Na ions in
the polymeric skeleton and residual NaCl (Figure S2b). The two peak components in Cl2p XPS,
at binding energy values of 198.7 eV and 200.2 eV ascribed to Cl2p3/2 and Cl2p1/2 further validated
–
the presence of Cl in the form of NaCl (Figure S2c). Two XPS peaks in the O1s region located at
5
31.6 eV and 532.4 eV were associated with surface adsorbed adventitious oxygens and –OH
groups (Figure S2d). The nature of C and N bonding in g-C and C was elucidated with
electron energy loss spectroscopy (EELS) (Figure 4c,d and Figure S312). The normalized EELS
N
3 4
3 5
N
Formatted: Highlight
1
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spectra of g-C
3
N
4
and C
3
N
5
exhibited two major symmetric peaks due to the contribution of C-K
3 4 3 5
and N-K edge loss. The C K-edges signal of both g-C N and C N was composed of two peaks
located at located at 284.6 and 293.2 eV corroborated to 1s-π* and 1s-σ* electronic transition of
2
sp hybridized carbons trigonally coordinated with nitrogens in s-heptazine nucleous (Figure
1
4a, b, 27
4
c).
higher than g-C
between bridging azo functionalities and heptazine motifs. The formation of extended π
conjugated network in C was also supported by increased UV-Vis absorption profile and
shorter TRPL lifetime decay (Figure 7 and 8). The N K-edges energy loss peaks for g-C
The relative intensity of π* C K-edge signal and π*/σ* peak area ratio of C
3 5
N was
3
N
4
suggesting increased conjugation in C due to extended π orbitals overlap
3 5
N
28
3 5
N
3
N
4
and
2
3 5
C N located at 399.8 and 408.5 eV, assigned to 1s-π* and 1s-σ* electronic transition of sp
2
hybridized nitrogens in heptazine ring and bridgining N, further verify sp hybridized nitrogen-
13
rich carbon nitride framework (Figure 4d). Absence of any new peak in N K-edge loss of C
3
N
5
3
3 5
demonstrate bridging nitrogens in C N have almost identical electronic environment like N(C)
29
nitrogens in g-C
than g-C
replacement of tertiary bridging nitrogens, N(C)
lone pair on azo nitrogens which contribute to σ* signal and relative intensity of π* signal was
suppressed. However, total peak area of N K-edge peak for C was increased which demonstrate
addition of extra nitrogens in carbon nitride framework. The N:C atomic ratio of C was
3
N
4
.
The relative peak intensity of N K-edge π* signal of C
3 5
N was slightly lower
3
N
4
demonstrating enhanced contribution of azo motifs in 1s-σ* transition The
in g-C via azo nitrogens, C-N=N-C render
3
3 4
N
3 5
N
3 5
N
calculated to be 1.62 which was in close agreement with theoretical value (1.66) and C:N value
obtained from CHNS analysis (1.65). Slightly lower N content might be due to cleavage of azo
bond resulting in loss of some nitrogens under high energy electron beam.
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Figure 4. The core level HR-XPS spectra of C
3
N
5
in (a) C1s region, (b) N1s region and normalized EELS spectra
of g-C and C N showing relative intensity of π* and σ* peaks for (c) C K-edge and (d) N K-edge loss.
3
N
4
3 5
Table 1. (a) Elemental analysis of C
3
N
5
showing C, H, and N wt% and empirical formula and (b) XPS elemental
composition.
analysis of C showing at% and empirical formula and their comparison with theoretical C N
3 5
3
N
5
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(a) Elemental analysis
Serial.
No.
N (wt%)
C (wt%)
H (wt%)
Empirical
formula
Nring:Nbridging
(at% ratio)
1
.
CHN
analysis
61.27
31.81
2.68
C
3
N
4.95
H
1.01
-
2
Theoretical
wt% value
66.02
33.98
-
C
3
N
5
3:2 (60:40)
(
b) XPS elemental analysis
N (at%)
63.24
C (at%)
H (at%)
Empirical
formula
N
ring:Nbridging
(at% ratio)
~ 3:2
3
.
