Synthesis of an electronically modified carbon nitride from a processable semiconductor, 3-amino-1,2,4-triazole oligomer,: Via a topotactic-like phase transition

Article abstract of DOI:10.1039/c7ta01714f

A thermally induced topotactic transformation of organic polymeric semiconductors is achieved using similarity of the chemical structures of two C,N,H-containing materials. Namely, the oligomer of 3-amino-1,2,4-triazole (OATA) is transformed into an elect

Full text of DOI:10.1039/c7ta01714f

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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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Synthesis of electronically modified carbon nitride from a
processable semiconductor, 3-aminotriazole-1,2,4 oligomer, via a
topotactic-like phase transition
Received 00th January 20xx,
Accepted 00th January 20xx
Aleksandr Savateev,a Sergey Pronkin,b Jan Dirk Epping,c Marc Georg Willinger,a,d Markus
DOI: 10.1039/x0xx00000x
Antonietti,a and Dariya Dontsovaa,*
www.rsc.org/
The thermally induced topotactic transformation of organic polymeric semiconductors is achieved using similarity of the
chemical structures of the two C,N,H-containing materials. Namely, the oligomer of 3-aminotriazole-1,2,4 (OATA) is
transformed into an electronically modified graphitic carbon nitride (OATA-CN) upon heating at 550 °C. During the
transition, the flat band potential of the organic semiconductor is only slightly shifted from -0.11 eV to -0.06 eV, while the
optical band gap is significantly expanded from 1.8 eV to 2.2 eV. The advantage of the suggested approach is the
processability of the starting semiconductor combined with the minor morphology changes during the heat-treatment
that enable preservation of the original oligomer micro- and macrostructures in the resulting carbon nitrides. As
illustration, different OATA morphologies, including spherical nanoparticles, nanobarrels, nanowires and self-assembled
macrospheres and composite sheets are synthesized and then transformed into OATA-CN with the retention of
morphology. The surface area of the final carbon nitrides reaches 66 m2/g, without using any templates, auxiliary reagents
or post treatment. As a consequence, the photocatalytic activity of the obtained carbon nitrides in the visible light driven
hydrogen evolution is up to 5 times higher than measured for the reference bulk carbon nitride prepared by pyrolysis of
melamine.
band gap (~2.7 eV) organic semiconductor that is used in
visible-light driven photocatalytic water splitting.2,3,4 Among
simple inorganic semiconductors, TiO2 is a wide band gap
Introduction
Among
a number of physico-chemical properties of a
photocatalyst broadly applied since 1970s5 in a number of
photocatalytic processes, but it is active only under UV-light
irradiation.6,7,8,9 Anatase and rutile are among the known
crystalline phases of TiO2,10 that particularly differ in their
band gap values. The band gap of anatase is 3.20 eV while that
of rutile is 3.03 eV.11 In addition, the CB of anatase is 0.2 eV
more negative than that of rutile.12 One of the consequences is
that these phases possess different photocatalytic activities in
a number of reactions.13 Rutile is known to be more
thermodynamically stable than anatase,14 and thus can be
obtained by heating anatase at 600 °C.15 Therefore, band
structure tuning of TiO2 and some other inorganic materials
can be simply achieved by heating. This is, however, totally
particular organic semiconductor, the band gap, conduction
band (CB) and valence band (VB) potentials are the most
important. Band gap designates the wavelength that should be
used to make catalyst photochemically active and generate
excitons. Thus, medium band gap semiconductors allow to use
the visible fraction of the sun light for diverse processes. On
the other hand, wide band gap semiconductors having
substantially negative CB or significantly positive VB potentials
come into play when pronounced reductive or oxidative
properties, respectively, are the prerequisites for
a
photocatalytic reaction. Graphitic carbon nitride (g-CN), a
representative of a class of conjugated polymers,1 is a medium
different
to
organic
semiconductors.
In
organic
semiconductors, band tuning is usually accomplished by
doping with elements, e.g. S,16 B,17 F,18 or by means of
molecular doping when combining different monomer-
comonomer pairs.19,20 To the best of our knowledge, there are
no reported studies describing thermally induced structural
transitions between two different organic semiconductors.
This is to be expected considering the usually large differences
in their chemical structures, cf. poly(p-phenylene vinylene)21
and poly(3-hexylthiophene).22
a.Max-Planck Institute of Colloids and Interfaces, Department of Colloid Chemistry,
Research Campus Golm, 14424 Potsdam, Germany.
b.Institut de Chimie et des Procédés pour l'Energie, l'Environnement et la Santé
(ICPEES), ECPM, CNRS-Université de Strasbourg (UdS) UMR 7515, 25, rue
Becquerel, 67087 Strasbourg, France.
c. Department of Chemistry, Technische Universität Berlin, Strasse des 17 Juni 135,
10623 Berlin, Germany.
d.Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck
Society, Faradayweg 4-6, 14195 Berlin, Germany.
