Photophysics and Photocatalysis of Melem: A Spectroscopic Reinvestigation

Article abstract of DOI:10.1002/asia.201800186

Graphitic carbon nitride (g-CN) is one potential metal-free photocatalyst. The photocatalytic mechanism of g-CN is related to the heptazine ring building unit. Melem is the simplest heptazine-based compound and g-CN is its polymeric product. Thus, studies on the photophysical properties of melem will help to understand the photocatalytic mechanism of heptazine-based materials. Herein, the spectroscopic features of melem were systematically explored through measuring its absorption spectrum, fluorescence spectrum, and fluorescence decay. Both fluorescence spectroscopy and fluorescence decay measurements show that the condensation of melamine to melem causes stronger photoluminescence, whereas the condensation of melem to g-CN causes weaker photoluminescence. In addition, all observations reveal that a mixture of monomer melem and its higher condensates is more easily obtained during the preparation of melem, and that the higher condensates of melem affect the photophysical properties of melem dominantly. The photocatalytic hydrogen evolution of melem has also been measured and the monomer melem has negligible photoinduced water-splitting activity.

Full text of DOI:10.1002/asia.201800186

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Accepted Article
Title: Photophysics and Photocatalysis of Melem: A Spectroscopic
Reinvestigation
Authors: Anchi Yu, Jing Wen, Ruiyu Li, and Rong Lu
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Photophysics and Photocatalysis of Melem: A Spectroscopic
Reinvestigation
Jing Wen, Ruiyu Li, Rong Lu*, and Anchi Yu*[a]
Abstract: Graphitic carbon nitride (g-CN) is one potential metal-free
photocatalyst. The photocatalytic mechanism of g-CN is involved
with its building unit heptazine ring. Melem is the simplest heptazine-
based compound and g-CN is its polymeric product. Thus, studies
on the photophysical property of melem will help to understand the
Melem (2,5,8-triamino-tri-s-triazine, Scheme 1) can be
obtained by thermal condensation of melamine, and it is also the
intermediate of the further thermal condensation product g-CN at
higher calcination temperature.[14-17] Kroke et al. have
summarized the formation of melem as well as its structure,
photocatalytic mechanism of heptazine-based materials. In this work, properties and applications.[15] In 2014, three groups respectively
we systematically explored the spectroscopic features of melem
through measuring its absorption spectrum, fluorescence spectrum
and fluorescence decay, respectively. Both fluorescence
spectroscopy and fluorescence decay measurements show that the
condensation of melamine to melem causes the stronger
photoluminescence, while the condensation of melem to g-CN
causes the weaker photoluminescence. In addition, all observations
reveal that a mixture of monomer melem and its higher condensates
is more easily obtained during the preparation of melem, and that
the higher condensates of melem affect the photophysical properties
of melem dominantly. The photocatalytic hydrogen evolution of
melem was also measured and it is found that monomer melem has
negligible photoinduced water splitting activity.
reported the photoinduced water splitting reaction by melem, but
they have got different opinions about the photocatalytic activity
of melem. Brꢀckner et al. systematically explored the structure-
activity relationship of g-CN at different calcination temperatures
and they found low catalytic activity for melem.[17] Later, Zou et al.
studied the photocatalytic H2 evolution of melem and they
concluded that melem is a metal-free unit for photocatalytic H2
evolution.[18] Almost at the same time, Lotsch et al. explored the
photocatalytic H2 evolution of melem and its higher condensates
and claimed that the oligomers of melem is the most active
species.[14] The understanding on the photocatalytic reaction
mechanism of
a photocatalyst closely depends on the
knowledge of its photophysical properties. As for the
photophysical properties of melem, there existed inconsistence
such as its fluorescence decay lifetime and fluorescence
16-20]
spectrum.[12,
Therefore, a systematic investigation on the
Introduction
photophysical properties of melem is still in need to correctly
understand its photocatalytic mechanism and its related
heptazine-based materials.