XPS
analysis
36.76
-
-
3 5.16
C N
(60.47:39.53)
3:2 (60:40)
4.
Theoretical
at% value
62.50
37.50
3 5
C N
To probe the proposed composition and structure of the synthesized C
3
N
5
material, CHNS
elemental analysis was performed which gave 61.27 wt% N, 31.81 wt% C and 2.68 wt% H
suggesting an empirical formula of C 1.01 which was in close proximity with the theoretical
wt% for C composition (Table 1). Slight difference between predicted and observed C:N ratio
might be due to the presence of unbonded -NH at the edge of sheets, formed by cleavage of
hydrazino group (-NH-NH ) at elevated temperature and loss of some azo nitrogens.
expected, sulfur was not present at measurable levels. Notably, the observed hydrogen might arise
from –NH and –OH groups present at the edge of the polymeric framework.
To elucidate the chemical structure of MH and C materials, solid-state nuclear magnetic
3
4.95
N H
3
N
5
2
9
b, 10b
2
As
2
3 5
N
resonance (NMR) spectroscopy using the cross-polarization magic-angle spinning (CPMAS)
technique was performed (Figure 5). CPMAS NMR enables the structural investigation of local-
1
and medium-range structure in micro- and nano-crystalline compounds. The C CPMAS NMR
3
spectra of melem hydrazine (MH) display three NMR signals at 164, 160 and 154 ppm (Figure
1
3
2 2
5a). The C NMR signals at 164 and 160 ppm originated from N C–NHNH carbons while the
1
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resonance at 154 ppm was observed from CN
3
carbons of the heptazine nucleus. The observed
signals were in good agreement with the reported NMR spectra for MH and melem based
18a 24, 30
13
structures.
The CPMAS NMR spectrum of C
C–N=N– and CN
2 2
C–NHNH carbon signal of MH located at 160 ppm arising due to C–H functionalities
3 5
N exhibits two C NMR signals of
2
4,
approximately equal intensity at 164 and 156 ppm for N
2
3
carbons (Figure 5b).
3
0-31
The N
13
disappeared in the C NMR of C
3 5 2
N , which confirms removal of –NHNH protons and formation
15
of an azide linkage during polymerization step agreeing with N CPMAS NMR, vide infra
.
13
13
Furthermore, the appearance of equally intense (Cc:Ce/1:1.07) C peaks in the C NMR spectrum
of C suggests that heptazine units are in the presence of a symmetric azo bridging motif (where
Cc corresponds to central carbons in ring and Ce to external carbons bonded to azo N). A slight
shift to higher frequency in CN carbon peaks from 154 ppm in MH to 156 ppm in C suggests
shielding of carbons due to N2p overlap of azo and aromatic system which extends the
3 5
N
3
3 5
N
π
π
32
conjugated network.
15
The N CPMAS NMR spectrum of MH exhibits four signals, –207, –252, –273 and –317
23a
15
ppm (Figure 5c). The N NMR signal at –207 ppm and another weak signal at –273 ppm were
2
3a, 31b, 32-33
assigned to (NC
and –317 ppm assigned to NH
transformation of MH to C proceeds with removal of NH
was evident from the disappearance of NH and NH peaks at –252 and –317 ppm in the N NMR
spectrum of C (Figure 5d). The two NMR peaks in the N NMR spectra of C
48 (weak) ppm were attributed to NC and NC nitrogens of heptazine skeleton while another
peak at –271 ppm arose from –N=N– (and residual NHs) type nitrogens. As the N atoms are in
2
) and (NC
3
) nitrogens of the heptazine motif,
and NH terminal nitrogens of hydrazino moiety.