*Email - dariya.dontsova@mpikg.mpg.de
Electronic Supplementary Information (ESI) available: SEM images, FT-IR spectra,
TGA-MS data, N2 sorption isotherms, UV-vis absorption spectra, room
temperature PL spectra, Mott-Schottky plots. See DOI: 10.1039/x0xx00000x
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In general, the idea to use an organic semiconductor as a
precursor for another semiconductor is highly appealing
because the texture and packing effect on the band structure
of the former may partially transfer to the final material during
transformation induced, for example, by pyrolysis. In order to
realize this idea we were searching for a couple of organic
semiconductors with similar chemical structures. A suitable
pair of semiconductors that can be regarded as a precursor
and a product of a chemical reaction could enable the
chemical transition between the materials, most favorably
accompanied by minimal structural changes.
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DOI: 10.1039/C7TA01714F
Results and discussion
OATA was prepared by the oxidative polymerization of 3-
amino-1,2,4-triazole. In order to obtain the products with
different morphologies, different isolation conditions and
posttreatment steps indicated in the Experimental Section
were used. The sample OATA-1 was obtained as nanorods with
the length varying from 100 nm to 500 nm as demonstrated by
scanning electron microscopy (SEM) studies (Fig. 2a). The
morphology of OATA-2 that was prepared using KOH instead
of NH4OH for the reaction mixture neutralization is
represented by 2-3 µm long nanowires tangled together and
resembling the ‘wings of a bat’ (Fig. 2c, S1a, S1b). On its turn,
OATA-3 sample prepared by the precipitation of OATA-1 from
its saturated solution in dimethyl sulfoxide, DMSO, has
spherical nanoparticle morphology with a diameter as small as
~30 nm (Fig. 2e and S1c). Finally, OATA-4 obtained by freeze-
drying an aqueous dispersion of OATA-1 adopts the
morphology of macrospheres and macrosheets composed of
0.1–1 µm long nanorods (Fig. 2g and S1d, e).
One suitable candidate to start with is shown in Fig. 1a. This is
a recently reported oligomeric 3-amino-1,2,4-triazole (OATA,
3-amino-1,2,4-triazole
tetramer),23,24
an
organic
semiconductor with a narrow band gap of ~1.7 eV, built of
aromatic 1,2,4-triazole rings with alternating C-N and N-N
bonds. On the other hand, monomeric substituted 1,2,4-
triazoles can serve as precursors to prepare carbon nitrides
with the structure represented by the cross-linked melon
polymer shown in Fig. 1b.25 Melon built of tri-s-triazine
(heptazine) units, converts into heptazine-based graphitic
carbon nitride upon subsequent condensation with elimination
of ammonia. Thus, a conversion of OATA into a carbon nitride
material heat treatment seemed to be possible.
The elemental composition of OATA-N samples determined by
the elemental analysis (Table S1) is close to the one expected
Traditionally, carbon nitrides are prepared by the pyrolysis of
small organic molecules such as cyanamide, melamine, etc.26,27
According to the accepted mechanism,28 the band structure of
the resulting semiconducting polymer is gradually developed
upon a sequence of condensation steps. In the present
contribution, heptazine-based carbon nitrides are prepared
starting from the already semiconducting oligomer OATA.
OATA possesses a number of advantages over traditional
monomeric carbon nitride precursors. First of all, it does not
sublime, e.g. as melamine, that provides a precise control of
the carbon nitride weight after pyrolysis, and does not melt.
This is particularly important for the preparation of carbon
nitride thin films with the controlled thickness and
morphology. Secondly, OATA to carbon nitride transformation
occurs with the significant retention of the flat-band potential
and morphology as it will be illustrated by a number of
examples. Thirdly, OATA is soluble in common organic solvents
making it possible to obtain precursor particles of the desired
morphology using either appropriate synthetic conditions, as it
was shown, for example, for pyrrole oxidative
polymerization,29
or
oligomer
posttreatment,
e.g.
recrystallization etc. The subsequent pyrolysis, in turn,
converts the precursor into a carbon nitride possessing a
similar morphology.
Fig. 2. SEM images of OATA-N (N = 1, 2, 3, 4) (on the left; a, c, e, g) and OATA-CN-N (N =
Fig. 1. Structures of 3-amino-1,2,4-triazole tetramer, OATA (a) and melon (b).
2 | J. Name., 2012, 00, 1-3
1, 2, 3, 4) (on the right; b, d, f, h), respectively.