Graphitic carbon nitride (g-CN) has become the most important
and potential visible light responsive metal-free photo-induced
water splitting catalyst over past several years.[1-11] However, the
mechanism of the photocatalytic water splitting reaction by g-CN
is not fully understood yet.[12-14] Recently, Lau et al. explored low
molecular weight g-CN for solar hydrogen evolution and found
that oligomers of melem have higher photocatalytic water-
splitting activity than that of g-CN as well as melem monomer.[14]
Later, Ehrmaier et al. applied first-principle computation and
proposed that water splitting with heptazine-based materials can
be understood as a molecular excited-state reaction taking place
in hydrogen-bonded heptazine-water complexes.[13] g-CN is the
polymeric product of melem and heptazine ring is their basic
building block (Scheme 1). Since the photocatalytic water-
splitting reaction by g-CN is related to the heptazine unit and
melem is the simplest heptazine-based compound, the
comprehensive investigation of the photophysical properties of
melem will help to more deeply understand the mechanism of
the photoinduced water splitting reaction of heptazine-based
materials.
For comparing with the photophysical data of melem
16-19]
presented in literature[14,
, in this work, purified and
unpurified melem were characterized by X-ray diffraction (XRD),
Fourier transform infrared (FTIR) spectroscopy, element analysis,
UV-Vis diffuse reflectance spectroscopy (UVDRS), fluorescence
spectroscopy, and fluorescence decay lifetime measurement.
The spectroscopic properties of the precursor melamine and its
thermal condensation product g-CN were reexamined. The
Brunauer-Emmett-Teller (BET) surface areas and the
photocatalytic H2 evolutions of melamine, melem and g-CN were
also measured. In addition, the fluorescence feature of melem in
aqueous solution was explored for the first time.
[a]
Jing Wen, Ruiyu Li, Prof. Dr. R. Lu, Prof. Dr. A. Yu
Department of Chemistry, Renmin University of China, Beijing
100872, P. R. China
E-mail: lurong@ruc.edu.cn, yuac@ruc.edu.cn
Scheme 1. Chemical structures of melamine, melem, and g-CN.
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Results and Discussion
shown in Figure 2, it is found that the FTIR spectrum of melem is
more structured than those of the as-synthesized melem and the
hot-water-washed melem in the range of 3000 ~ 3700 cm−1. The
elemental analysis data of melamine, as-synthesized melem,
hot-water-washed melem, melem, and g-CN are presented in
Table 1. The C:N ratio (0.601) of melem (C6N10H6) shown in
Table 1 is extremely close to the theoretical value, further
indicating that the high quality of pure melem is obtained in this
work.
XRD, FTIR and elemental analysis characterization of melem:
Figure 1 shows the XRD patterns of the as-synthesized melem,
hot-water-washed melem, and melem. In Figure 1, the XRD
patterns of melamine and g-CN are also displayed for reference.
From Figure 1, it is found that the XRD pattern of the as-
synthesized melem in this work is consistent with the reported
XRD pattern of the as-synthesized melem,[14, 18-19] and the XRD
pattern of the hot-water-washed melem is consistent with that of
the DMSO soluble melem[14] and the dehydrated melem[21]. The
obvious difference of the XRD patterns between the as-
synthesized melem and the hot-water-washed melem indicates
that the as-synthesized melem contains a certain amount of
melamine and that hot water can remove the non-reacted
melamine from the as-synthesized melem. According to the
reported method, the hot-water-washed melem was further
boiled with HCl and then with NaOH solution,[22] and finally
melem power was obtained. The XRD pattern of melem shown
in Figure 1 is similar to the calculated pattern based on single-
crystal data[21] and the temperature-programmed XRD pattern of
melem[23], as well as the observed and calculated XRD pattern
of melem[16], indicating that high quality of pure melem is
obtained in this work.