and formation of azo linkage which
while the peaks at –252
3
3-34
2
The
3
N
5
3
1
5
2
1
5
3
N
5
3 5
N at -197 and -
2
2
3
15
similar chemical environments, a semi-quantitative CPMAS NMR analysis of the N peak areas
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achieved by peak integration of NC
2 3
and NC and –N=N– resonances was found give a ratio of
1
.00:0.18:0.54 which was in good agreement with the theoretical value (1.00:0.17:0.5) calculated
for C
3
N
5
polymeric structure containing heptazine units interconnected with azo linkage (Figure
1
S1). Furthermore, H NMR of MH gave an intense peak at 5.11 ppm due to NH and NH
2
hydrogens
1
Figure S4). This intense peak disappeared in the H NMR spectra of C
(
3
N
5
further confirming the
removal of NH hydrogens and a very broad peak centered at 9.18 ppm appeared due to intercalated
hydrogen, and residual carboxy and aldehyde hydrogens (essential for the CPMAS approach to
1
function whereby H magnetization is transferred to C and N). All these NMR results validate
13
15
the successful synthesis of a modified carbon nitride framework.
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15
3 5 3 5
Figure 5. CPMAS NMR spectra (a) C of MH, (b) C of C N , (c) N of MH and (d) N of C N .
0
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Fourier transform infrared (FTIR) spectroscopy was employed to determine the change in
functional moiety in the material (Figure 6a-d). The FTIR spectrum of melem shows characteristic
–1
broad peaks at 3109 cm due to the combined symmetric and antisymmetric stretch vibrations of
–
1
–NH
ascribed to the C–N stretch (
The N–H stretch band ranging from 3153–2895 cm for MH was found to become broader due to
combinational symmetric and asymmetric N–H stretches of –NH–NH group in MH which
confirms the successful transformation of –NH moiety in melem to –NH–NH in melem
2
and –OH (νN–H and νO–H) groups. The IR bands at 1595, 1411, 1230 and 1078 cm are
18a, 31b, 34a, 35
ν
6 7
C–N) of heptazine (C N ) aromatic nucleus (Figure 6a).
–1
2
2
2
hydrazine (Figure 6b). The broadening of NH peak was attributed to strong intermolecular
35-36
hydrogen bonding in MH molecules.
However, all stretching and bending peaks due to
heptazine aromatic ring skeleton remain preserved which indicates that the heptazine motif
remains unchanged during the hydrazine treatment. Additionally, some new peaks emerged at
–1
18a, 36-37
1
095 and 965 cm implicating the N–N stretch and –NH
2
rocking vibration respectively.
–1
2
Graphitic carbon nitride shows characteristic peaks at 3145 cm due to residual –NH and –OH
–1
–1
stretch and 1639–1145 cm due to triazine ring stretch and 798 cm for triazine ring bending
37-38
vibration was in good agreement with the reported literature (Figure 6c).
MH to C by thermal annealing, the intensity of –NH–NH peak of MH was diminished which
implicated the transformation of –NH–NH group into azo (–N=N–) linkage through the removal
of NH (Figure 6d). It is important to note that vibration of symmetrical –N=N– azo linkage is
After conversion of
3
N
5
2
2
3
forbidden due to which no new sharp peak due to azo functionalities was observed. The possibility
of –NH–NH– bond can be neglected due to the absence of any strong N–H band; however very
1
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weak broad peaks arise due to some residual –NH present at the edge of the polymeric framework.
This fact was well supported by CHNS analysis which showed the presence of only one H for each
-1
stoichiometric C
stretch and –NH
hydrazine group into azo moiety. Peaks corresponding to the C
3
N
5
unit (Table 1). Further, other peaks of MH at 1095 and 965 cm due to N–N
2
rocking vibration disappear in C
3
N
5
, which confirmed the transformation of
framework at 1542, 1315 and
which suggests an entirely different network of C in comparison
3 4
N
–1
8
87 cm were absent in C
3
N
5
3 5
N
to g-C
N .