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for the OATA structure shown in Fig. 1a.
ARTICLE
470 °C is attributed to C2N2 fragments that coVuieldw AortricilgeiOnnalitnee
+
A somewhat increased C/N ratio in the samples is attributed to from the decomposition of the triDaOzoI:l1e0-.1r0in3g9/C7aTtA01h7i1g4hF
the hydrolysis of the side NH2-group to OH-group in alkaline temperatures or from the dimerization of CN+ fragments
reaction medium. The increased H content and the presence mentioned above (Fig. S3i). Some C2N2-related species, such as
+
of oxygen in the materials are due to partial hydration of the C2 fragments (m/z = 24) are found to be produced in small
products surface and the hydrolysis mentioned above. This is quantities at temperatures close to 470 °C (Fig. S3j).
further supported by the TGA-MS studies of OATA discussed The results of the OATA TGA-MS studies indicate clearly that
below. The identity of the Fourier transform infrared (FTIR) the mechanism of carbon nitride formation from OATA differs
spectra of the OATA samples (Fig. S2a) with each other and significantly from the one reported for the polymer synthesis
with the one reported in literature confirms the successful from melamine.31 In the latter case, melem (2,5,8-triamino-tri-
preparation of the tetramers.23
s-triazine) is formed as an intermediate at about 450 °C, and its
All four OATA-N precursors were then subjected to pyrolysis at subsequent condensation reaction at higher temperatures
550 °C for 4 hours. Based on the results of the FTIR leads to the formation of the cross-linked melon polymer,
spectroscopy, powder X-ray diffraction (PXRD), optical typically referred to as ‘graphitic carbon nitride’ (ref-CN). Both
absorption and emission spectroscopies and elemental melem formation from melamine and its further
analysis studies that are discussed below, these obtained polycondensation are accompanied and actually driven by the
products were identified as heptazine-based carbon nitrides, elimination of ammonia gas. OATA, compared to melamine, is
however with a modified electronic structure, namely, with a more nitrogen-rich and hydrogen-poor precursor, thus
very ‘low-lying’ CB.
formation of a carbon nitride from OATA by polycondensation
In order to shine light on the mechanism of OATA mechanism that is accompanied by ammonia release, is highly
transformation to carbon nitride, TGA-MS studies were unlikely from the standpoint of stoichiometry. Instead,
conducted (Fig. S3). OATA is known to be stable up to 200 °C,30 elimination of nitrogen gas appears much more probable and
and this is in agreement with the results of the present study. is supported by the conducted TGA-MS studies. Besides, OATA
Further increase of temperature is accompanied by a gradual contains C‒H bonds, which are absent in melamine, therefore
loss of weight and progressing elimination of species with the production of HCN detected by TGA-MS comes as no surprise.
+
m/z = 27 and 28 that are assigned to HCN+ and N2 , Based on the results of the TGA-MS studies, we suggest a
respectively (Fig. S3a,b). The elimination of HCN is supported plausible mechanism of the tri-s-triazine unit formation from
by the presence of its CN+ fragment with the m/z = 26 (Fig. OATA as depicted in Fig. S4a. The reaction presumably starts
S3c), while the assignment of the m/z = 28 species to N2 is with a nucleophilic attack of the nitrogen atom at the carbon
based on the detection of 15N14N+ species with the m/z = 29 in atom of the adjacent triazole ring resulting in a highly strained
a
ratio, close to the one expected from the relative intermediate ‘A’. The latter is transformed into a zwitterionic
occurrences of the two nitrogen isotopes (Fig. S3d). Besides, intermediate ‘B’ by the cleavage of the C-N bond and thus
the ion current curves for the m/z 28 and 29 species have fully releasing the constraint. Upon the nucleophilic attack of the
identical behavior. The production of HCN reaches maximum imide nitrogen atom at the carbon atom of the adjacent
at about 470 °C, then decreases slightly upon further rise of triazole ring, ‘B’ is converted into the intermediate ‘C’. At this
temperature to 550 °C, and then increases again. The ion point, the structure of the intermediate already resembles a
currents of N2-related species (m/z = 28 and 29) exhibit two tri-s-triazine ring. A migration of 3,5-diamino-triazole-1,2,4
local maxima, at about 220 °C and 470 °C. A steeper drop of moiety with the simultaneous proton shift gives rise to the
the sample weight observed between 440 and 500 °C is intermediate ‘D’. The nucleophilic attack of the nitrogen atom