Figure 2. FTIR spectra of melamine, as-synthesized melem, hot-water-
washed melem, melem, and g-CN.
Table 1. Elemental analysis of melamine, as-synthesized melem, hot-water-
washed melem, melem, and g-CN.
Sample
melamine (C3N6H6)
as-synthesized melem
hot-water-washed melem
melem (C6N10H6)
g-CN
C (ω%)
N (ω%)
C:N atomic ratio
0.501±0.001
0.605±0.001
0.604±0.001
0.601±0.002
0.665±0.04
28.85±0.01
32.46±0.09
30.67±0.04
30.28±0.01
34.60±0.12
67.14±0.02
62.53±0.09
59.25±0.08
58.74±0.02
60.66±0.09
Figure 1. X-ray diffraction patterns of melamine, as-synthesized melem, hot-
water-washed melem, melem, and g-CN.
UVDRS and fluorescence spectra of melem: Figure
3
Figure 2 shows the FTIR spectra of as-synthesized melem,
hot-water-washed melem, and melem. In Figure 2, the FTIR
spectra of melamine and g-CN are also displayed for reference.
As shown in Figure 2, all FTIR spectra of the as-synthesized
melem, the hot-water-washed melem and melem present similar
features, which agree with the reported FTIR data of melem.[14,
displays the normalized UVDRS spectra of melamine, as-
synthesized melem, hot-water-washed melem, melem, and g-
CN. As shown in Figure 3, the absorption maximum wavelength
of melem locates at around 330 nm. Lotsch et al. reported the
UVDRS of the DMSO soluble melem powder maximized around
350 nm[14], and Zou et al. reported the UVDRS of melem powder
maximized at around 390 nm[18]. According to the DRS spectral
peaks of the as-synthesized melem, the hot-water-washed
melem, and melem shown in Figure 3, the inconsistences
among UVDRS data of the obtained melem from different
groups are due to the different constitutes of the obtained melem.
As shown in Figure 3, the UVDRS spectral band of the prepared
melem presents monomer characteristic, indicating that the
prepared melem in this work is monomer melem, while the as-
18-19, 21, 23]
In general, the FTIR spectra of all melem samples
present characteristic bands of aromatic CN heterocycles in the
range of 1200 ~ 1700 cm−1 and one breathing mode of tri-s-
triazine unit at 805 cm−1. The broad bands in the range of 3000 ~
3700 cm−1 are attributed to the stretching modes of secondary
and primary amines, or their intermolecular hydrogen-bonding
interactions.[24] Comparing the FTIR spectra of the as-
synthesized melem, the hot-water-washed melem, and melem
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Figure 4. Fluorescence spectra of melamine (a), as-synthesized melem (b),
hot-water-washed melem (c), melem (d), and g-CN (e) under 250 nm light
excitation.
synthesized melem as well as the hot-water-washed melem
contains both monomer melem and its higher condensates.
Figure 4 shows the fluorescence spectra of melamine, as-
synthesized melem, hot-water-washed melem, melem, and g-
CN under 250 nm light excitation. As shown in Figure 4, it is
found that the fluorescence emission of melem is peaked at
around 395 nm, which is consistent with Shigesato’s report but
is inconsistent with the Ricci’s and Brꢀckner’s reports. Shigesato
et al. reported the emission maximum of melem at 390 nm,[25]
Ricci et al. reported the emission maximum of melem at 415
nm,[20] and Brückner et al. reported the emission maximum of
melem at around 420 nm.[17] In Figure 4, we also found that the
fluorescence peak positions of the as-synthesized melem, hot-
water-washed melem, and g-CN are all red shifted comparing
with that of melem. Thus, it is reasonable to suggest that the
different emission maxima of melem among Ricci’s (415 nm),
Brꢀckner’s (420 nm) and ours (395 nm) are due to the effect of
the higher condensates of melem on melem. In addition, it is
also found that the photoluminescence intensities of melamine,
melem and g-CN present the decreasing sequence as melem >
g-CN > melamine, indicating that the condensation of melamine
to melem causes the stronger photoluminescence, while the
condensation of melem to g-CN causes the weaker
photoluminescence.