3 4
3 4 3 5
The changes in phase structure and crystalline nature of melem, MH, g-C N , and C N
were investigated through the measurement of X-ray diffraction (XRD) (Figure 6). The XRD
pattern of melem demonstrated a series of peaks located at 12.5º, 13.6º, 16.7º, 18.4º, 19.7º, 22.0º,
3
8-39
2
5.2º, 27.2º and 30.4º, in close agreement with previous reports (Figure 6e).
The XRD results
Due to the
18b, 39a
indicate the absence of any melamine impurity in the melem sample.
transformation of melem into melem hydrazine, the XRD pattern of MH changed, with new peaks
being observed at 2 values of 7.3º, 7.9º, 8.4º, 12.9º, 13.7º, 14.8º, 25.1º and 28.0º (Figure 6f). Bulk
g-C shows two distinct XRD peaks at 2 values of 27.1º and 13.0º indexed to the 002 and 100
planes of carbon nitride materials (Figure 6g). The 002 peak with a 0.32 nm interplanar spacing
θ
3
N
4
θ
d
was correlated to interplanar stacking of sheets while 100 peaks with a 0.68 nm spacing was
18b, 39b, 40
specific to in-plane structural packing of heptazine units (Figure 6g).
The XRD pattern of
C
3
N
5
exhibits one broad 002 peak at 27.6º corresponded to 0.33 nm interplanar sheet distance. The
slight increase in 2 value and spacing can be explained due to repulsion between electron rich
conjugated C sheets as in graphite (0.34 nm) (Figure 6h). Further, the absence of 100 peak –
θ
d
π
3 5
N
a specific feature of in-plane packing, suggests distortion in the carbon nitride framework and
broadening of the nanochannel distance between heptazine units due to azo (–N=N–) bridging
1
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linkage, further consistent with C and N NMR resonance broadening above, suggesting
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local/medium-range disorder. Also, bridging of two heptazine units with two nitrogens through in-
3 5
plane lattice packing is less efficient in C N which was responsible for the absence of any
3 5
expected peak at lower 2θ values. These XRD results clearly support the distinct structure of C N
possessing azo linkage.
Figure 6. FTIR spectra of (a) Melem, (b) Melem hydrazine, (c) g-C
of (e) Melem, (f) Melem hydrazine, (g) g-C , (h) C
3 4 3 5
N , (d) C N , and XRD diffraction pattern
3
N
4
3 5
N .
Raman spectra of melem acquired using 632 nm laser excitation show characterstic
–1
fingerprint peaks of melem at 435 and 697 cm due to heptazine ring (C
6
N
7
) breathing modes and
–1
18a, 40a
a broad hump at 1452 cm due to –NH
2
bending mode (Figure S5a).
Raman spectra of MH
–1
demonstrate many signature peaks correlated to the core at 472, 744 and 1529 cm which were
shifted in comparison to melem due to functionalization while other peaks due to various
2
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vibrations of the heptazine nucleous and hydrazine group were observed at 127, 342, 537, 985,
–1
18a, 23a, 40b,
1
159, 1314 and 3071 cm , in good agreement with the reported literature (Figure S5b).
41
The Raman spectra of g-C
3
N
4
display many prominent peaks due to the heptazine framework at
−1
471, 697, 706 cm (heptazine ring breathing modes) and two additional peaks at 1233 and 1567
-
cm corresponding to the –NH
1
23a, 42
2
bending mode and graphitic G band (Figure S5c).
Further,
-
the presence of a broad hump extended from 1100-1600 cm suggests multilayer stacking of g-
1
41, 43
C
3
N
4
sheets.
In the Raman spectra of C
3
N
5
, only trace peaks of melem hydrazine motif are
. Two small peaks were
observed at 1085 and 1161 cm due to the mixed vibration of heptazine motif and azo stretch
3 5
observed which indicates the complete transformation of MH to C N
-
1
–1
(
Figure S5d). A sharp peak at 1609 cm originated due to the C=N stretching mode.