followed by another gradual decrease of the mass upon of the preorganized tri-s-triazine ring at the triazole ring
temperature increase, yielding a solid residue of about 30 % of carbon atom with the simultaneous elimination of HCN gives
the starting amount at 600 °C. The real residual mass of OATA intermediate ‘E’. The elimination of HCN is in agreement with
at 550 °C under the synthesis conditions used is, however, TGA-MS studies discussed above (see Fig. S3). The evolution of
close to 50 % of the original sample weight. Trace amounts of HCN starts at 210 °C and reaches maximum at 470 °C, then
water in the analytical system are observed as characteristic slowly ceases. ‘E’ is transformed into ‘F’ via the hydride shift of
ionization species with the m/z = 18 (H2O+) and 17 (OH+) the Hb hydrogen atom to diazonium group. This step increases
exhibiting a single ion current maximum at 100 °C (Fig. S3e,f). the number of double bonds involved in the conjugation and
The absence of the sample weight loss at 100 °C proves the thus stabilizes the whole structure. The further migration of
absence of water in the sample itself, while the identical the imidoguanidine-like backbone (highlighted in green)
behavior of the ion current curves of m/z 17 and 18 enables us accompanied by electrocyclization reaction yields the
to attribute the species with the m/z =17 to OH+ only, and intermediate ‘G’. Finally, the formation of the structure ‘H’ is
exclude the presence of NH3 among the products of OATA achieved by the aromatization and therefore stabilization of
+
+
pyrolysis. Oxygen (O2 , m/z = 32) and carbon dioxide (CO2 , the tri-s-triazine structure that obeys Hückel's rule possessing
m/z = 44) originate from the impurities in the carrier gas flow, 14 π-electrons. The formation of ‘H’ is realized by the
as suggested by the absence of the peaks in the corresponding expulsion of N2 and H2 molecules from ‘G’. The evolution of
ion current curves (Fig. S3g,h). The presence of few species nitrogen is confirmed by the TGA-MS studies. Differently from
with the m/z = 52 whose ion current exhibits a maximum at melem which has three amino-groups attached to the
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heptazine ring, the intermediate ‘H’ possesses two different OATA-2 derived carbon nitride (OATA-CN-2) demonstrates two
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reactive groups and a C–H bond, which can further react with peaks at ~161 ppm and ~153 ppm relatedDtOoI:C10.1039/C7TA01714F
N2(NHx) and CN3
OATA pyrolysis intermediates to yield a cross-linked melon carbons of tri-s-triazine unit, respectively (Fig. 3a).33 The
polymer at 550 °C. It is highly probable that some of these absence of a low field signal at 168 ppm, as well as a lower-
reactive groups or C–H bonds in ‘H’ will be preserved in the field shoulder at 161 ppm signal excludes the presence of
structure of the final polymer causing alterations in its surface triazine groups in the polymer. For comparison, SS 13C NMR
properties, such as flat band potential, as will be demonstrated spectrum of the precursor oligomer OATA-2 (Fig. S5) is
below. The triazole ring decomposition can, in principle, be a represented by a broad peak between 135 and 178 ppm
source of the aforementioned C2N2, presumably dicyan composed of a number of overlapping signals of different
species. The proposed mechanism of this process is presented carbon atoms in the tetramer structure. Here, the peaks at
in Fig. S4b and includes homolysis of C-N bond leading to the 135‒140 ppm are presumably due to N2
C-H, N2C-NH2 and
intermediate ‘J’ followed by the radical isomerization to ‘K’. CN2 -OH carbons, while those at 150‒60 ppm are related to
C
Upon expulsion of N2 molecule from ‘L’ and subsequent carbons surrounded by higher number of electron withdrawing
recombination of radicals, intermediate ‘M’ with an azirine (deshielding) substituents (triazole groups). SS 15N NMR
cycle is obtained. The cleavage of C-N bond gives rise to spectrum of OATA-CN-2 (Fig. 3b) shows two peaks at about ‒
azirinium cation and tri-s-triazine predecessor ‘N’. The 178 ppm and about ‒244 ppm that are typically assigned to
migration of proton from the azirinium cation to ‘N’ and
elimination of H2 molecule leads to the rearrangement of Thus, the results of NMR study suggest that the pyrolysis of
azirine ring to dicyan (C2N2). OATA samples at 550°C leads to formation of CN-polymers
NC3 and N
Hx nitrogen atoms in carbon nitride, respectively.34
The limited thermal stability of 1,2,4-triazole ring ensures the built of tri-s-triazine units.