Fluorescence decay measurement of melem: For further
explore the photoluminescence property of melem, the
fluorescence decays of melamine, as-synthesized melem, hot-
water-washed melem, melem, and g-CN were measured under
250 nm light excitation by using a home-made time-correlated
single photon counting setup, as shown in Figure 5. All the
fluorescence decays can be fitted with a two-exponential decay
function and their fitting parameters are summarized in Table 2.
In Figure 5, the symbols denote the experimental data and the
solid lines are their respective fitting curves. The instrumental
response function (IRF) curve is also displayed for reference.
From Table 2, it is found that the fluorescence lifetimes of
melamine, melem and g-CN present the decreasing sequence
as melem (9.78 ns) > g-CN (0.89 ns) > melamine (0.20 ns),
which is consistent with their respective steady-state
fluorescence spectroscopic results shown in Figure 4. Zhang et
al. reported a more than 300 ns fluorescence lifetime for melem
powder,[19] which might be due to the instrument response time
limit
of
their
luminescence
lifetime
setup.
The
photoluminescence decay lifetime of g-CN measured in this
work is close to reported photoluminescence lifetime of g-CN
prepared at 550 oC calcination temperature[26]. In addition, the
fluorescence lifetime of melamine is quiet short and close to the
IRF time scale, indicating there is almost no fluorescence
emission from melamine, which agrees with the reported result
from the study of its excited dynamics.[27] Kohler et al. explored
the excited-state dynamics of melamine in aqueous solution and
observed that the excited melamine molecules can decay to its
electronic ground state with tens of picoseconds after excitation
at 240 nm.[27]
Figure 3. Normalized UVDRS spectra of melamine (a), as-synthesized melem
(b), hot-water-washed melem (c), melem (d), and g-CN (e).
Figure 5. Fluorescence decays of melamine, as-synthesized melem, hot-
water-washed melem, melem, and g-CN. Excitation wavelength: 250 nm;
detection wavelength: 420 nm.
Table 2. Fluorescence decay fitting parameters for melamine, as-synthesized
melem, hot-water-washed melem, melem, g-CN, and 0.003 mg/ml aqueous
melem solution.
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From Figure 7A, it is found that the fluorescence maximum
excitation wavelength of aqueous melem solution locates at
around 250 nm, and the fluorescence maximum emission
wavelength of aqueous melem solution locates at around 365
nm. Meanwhile, it is also found that there is no spectral shift of
the fluorescence spectra of 0.003 mg/ml aqueous melem
solution under different wavelength light excitation, implying that
the component of aqueous melem solution is uniform, and thus
the component of the prepared melem powder in this work is
also uniform. In addition, it is further found that there is very
weak fluorescence excitation and fluorescence emission for
aqueous melamine solution (Figure 7B), which agrees well with
the florescence spectrum of melamine shown in Figure 4 and
the fluorescence decay of melamine shown in Figure 5.
Sample
1
τ1 / ns
2
τ2 / ns
<> / ns [a]
melamine
as-synthesized melem
hot-water-washed melem
melem
1.00
0.90
0.86
0.89
0.90
0.41
0.20
7.11
9.19
7.06
0.44
6.61
0.20
9.79
0.10
0.14
0.11
0.10
0.59
33.93
33.53
31.78
4.98
12.59
9.78
g-CN
0.89
aqueous melem solution
48.29
31.20
  (11 22 )/(1 2 ).
[a]
UV-Vis absorption spectra, fluorescence spectra and
fluorescence decay of aqueous melem solution: Due to the
low solubility of melem in aqueous solution, the photophysical
property of melem in aqueous solution is rarely studied in
previous report. Figure 6 shows the UV-Vis absorption spectrum
of 0.003 mg/ml aqueous melem solution in the wavelength range
of 190 - 400 nm. The UV-Vis absorption spectrum of 0.003
mg/ml aqueous melamine solution is also present for reference.