Figure 7a displays the diffuse reflectance UV-Vis (DR-UV-Vis) spectra of g-C
3 4
N and
C
3
N
5
. The DR-UV-Vis spectra of g-C
3 4
N shows a characteristic absorption peak between 200–
400 nm with a band tail extended up to 450 nm due to charge transfer from the populated valence
band of the nitrogen atom (2p orbitals) to the conduction band of the carbon atom (2p orbitals) of
carbon nitride. The less intense absorption band at 330 nm is due to π→π* transition in the
conjugated network while another intense peak at ca. 387 nm appeared due to n→π* transition
12b, 42-44
from nitrogen nonbonding orbital to the aromatic nonbonding orbital.
spectrum of C demonstrates a drastic change in the UV-Vis absorption profile in comparison
to g-C due to a more extended π conjugated network (Figure 7a).
around 393 nm in UV-Vis spectrum of C
was attributed to n→π* transition from nitrogen
nonbonding orbital to the π conjugated nonbonding orbital. The absorption spectrum of C was
The DR-UV-Vis
3 5
N
12b, 45
3
N
4
A broad absorption peak
3 5
N
3 5
N
red shifted showing band tailing up to 670 nm, due to an extended π conjugated network arising
from the overlap between N2p orbitals of bridging azo moieties and N2p in heptazine π conjugated
2
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system. Further residual –NH
2
also contributes to the delocalized aromatic π conjugated system.
Due to this, the position of the valence band gets upshifted and π→π* transition occurs at a
relatively low energy which facilitates the absorption of a large fraction of the visible spectrum
and results in the sample displaying an orange color. Further, the optical bandgaps of g-C
3
N
4
and
αhν) vs hν and
extrapolation of the linear tangent to abscissa; where α is absorption coefficient, h is plank constant
and ν is light frequency (Figure S6a). From the Tauc plot, the value of bandgap for g-C was
estimated to be 2.65 eV corresponding to a band-edge at a wavelength of 467 nm, in good
1
/2
C
3
N
5
were determined using a Tauc plot by plotting a graph between
(
3 4
N
4
6
agreement with the bandgap values reported in the literature. The bandgap value of C
3 5
N was
calculated to be 1.76 eV corresponding to a band-edge at a wavelength of 707 nm.
Photoluminescence (PL) spectra were collected by exciting samples using 360 nm photons
to probe radiative recombination (Figure 7b). The PL spectrum of melem consists of an intense
emission peak centered at 441 nm which is indicative of radiative recombination of carriers within
39a, 44
melem unit.
It is important to note here that melem exhibits excitation wavelength-dependent
showed a sharp emission peak at 468 nm which did not
shift upon changing the excitation wavelength. This peak is attributed to fast interlayer carrier
3 4
PL emission. On the other hand, g-C N
4
5, 47
recombination in multilayered sheets of bulk g-C
3
N
4
.
3 5
Surprisingly, C N does not exhibit any
distinguishing PL peak which might be indicative of efficient charge separation between the bulk
and the surface. Such charge transfer excitonic states involving the bulk and the surface, have also
been observed in other conjugated organic semiconductors that possess an extended π-conjugated
network that prevents radiative recombination by delocalizing the Frenkel exciton. However, due
to conductive conjugated surface non-radiative charge recombination can take place over new
4
8
localized states in the sheets scaffold.
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Figure 7. (a) DR-UV-Vis spectra of g-C
and C samples and (b) Steady- state PL spectra of melem (black), g-C
using ant excitation wavelength of 360 nm.