full conversion of the precursor into the CN-polymer at The first hint on the microstructure retention upon
synthesis temperature used (T = 550 °C). Besides, the transformation of OATA to carbon nitride was provided by the
homogeneity of the precursor phase, implied e.g. by the results of the PXRD studies summarized in Fig. 4. The PXRD
uniformity of its particle morphology, together with the fact patterns of OATA-N samples are similar to each other, with a
that the chemical transformations of the precursor occur in pronounced diffraction peak at 2θ = 27.5° that corresponds to
the solid state (see below), lead to the chemical homogeneity the d-spacing of 0.324 nm (Fig. 4a). It is assigned to the
of the resulting products. The local structural order in the interlayer distance in OATA built of 1,2,4-triazole units that
OATA-derived products, however, is shown to be interact strongly via π-π stacking. The peak broadening
inhomogeneous (see below).
observed for OATA-3 sample is due to the decrease of the
The elemental compositions of the carbon nitrides (OATA-CN- OATA crystallite size, while lowering of the peak intensity,
N, N = 1, 2, 3, 4) obtained by the pyrolysis of the OATA-N most prominent in the case of OATA-4 precursor, reflects the
samples are given in Table S1, as found by the elemental decreased number of the stacked layers, i.e. a preferred lateral
analysis. The C/N weight ratios in the products range between orientation in the morphology. In the OATA-CN-N products
0.603 and 0.623 (a hypothetical fully condensed carbon nitride (Fig. 4b), the position of the interlayer stacking peak is slightly
would have 0.64, condensed melon has 0.57); these are higher shifted to the lower angles, 2θ = 26.0–26.5°, implying that the
than in the reference sample obtained by the pyrolysis of layered arrangement of the oligomer is slightly expanded
melamine (ref-CN, C/N
= 0.576) suggesting a higher during heating. In addition, the peak became much broader
condensation degree, as also supported by the much smaller illustrating a decrease of local structural order, typical for the
IR absorption band of the residual amino-groups at 3000–3500 rather complicated thermal elimination scheme. Indeed,
cm-1 (Fig. S2b). The presence of hydrogen is due to the transmission electron microscopy (TEM) studies show the
incomplete condensation of amino-groups and the presence of presence of ‘ultrasmall’ crystallites composed of only few (2 to
the surface adsorbed water. The latter is also responsible for 8) layers in the products (see below). The second typical
the presence of oxygen in the products. The FTIR spectra (Fig. carbon nitride reflection due to the in-plane periodicity of the
S2b) of OATA-CN-N materials are identical to each other and to heptazine units appears at 2θ = 13.2-13.9° and is also rather
the one of the reference carbon nitride. The peak at 810 cm-1
corresponds to the breathing mode of the tri-s-triazine unit.
The group of peaks in the range 1100-1500 cm-1 is due to the
stretching vibrations of C–N and C=N bonds in tri-s-triazine
unit. Two peaks at 1550 and 1650 cm-1 are due to N-H bending
in the residual/terminal secondary and primary amino groups
in the polymers. A broad peak between 2500 and 3600 cm-1 is
attributed to H-X (X = N, O) stretching of the uncondensed
amino-groups and hydroxyl groups at the products surface.
The absence of the peak at 650 cm-1 typical for s-triazine ring
suggests that the polymers are built of heptazine units.32
The cross polarization magic angle spinning (CP-MAS) solid
state (SS) 13C nuclear magnetic resonance (NMR) spectrum of
Fig. 3. 13C (a) and 15N (b) CP-MAS NMR spectra of OATA-CN-2. Asterisks denote spinning
sidebands.
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Fig. 4. PXRD patterns of OATA-N (a) and OATA-CN-N samples obtained after calcination
(b)
broadened.
The direct evidence of the preservation of the original OATA
micro- and macromorphologies in the resulting carbon nitrides
can be provided by SEM studies. Thus, carbon nitrides in the
form of nanorods, nanowires, spherical nanoparticles and
assembled macrospheres and macrosheets are obtained from
OATA-1, OATA-2, OATA-3 and OATA-4 precursors, respectively,
as illustrated in Fig. 2b, d, f, h and S5. The retention of
morphology is a highly appealing feature of the OATA to
carbon nitride transformation that can be used to prepare CN-
polymer structures with the desired morphology. The latter is
important, in turn, since the optical and electronic properties
of carbon nitrides are known to depend on morphology. For
example, carbon nitride nanorods prepared using porous
anodic aluminium oxide as a template were shown to have
improved electronic structure compared to the bulk material
resulting in better performance in hydrogen and oxygen
evolution reactions.35
Another useful property of OATA is its much easier
processability compared to the final carbon nitrides. Thus,
OATA can be dissolved in organic solvents such as DMSO and
N-methyl-2-pyrrolidone (NMP) at room or slightly elevated
temperature. This can be used to prepare thin films which, in
turn, can be converted into carbon nitride upon pyrolysis. An
additional advantage of the morphology retention is that the
surface area of the precursors can be at least partially
preserved in the products (Table S2). Among the starting
compounds, OATA-3 prepared by precipitation from the
concentrated DMSO solution has the highest surface area of
89 m2·g-1. During pyrolysis, the surface area decreased by 26%
to 66 m2·g-1 in the resulting product, OATA-CN-3, probably due
to a partial sintering of the nanoparticles (Fig. 5a). At the same
time, both OATA-3 and the resulting carbon nitride are
characterized by the increased N2 uptakes at the relative
pressures close to 1 that suggests the presence of some
macropores in the solids. The presence of macropores in
OATA-CN-3 product, ranging from 20 nm to few hundred
nanometers, can be further visualised in a TEM image in Fig.