In Figure 6, the insert is their respective normalized absorption
spectra, which agree with the reported UV-Vis absorption
spectra of aqueous solutions of melamine (1.25 10-4 M) and
melem (210-5 M).[28] Compared with the UVDRS spectra of
melamine and melem shown in Figure 3, both the absorption
spectra of their respective aqueous solution present large blue
shift, which might be due to the condensed phase characteristic.
Figure 7. (A) Fluorescence spectra of 0.003 mg/ml aqueous melem solution
under different wavelengths light excitation. Insert: fluorescence excitation
spectra of 0.003 mg/ml aqueous melem solution at 365 nm light emission. (B)
Fluorescence spectra of 0.003 mg/ml aqueous melamine solution under
different wavelengths light excitation. Insert: fluorescence excitation spectra of
0.003 mg/ml aqueous melamine solution at 365 nm light emission.
Figure 6. UV-Vis absorption spectra of 0.003 mg/ml aqueous melamine
solution (a) and 0.003 mg/ml aqueous melem solution (b). Insert: Normalized
UV-Vis absorption spectra of 0.003 mg/ml aqueous melamine solution (a) and
0.003 mg/ml aqueous melem solution (b).
Figure 8A shows the normalized fluorescence spectra of
0.003 mg/ml aqueous melamine solution and 0.003 mg/ml
aqueous melem solution, and Figure 8B shows the fluorescence
decays of 0.003 mg/ml aqueous melamine solution and 0.003
mg/ml aqueous melem solution. The fluorescence decay fitting
parameters are also listed in Table 2. From Figure 8, it is found
that both the fluorescence spectrum of 0.003 mg/ml aqueous
melamine solution and the fluorescence decay profile of 0.003
mg/ml aqueous melamine solution overlap perfectly with those
Figure 7A shows the fluorescence emission spectra of 0.003
mg/ml aqueous melem solution under different wavelength light
excitation, and the insert is the fluorescence excitation spectrum
of 0.003 mg/ml aqueous melem solution under 365 nm light
emission. For reference, Figure 7B also displays the
fluorescence emission spectra of 0.003 mg/ml aqueous
melamine solution under different wavelength light excitation,
and the insert is the fluorescence excitation spectrum of 0.003
mg/ml aqueous melamine solution under 365 nm light emission.
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of 0.003 mg/ml aqueous melem solution. Both the fluorescence
spectrum of melamine powder shown in Figure 4 and the
fluorescence decay of melamine powder shown in Figure 5
clarify that there is no fluorescence emission from melamine.
Therefore, it is reasonable to conclude that the fluorescence
emission of 0.003 mg/ml aqueous melamine solution (Figure 7B)
originates from the fluorescence emission of the melem impurity
containing in melamine. Then, the previous reports about the
fluorescence characteristic of melamine [29-30] should be carefully
interpreted. Besides, the fluorescence decay lifetime of 0.003
mg/ml aqueous melem is much longer than that of melem
powder (Table 2), which is due to the stronger molecular
interaction in condensed phase of melem.