3
N
4
(blue) and C
3
N
5
(red), with inset showing photographs of g-C
3 4
N
3
N
5
3
N
4
3 5
(blue) and C N (red) obtained
In order to investigate the lifetime of excited charged species, and charge separation processes,
we collected time resolved photoluminesece (TRPL) spectra of g-C and C using a single
photon picosecond pulsed laser at a wavelength of 405 nm. Figure 8 displays the PL lifetime decay
3
N
4
3 5
N
curves of g-C
equation:
3 4 3 5
N and C N . The PL decay curve was fitted tri-exponentially using the following
−t/τ1+ A
e−t/τ2 + A
e−t/τ3
(1)
1
I(t) = A e
2
3
where, A
and τ are values of the lifetime components respectively. The existence of three radiative lifetimes
in the fitted PL lifetime spectra of g-C and C was in good agreement with previously
1
, A
2
, and A
3
represent the normalized amplitudes of each decay component and τ
1
, τ
2
3
3
N
4
3 5
N
2
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reported carbon nitride- based materials. The obtained values of lifetimes and their fractional
9
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2
3
4
5
6
7
8
9
0
components are given in Table 2.
3 4 3 5
Table 2. The PL lifetime of phogenerated charge carrier and their relative contribution in g-C N and C N .
Sample
1
τ (ns) [A
1
]
1
τ (ns) [A
1
]
1
τ (ns) [A
1
]
Average lifetime (τavg) ns
g-C
3
N
4
3.31 [0.34] 0.75 [0.63] 25.02 [0.05]
8.10 [0.07] 2.11 [0.26] 0.28 [0.73]
12.43
4.40
3 5
C N
3 4
The three components in the PL lifetime decay curve of g-C N can be assigned to various energy
2
3
formed by the overlap of C and N sp and sp hybridized orbitals and the presence
states in g-C
3
N
4
of lone pairs of electrons which allow for various radiative transitions. g-C
s-triazine (C
3
N
4
is composed of tri-
3
) units inter-connected with tertiary nitrogen atoms where C-N sp hybridized state
6
N
7
2
constitute high energy σ and σ* molecular orbitals while C-N sp hybridization gives rise to a
conjugated network resulting in low energy π bonding and π* antibonding orbital which constitutes
5
0
the valence and conduction bands respectively. The presence of unbonded lone pairs of electrons
on pyridinic N atoms creates energy levels just below the π bonding orbital and their overlap with
the π conjugated system can further decrease the energy of the π molecular orbital resulting in the
51
reduction of the bandgap. The first two shorter lifetime components of 3.31 and 0.75 ns with 34
%
and 63 % contribution in g-C correspond to charge carrier recombination from σ* and π*
3 4
N
52
antibonding to π MO. The third longer lifetime component of 25.02 ns with a relative low
contribution originated due to intersystem crossing (ISC) of electron from σ* and π* orbital
followed by radiative relaxation to conjugated π orbital and trap- assisted radiative
2
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recombination. The first two lifetimes of C
contributions in the PL decay curve were significantly longer lived in comparison to g-C
strongly suggesting that the introduction of azo moiety extends π conjugated network which
facilitates better charge carrier mobility on C sheets (delocalized the exciton, as mentioned
3 5
N at 8.10 and 2.11 ns with 7 % and 26 %
N ,
3 4
3
N
5
54
previously) and prevents faster charge carrier recombination. Further, due to extended
conjugation, the difference between σ* and π* band get decreased which is also evident in Mott-
51a
Schottky measurement (Figure S6b). The low energy difference between σ* and π* accelerates
the transfer of electrons from σ* and π* orbital via intersystem crossing followed by radiative
relaxation which was evident from higher percentage contribution of the third lifetime component
(
73 %).
The average lifetime (τavg) which is regarded as coherent measure to evaluate the rate of
spontaneous emission was calculated from the three lifetime components using the following
expression.