5b. The pyrolysis of OATA-1 and OATA-2 with their specific
surface areas of 38 and 28 m2·g-1, respectively, did not change
the surface area, while the surface area of OATA-4 decreased
from 43 to 35 m2·g-1 upon heat-treatment (Fig. S7).
Fig. 5. N2 sorption isotherms of OATA-3 and OATA-CN-3 (a); TEM image of OATA-CN-3
(b) and HR-TEM images of OATA-CN-2 (c,d). Inset in (c) shows EEL spectrum of carbon
and nitrogen K edges.
presence of some ‘ultrasmall’ crystallites made of 2-8 stacked
layers in the structure that can be regarded as ‘clusters’ or
‘quantum dots’ (Fig. 5d), though the major part of the product
seems to be amorphous. These nano-sized crystallites were
presumably formed in the products as a result of the
favourable orientation of a certain number of precursor
molecules relative to each other. The presence of some
species with quantized energy levels is also reflected in the
core-level electron energy loss (EEL) spectrum of the material
shown in the inset of Fig. 5c exhibiting very pronounced, i.e.
localized, 1s
 π* carbon transition. These nano-sized
crystallites are likely responsible for the enhanced
photocatalytic activity of the materials discussed below. Such
‘clusters’ are found to be extremely sensitive to the high
energy electron beam and, therefore, are very hard to detect.
This follows from the substantial decrease of the intensity of
the 1s
 π* carbon transition in EEL spectrum upon the
extended exposure of the sample to the electron beam. Thus,
Fig. S8 shows two EEL spectra of the same region of OATA-CN-
3 measured one after another without a delay. We assume
that the role of the amorphous carbon nitride support is to
efficiently stabilize the incorporated ultrasmall crystallites that
are mainly responsible for the photocatalytic activity. This is
different from the recently reported cases of the formation of
a so-called ‘isotype nanojunction’.36,37 In the reported cases, g-
CN/g-CN heterojunctions were designed by the co-
polymerization of two different CN-polymer precursors,
yielding products with the different electronic structures. In
our case, however, a single precursor was used; besides, we
find no driving force for a microphase separation, since the
reactions proceed in the solid state, as suggested by the full
preservation of the morphology during the synthesis.
The optical properties of OATA and OATA-CN-N samples were
studied using UV-Visible absorption spectroscopy and steady-
state photoluminescence spectroscopy. The absorption
High resolution (HR) TEM studies demonstrated a segmented
structure of OATA-CN-2 nanowires composed of fused
together spherical nanoparticles (Fig. 5c) as well as the
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spectra of OATA-CN-N materials (Fig. 6, S9) are characterized the presence of different unreacted surfaceViegwrAoruticplesO, nlianes
by the presence of the two bands typical for some recently- discussed above. At the same time, VB poDteOnI:t1ia0.l1o03f9O/CA7TTAA0-C17N14-1F
prepared heptazine based polymers:22 a sharp stepwise one (+2.16 eV) is much more positive compared to that of the ref-
with the edge at ~450 nm which is due to the π
semiconductor transitions and the broad one at ~520 nm Thus, another principal advantage of OATA compared to the
originating from n π* transitions that are only allowed at traditional monomeric carbon nitride precursors, is the
some imperfections of the carbon nitride lattice.38 The π
π* increase of the oxidation potential of the resulting polymer.
transitions provide the main contribution to the photocatalytic During chemical transformation of OATA precursor into carbon
activity of the products, while the n π* transitions may still nitride, OATA loses much more nitrogen than carbon (Fig. S4).

π* CN (+1.6 eV)2,40 suggesting a much stronger oxidation power.