summarized in Table 3. The determined photocatalytic H2
evolution rate for g-CN under Xeon lamp full light irradiation is
31]
consistent with the previous reports.[18,
From Figure 9 and
Table 3, it is found that the photocatalytic H2 evolutions of
melamine and melem under Xeon lamp light irradiation are
almost negligible. Since the photocatalytic H2 evolution efficiency
is related to the surface area of photocatalyst, the BET surface
areas of melamine, as-synthesized melem, hot-water-washed
melem, melem, and g-CN are also measured and summarized
in Table 3. Table 3 shows that melamine has very low surface
area and negligible photocatalytic activity. The negligible
photocatalytic activity of melamine is mainly due to its absent
absorbance in the Xeon lamp spectral region. As for melem, its
surface area is larger than that of g-CN, but its H2 evolution rate
is also negligible compared to g-CN, which is due to the
unsuitable oxidation and reduction levels of melem for water
splitting reaction.[4, 18] However, the higher photocatalytic activity
of the as-synthesized melem as well as the hot-water-washed
melem as shown in Table 3 probably originates from the higher
condensates of melem in the product.[14]
Figure 9. Photocatalytic H2 evolutions of melamine, as-synthesized melem,
hot-water-washed melem, melem, and g-CN under 300 W Xeon lamp light
(300 - 700 nm) irradiation.
Table 1. BET surface areas and photocatalytic H2 evolution rates of
melamine, as-synthesized melem, hot-water-washed melem, melem, and g-
CN.
Figure 8. (A) Normalized fluorescence spectra of 0.003 mg/ml aqueous
melamine solution (black line) and 0.003 mg/ml aqueous melem solution (red
line) under 250 nm light excitation; (B) Fluorescence decays of 0.003 mg/ml
aqueous melamine solution (black line) and 0.003 mg/ml aqueous melem
solution (red line). Excitation wavelength: 250 nm; detection wavelength: 420
nm.
Sample
BET surface area
H2 evolution rate
(μmol•g-1•h-1)
(m2•g-1)
melamine
as-synthesized melem
hot-water-washed melem
melem
0.3±0.01
3.9±0.1
0
135
170
1.4
400
Photocatalytic H2 evolution: g-CN is a potential function
material for photoinduced water splitting catalyst and its building
block heptazine ring is proposed to contribute to its
photocatalytic mechanism. In this work, we also measured the
photocatalytic H2 evolutions of melamine, as-synthesized melem,
hot-water-washed melem, melem, and g-CN under Xeon lamp
(spectral region around 300 - 700 nm) light irradiation, as shown
in Figure 9. The determined photocatalytic H2 evolution rates for
melamine, as-synthesized melem, hot-water-washed melem,
melem, and g-CN under Xeon lamp light irradiation are all
13.9±0.1
19.4±0.1
7.0±0.1
g-CN
Conclusions
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The X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-
7000 diffractometer with Cu-Kα radiation(λ = 1.54056 Å) under 40 KV
and 30 mA using a scan rate of 2degree/min. Fourier transform infrared
(FTIR) spectra were collected with a Brucker Tensor 27 in a KBr disk in
the scan range of between 400 and 4000 cm-1. UV-Vis diffuse reflectance
spectrometry (UVDRS) spectra were recorded on a Shimadzu UV-2600
spectrophotometer equipped with a diffuse reflectance accessory with
BaSO4 as the reference. UV-Vis absorption spectra were recorded with a
Shimadzu UV-3600 spectrophotometer. Fluorescence excitation and
emission spectra were recorded on a Hitachi F-4600 spectrophotometer.
The chemical compositions of the sample were determined by using an
elemental analyzer (Flash-EA-1112). The Brunauer−Emmett−Teller
(BET) surface areas of samples were determined with a Belsorp-Mini II
equipment through measuring its N2 adsorption−desorption isotherm at
77 K.
In conclusion, monomer melem was prepared and characterized
by XRD, FTIR and element analysis, as well as by absorption
spectroscopy, fluorescence spectroscopy and fluorescence
decay measurement. All observations verify that a mixture of
monomer melem and its higher condensates is more easily
obtained during the preparation of melem, and that the higher
condensates of melem in the mixture will affect the
spectroscopic feature of melem. The photocatalytic H2 evolution
of melem data indicates that monomer melem has negligible
photoinduced water splitting activity under 300 - 700 nm light
irradiation, which is due to the unsuitable oxidation and
reduction levels of monomer melem for water splitting reaction.