2
1
2
2
2
3
τ
avg = (A
1
τ
+ A
2
τ
+ A
3
τ
)/ (A
1
τ
1
+ A
2
τ
2
3
+ A τ
3
)
(2)
From Eq (2), the average lifetimes of g-C
respectively. The decreased lifetime of the C
weak photoluminescence of C (as seen in Figure 7b) is indicative of fast quenching of the C
luminescence. The fast quenching might originate from improved charge separation in C due
3
N
4
and C
3
N
5
were calculated to be 12.43 and 4.40 ns
3
N
5
in comparison to g-C
3 4
N
coupled with the very
3
N
5
3 5
N
3 5
N
to a larger conjugated π network but might also be due to stronger non-radiative transitions. Fast
exciton dissociation with concomitant high carrier mobility can result in photogenerated electrons
48a
finding trap sites (and moving to them) and recombining by non-radiative processes. The
aforementioned processes are highly likely in C
N
3 5
since the presence of azo bonds extends the π
2
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network because of overlap of N2p orbital on azo nitrogens with the π network of heptazine motif
due to which electrons can move within C
3
N
5
scaffold freely. The lower PL lifetime of C
3 5
N in
comparison to g-C was consistent with steady- state PL where C N
3 5
3
N
4
shows prodigious
quenching in its PL spectrum.
3 4 3 5
Figure 8. (a) PL lifetime decay curves of g-C N (red; tri-exponential fit, yellow line) and C N (black, tri-
exponential fit, cyan), (b) Schemaetics of various energy levels bands and possible route of charge carriers
recombination (c) X-band EPR spectra of g-C in the dark (blue), after light irradiation (orange dots) and C
in the dark (red) and after light irradiation (black dots) at room temperature, (d) Plausible molecular orbital
overlap representation of C
3
N
4
3 5
N
N .
3 5
2
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Electron paramagnetic resonance (EPR) spectra of g-C
nature and band excited paramagnetic species were collected under dark and UV irradiation at
room temperature (Figure 8b). The EPR spectra of g-C under dark conditions exhibits an
intense Lorentzian EPR resonance signal located at a g-factor of 2.003. The observed EPR signal
originated due to the presence of unpaired electrons in the sp hybridized aromatic π-system which
The EPR signal intensity of g-C increased
3 4 3 5
N and C N to elucidate electronic
3
N
4
2
36, 55
was in good agreement with previous reports.
3 4
N
after UV irradiation, attributed to populated unpaired electrons in the conduction band due to π-π*
and N non-bonding to π* (n-π*) transition followed by slow relaxation via ISC. The observed EPR
signal of C
reamains intact in C
intensity of C was also enhanced due to increased numbers of unpaired electrons in the
conduction band. However, the overall EPR signal intensity of C
UV illumination was significantly weaker in comparison to g-C
5
lesser number of unpaired electrons in C N , which in turn can be taken as evidence of the presence
3 5
N was also observed at 2.003 g-value which implies basic graphitic heptazine skeleton
5
6
3
5
N framework. Further, after irradiation with UV light, the EPR signal
3
N
5
3
N
5
in both the dark and under
3
N which was attributed to a
4
3
of extra N atoms outside the heptazine nucleus in comparison to conventional N-rich carbon nitride
materials where N atoms substitute C atoms in the heptazine motif. It is well documented in the
2
literature that substitution of sp hybridized +4 state C atom in heptazine motif with sp hybridized
2
+3 state N atom will liberate extra electrons in the aromatic system which will distort electronic
16, 47, 57
symmetry
3 5
and also increase EPR signal intensity. However, in the case of C N , the
additional N atom makes an azo bond with an N atom outside the ring via π overlap and the extra
electrons remain in the form of lone pairs (Figure 8d).
Fluorescence lifetime imaging microscopy (FLIM) of samples at different spots was used
to probe the homogeneity of samples and to determine the nature of the fluorescence (Figure S79).