constitute about 10% of the overall visible-light induced Thus, the position of the valence band of the resulting carbon
photocatalytic activity, as it was recently demonstrated for the nitride composed of nitrogen pz orbitals differs more
carbon nitride related polymers.39 All OATA-CN-N products significantly from that of the precursor. Differently, the
absorb much larger fraction of visible light compared to ref-CN position of conduction band of the final polymer made of
(Fig. 6), mainly due to the pronounced absorption at λ > 500 carbon pz orbitals is close to the CB position of OATA.
nm. At the same time OATA-CN-3 absorbs the largest fraction Considering the significant differences in the electronic
of visible light among the differently structured materials (Fig. structures of OATA-derived polymers and traditional carbon
S9b). The optical band gaps of OATA-N precursors and OATA- nitrides, OATA can potentially be used to design hybrid
CN-N products calculated from the corresponding Tauc plots materials with a heterojunction between different carbon
are summarized in Table S2. All OATA-derived carbon nitrides nitride phases aimed at improvement of the charge
have wider optical band gaps compared to their oligomeric separation.
precursors, 2.17
significantly smaller than that of the reference carbon nitride assisted visible light driven hydrogen evolution reaction (HER)
(2.7 eV). using triethanolamine (TEOA) as a sacrificial hole scavenger
‒2.23 eV vs. 1.76‒1.92 eV, but the band gap is The photocatalytic activity of OATA-CN-N was assessed in Pt-
The emission of OATA-CN-N materials is negligible compared (Table S2, Fig. 7b and S12). OATA itself is not active in HER
to that of the reference polymer (Fig. S10a). We attribute this under the selected reaction conditions due to its dissolution in
to the more efficient separation of the photogenerated charge the aqueous TEOA. In general, OATA-CN-N solids exhibit at
carriers, as suggested by the significant photocatalytic activity least twice higher activity in this reaction compared to the
of the OATA-derived products (see below).
grinded reference polymer. The superior activity of the OATA-
The flat band potentials of OATA-1 and OATA-CN-1, EFB, derived polymers compared to ref-CN is a result of the
estimated using electrochemical impedance spectroscopy and interplay of the three factors, including higher surface areas of
Mott-Schottky analysis (Fig. S11) are -0.11 eV and -0.06 eV in the solids, their improved visible light absorption and presence
the reverse hydrogen electrode (RHE) scale, respectively, of ultrasmall crystallites in their structure. These factors
implying that thermal treatment did not cause significant efficiently overbalance the unfavourable reaction kinetics
changes in the flat band position. The band structures of caused by the proximity of the CB potentials of the products to
OATA-1 and OATA-CN-1 established from EFB and optical band the water reduction potential. In particular, OATA-CN-1 and
gap values are schematically represented in the Fig. 7a. The CB OATA-CN-3 are found to be ~5 times more active producing
potential of OATA-CN-1 (-0.06 eV) is significantly less negative ~3.2 µmol H2·hour-1 that can be attributed to the better
than that of the ref-CN (-1.1 eV)2,40 but still thermodynamically structural order in the two products (Fig. 4b) and higher
enables water reduction as demonstrated below. From this surface area possessed by OATA-CN-3 (Table S2). The reaction
point of view, OATA-derived carbon nitrides can be regarded rates can be further increased up to 14.7 µmol H2·h-1, if OATA
as materials with a low-lying CB. The difference of the CB pyrolysis is carried out in LiCl/KCl eutectic salt mixture. Under
potentials of OATA-CN and ref-CN can partially be explained by these conditions, however, the reaction leads to potassium
poly(heptazine imide).25,39
Thus, the low-lying CB position of OATA-derived bulk CN-
polymers still enables efficient hydrogen evolution, which is
Fig. 7. A schematic representation of the band structures of OATA-1 and OATA-CN-1
(a); activity of OATA-CN-N and ref-CN in Pt-assisted HER upon visible light irradiation,
using TEOA as a sacrificial agent (b).
Fig. 6. UV-Vis spectra of OATA-1, OATA-CN-1 and reference CN-polymer.
6 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
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kinetically unfavored in this case. However, the advantage of Preparation of OATA-1: To a solution of 3-aminoVtireiwazAortilcele-1O,n2lin,4e
OATA-CN is their enhanced oxidation power, i.e. higher values (12.47 g, 0.15 mol) and hydrochloric acidD(7O.I2: 1m0.1L0,3397/Cw7TtA.0%17)14inF
of the VB potentials. This gives a chance to overcome the water (450 mL), a solution of ammonium persulfate (42.34 g,
typical bottleneck of CN-polymers, namely much lower 0.19 mol) in water (250 mL) was added. The solution was
reaction rates in water oxidation compared to water stirred at room temperature overnight upon which color
reduction, and would be a subject of future studies.
gradually evolved from colorless to dark red. A solution of
The OATA-derived catalysts are found to be essentially stable NH4OH (50 mL, 30 wt. %) was added to the reaction mixture
during photocatalytic hydrogen evolution, as follows from the until pH=9-10 and mixture was left at stirring for 4 days. Solid
comparison of the PXRD patterns and FTIR spectra of a sample was separated by filtration, washed extensively with water and
catalyst, OATA-CN-X, before and after HER experiment (Fig. dried in vacuum. Yield: 3.03 g (brown solid).