In agreement with the literature, the higher condensates of
melem will contribute to the photocatalytic water splitting activity.
In addition, the photophysical properties of aqueous melem
solution and aqueous melamine solution are also explored, and
the experimental data reveals that melamine molecule has no
fluorescence emission.
The fluorescence decays were measured using a homemade time-
correlated single photon counting (TCSPC) apparatus.[33] Briefly, the
second harmonic (250 nm) of the output (500 nm) from an optical
parametric amplifier pumped by a Spectral Physics 1 kHz amplified
Ti:sapphire laser was used as the excitation source. The emission was
collected and sent into a Princeton Instruments SP2358 monochromator
and detected with a Hamamatsu R3809U-50 MCP-PMT. The signal from
the R3809U-50 MCP-PMT was amplified by a Becher&Hickl GmbH
HFAC-26 preamplifier. Then the output of the HFAC-26 preamplifier and
the output of a fast PicoQuant TDA 200 photodiode were, respectively,
connected to a Becher&Hickl GmbH SPC-130 module as the start and
stop pulses. The instrumental response function (IRF) of the TCSPC
setup was about 70 ps.
Experimental Section
Chemicals
Melamine (99%) was purchased from Sigma-Aldrich (St. Louis, MO,
USA) and used without further purification. HCl (36%-38%) and NaOH
(96%) were purchased from Sinopharm Chemical Reagent Co., Ltd.
(Beijing, China). Pure water (18 MΩ•cm) was obtained through a Milli-Q
water purification system (Millipore, Billerica, MA, USA).
Photocatalytic H2 evolution
Photocatalytic H2 evolution reaction was conducted on a Pyrex top-
irradiation reaction vessel connected to a closed gas system.[34] In detail,
100 mg catalyst powder was dispersed in 100 ml of aqueous solution,
which contains 10% (volume) triethanolamine. A total of 1% (weight) Pt
cocatalyst was deposited on the surface of catalyst powder directly
adding H2PtCl6 in 100 ml of aqueous solution. The air was completely
removed before the reaction solution was irradiated with a 300 W
LX300F Xeon lamp without any filter. A gas chromatography was used to
determine the amount of H2.
Sample preparation
Melamine (10 g) was heated in an alumina crucible with a cover at 400
oC for 4 h within a muffle furnace with a heating rate of 2.3 oC/min to get
as-synthesized melem.[17] To remove a certain amount of non-reacted
melamine, the obtained as-synthesized melem (500 mg) was placed in
200 ml pure water, and boiled for 1 h under 100 oC due to that the
solubility of melamine in hot water is higher than that of melem,[28] then
centrifuged quickly and dried at 70 oC and hot-water-washed melem was
obtained. The yield for the hot-water-washed melem is about 90%.
Following the reported method,[22] the obtained hot-water-washed melem
(200 mg) was further boiled in 36% HCl (15 ml) solution for 10 min,
successively boiled in 2% NaOH solution (20 ml) for 10 min, and then
sufficiently washed with water and dried at 70 oC, and finally melem
power was obtained. The obtained melem was fully ground in an agate
mortar for further characterization. The yield for melem is about 85%.
Acknowledgements
Financial supports by the National Natural Science Foundation
of China (21773306, 21373269) are gratefully acknowledged.
Keywords: melem, melamine, carbon nitride, heptazine ring,
According to the reported method,[32] g-CN was prepared by heating
melamine in an alumina crucible with a cover at 550 oC within a muffle
furnace for 4 h with a ramping rate of 2.3 oC /min and then cooled
naturally to room temperature. The as-prepared g-CN was washed in hot
water and dried at 70 oC, and then fully ground in an agate mortar for
further characterization.
photocatalytic hydrogen evolution
[1] T. S. Miller, A. B. Jorge, T. M. Suter, A. Sella, F. Cora, P. F. McMillan,
Phys. Chem. Chem. Phys. 2017, 19, 15613-15638.