Formatted: Highlight
2
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The PL spectra of g-C
3
N
4
samples obtained from different spots exhibited identical emission
profiles with a sharp intense peak at 480 nm which was in good agreement with the steady- state
PL spectrum (Figure 7b). The slight red shift in the emission peak (Figure S79a) is attributed to
the difference in the mechanism of excitation (750 nm two- photon excitation source for FLIM,
Formatted: Highlight
3
60 nm single photon excitation in Figure 7b). Furthermore, the emission spectrum of C
displays two relatively weak peaks centered around 410 and 490 nm which likely originated from
some relatively smaller C polymeric fragments and heptazine networks (Figure S79cb). The
smaller fragments are consistent with a lesser number of MH units and therefore exhibit PL
properties closer to melem. FLIM images of g-C were brighter than C which further
supports our inference that the charge separation process was dominant in C samples (Figure
S79bc and Fig. S79d). The C /MB samples obtained after methylene blue (MB) dye adsorption
displayed relatively strong PL and brighter FLIM images due to the presence of MB in the
composite (Figure S79e and Figure S79f). The absence of PL quenching in the C /MB
composite further suggests the absence of photo-induced charge transfer between the methylene
blue and C
The synthesized C
3 5
N
3
N
5
Formatted: Highlight
Formatted: Highlight
3
N
4
3 5
N
N
3 5
Formatted: Highlight
3
N
5
Formatted: Highlight
Formatted: Highlight
3
N
5
Formatted: Highlight
Formatted: Highlight
N .
3 5
3
N
5
material was explored for dye adsorption studies using methylene
Formatted: Normal (Web), Font Alignment: Baseline
Formatted: Highlight
blue (MB) as a model dye. Methylene blue is a staining dye widely used in the paper, textile and
leather industries which also constitutes a good example of a colored water contaminant which
due to its excellent visible light absorption, reduces light penetration in aqueous ambients and
adversely affects aquatic flora and fauna. All dye adsorption studies were carried out at room
temperauture and under dark conditions. UV-Vis spectra of samples were collected for
determining the concentration of MB solutions during dye adsorption experiments (For
experimental details, see supporting information). MB has a sharp peak at 664 nm due to π-π*
2
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transition and a shoulder around 614 nm which represents MB present in dimeric and polymeric π
stacked forms in water (Figure 9a). After the addition of C sample into methylene blue solution,
3 5
N
the color of the solution instantaneously turned green. The green solution after centrifugation
turned completely colorless which demonstrated the prompt adsorption of MB dye over the surface
of C
solid after centrifugation (denoted as C
between C and MB with a broad peak centered at 680 nm. The red shifting in the peak of C
from 664 to 680 nm is attributed to the transformation of MB into monomeric form and some
degree of ground state charge transfer from C to MB during adsorption on the surface of C
The dye adsorption performance of C was much higher than g-C . MB is a well known
cationic dye possessing positive charge centered on the S atom in aqueous solutions. On the other
hand, the surface of C material has electron rich character due to the presence of secondary N
NC ) in heptazine moieties, terminal –NH and π extended network. Therefore, electrostatic
interactions between the positively charged MB molecule and negatively charged C are likely
To confirm negative charge on the surface of
, zeta potential measurement was performed which depicts average surface charge –36.2 mV
prove the electron- rich surface of C (Figure S810). Further MB can also adsorb on the surface
of C via π-π stacking between aromatic conjugated network of MB and π framework of C
(Figure 9b). To investigate the role of surface specific properties in the enhanced adsorption
profile, Brunauer-Emmett-Teller (BET) surface area (SBET), pore volume (V ) and pore diameter
) of g-C and C were measured by N adsorption and desorption. The obtained BET
surface area, pore volume and pore diameter for g-C
and 19.13 nm while these values for C were found to be 1.78 m g , 0.002 cm g and 16.98
3
N
5
and subsequent settling of the MB adsorbed C
3 5
N during centrifugation. The obtained
3 5
N /MB) exhibits a sharp absorption peak intermediate
3
N
5
3 5
N
3
N
5
3 5
N .
3
N
5
3 4
N
5
8
3 5
N
(
2
2
3 5
N
54, 59
responsible for the instantaneous adsorption.
3 5
C N
3
N
5
Formatted: Highlight
3
N
5
3 5
N
60
p
(
r
p
3
N
4
3
N
5
2
2
-1
3
3
4
N were found to be 11.47 m g , 0.095 cm ,
2
-1
3
-1
3 5
N
2
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