S13). FTIR studies reveal the slight increase of the absorption Preparation of OATA-2: To a solution of 3-aminotriazole-1,2,4
band at 3000–3600 cm-1 corresponding to the stretching (4.16 g, 0.05 mol) and hydrochloric acid (2.4 mL, 37 wt. %) in
vibrations of the surface OH-, NH-, and NH2-groups. This is due water (100 mL), a solution of ammonium persulfate (14.11 g,
to the fact that TEOA works as an efficient dispersing agent for 63 mmol) in water (100 mL) was added. The solution was
the CN-polymers and thus causes partial exfoliation of the stirred at room temperature overnight upon which color
solid, which is accompanied by the creation of new catalyst – gradually evolved from colorless to dark red. A solution of KOH
reaction solution interfaces. The newly created catalyst in water was added to the reaction mixture until pH=9-10 and
surfaces are then stabilized by bearing terminal OH-, NH-, and reaction mixture was left at room temperature for 2 days.
NH2-groups. The materials can be recycled after the Brown solid was separated by centrifugation, washed with
photocatalytic test and are shown to maintain the original water (3x45 mL) and water (30 mL) was added to the brown
activity in three consecutive photocatalytic runs (Fig. S14).
residue. The material was obtained as a fluffy powder after
freeze-drying; yield: 535 mg.
Preparation of OATA-3: OATA-1 (400 mg) was dissolved in
DMSO (20 mL) at stirring at +60 °C for 15 min and cooled to
room temperature. Water (500 mL) was added to the dark red
solution. Within 2 days a fine powder precipitated. It was
separated by centrifugation and washed with water (5x1.5
mL). Water (15 mL) was added to the brown residue and
freeze dried. Yield: 232 mg (brown solid).
Preparation of OATA-4: OATA-1 (212 mg) and water (30 mL)
were placed into a flask and kept in a sonication bath for 30
min. Suspension was frozen in a liquid nitrogen bath and
freeze-dried.
Conclusions
In the current contribution we showed that 3-aminotriazole-
1,2,4 tetramer OATA can be transformed into carbon nitrides
by heat treatment. The reaction mechanism involves a number
of condensation steps accompanied by the elimination of HCN
and N2 as revealed by the TGA-MS analysis. The OATA to
carbon nitride transformation occurs in a topotactic-like
fashion, with preservation of the macro- and
micromorphology. The habitus of the final products is defined
by the morphology of the original OATA nanoparticles, and can
be tuned in a wide range, from 30 nm spherical nanoparticles
to few micrometers long nanowires. The flat band position of
the semiconductor is substantially preserved during the
thermal semiconductor-to-semiconductor transition, while the
optical band gap is getting bigger, due to the higher stability of
the annealed samples. Therefore, using OATA, a class of
carbon nitrides with low-lying CB and potentially higher
Preparation of OATA-CN-1, OATA-CN-2, OATA-CN-3 and
OATA-CN-4: Caution! The exhaust fumes should be properly
neutralized, e.g. by passing the exhaust flow through alkaline
dilute bleach solutions, as they might contain HCN, produced
upon OATA calcination. A porcelain crucible equipped with a
lid and containing a precursor (OATA-1 –335 mg, OATA-2 – 510
mg, OATA-3 – 232 mg, OATA-4 – 207 mg) was placed into the
oven operated under N2 flow (20 L/min). The temperature in
the oven was increased from 20°C to 550°C within 4 h
(~2.2°/min), maintained at 550°C for 4 h and then allowed to
cool slowly to room temperature. Yield: OATA-CN-1 – 157 mg
(47% of the original weight), OATA-CN-2 – 263 mg (52%),
OATA-CN-3 – 111 mg (48%), OATA-CN-4 – 87 mg (42%).
oxidative power (above
2
Volt) are obtained. The
photocatalytic activity of OATA-derived products in HER is up
to 5 times higher than that of the polymer obtained by
pyrolysis of melamine. This is due to the number of factors,
including better spectral coverage, presence of highly active
‘ultrasmall’ crystallites and higher surface area of the catalysts
which, in turn, is a consequence of the morphology retention.
Potentially, OATA is an excellent precursor for the preparation
of carbon nitride films with the controlled thickness and
morphology, and can be used to prepare composite carbon
nitride materials with a heterojunction.
Notes and references
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Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C7TA01714F
An organic semiconducting tetramer is converted by pyrolysis at 550 °C into carbon nitride
semiconductor with the improved photocatalytic activity.