[2] W.-J. Ong, L.-L. Tan, Y. H. Ng, S.-T. Yong, S.-P. Chai, Chem. Rev. 2016,
116, 7159-7329.
[3] Y. Wang, X. Wang, M. Antonietti, Angew. Chem. Int. Ed. 2012, 51, 68-89.
[4] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K.
Domen, M. Antonietti, Nat. Mater. 2009, 8, 76-80.
Aqueous solution was prepared as following: first, 3.0 mg melamine or
melem was dissolved into 100 ml water as stock solution, then 1 ml stock
solution was diluted 10 times with water, and finally 0.003 mg/ml
melamine or melem aqueous solution was obtained.
[5] S. Cao, J. Yu, J. Phys. Chem. Lett. 2014, 5, 2101-2107.
[6] J. Zhang, G. Zhang, X. Chen, S. Lin, L. Moehlmann, G. Dolega, G. Lipner,
M. Antonietti, S. Blechert, X. Wang, Angew. Chem. Int. Ed. 2012, 51,
3183-3187.
Characterization
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[7] Y. Zheng, L. Lin, X. Ye, F. Guo, X. Wang, Angew. Chem. Int. Ed. 2014, 53,
11926-11930.
[20] L. Stagi, D. Chiriu, C. M. Carbonaro, R. Corpino, P. C. Ricci, Diamond
Relat. Mater. 2016, 68, 84-92.
[8] J. Zhang, M. Zhang, R.-Q. Sun, X. Wang, Angew. Chem. Int. Ed. 2012, 51,
10145-10149.
[21] S. J. Makowski, P. Koestler, W. Schnick, Chem. Eur. J. 2012, 18, 3248-
3257.
[9] H. Ou, P. Yang, L. Lin, M. Anpo, X. Wang, Angew. Chem. Int. Ed. 2017, 56,
10905-10910.
[22] T. Komatsu, Macromol. Chem. Phys. 2001, 202, 19-25.
[23] B. V. Lotsch, W. Schnick, Chem. Eur. J. 2007, 13, 4956-4968.
[24] Y. Cui, Z. Ding, P. Liu, M. Antonietti, X. Fu, X. Wang, Phys. Chem. Chem.
Phys. 2012, 14, 1455-1462.
[10] L. Lin, C. Wang, W. Ren, H. Ou, Y. Zhang, X. Wang, Chem. Science
2017, 8, 5506-5511.
[11] J. Sun, J. Zhang, M. Zhang, M. Antonietti, X. Fu, X. Wang, Nat. Commun.
2012, 3.
[25] M. Hasegawa, A. Ishii, K. Habu, H. Ichikawa, K. Maeda, S. Kishi, Y.
Shigesato, Sci. Technol. Adv. Mater. 2006, 7, 72-76.
[26] H. Zhang, A. Yu, J. Phys. Chem. C 2014, 118, 11628-11635.
[27] Y. Zhang, A. A. Beckstead, Y. Hu, X. Piao, D. Bong, B. Kohler, Molecules
2016, 21, 1645(1-11).
[12] C. Merschjann, T. Tyborski, S. Orthmann, F. Yang, K. Schwarzburg, M.
Lublow, M. C. Lux-Steiner, T. Schedel-Niedrig, Phys. Rev. B 2013, 87,
205204(1-8).
[13] J. Ehrmaier, T. N. V. Karsili, A. L. Sobolewski, W. Domcke, J. Phys.
Chem. A 2017, 121, 4754-4764.
[28] S. Uemura, M. Aono, K. Sakata, T. Komatsu, M. Kunitake, J. Phys. Chem.
C 2013, 117, 24815-24821.
[14] V. W.-h. Lau, M. B. Mesch, V. Duppel, V. Blum, J. Senker, B. V. Lotsch, J.
Am. Chem. Soc. 2015, 137, 1064-1072.
[29] C. Yang, Y. Liu, L. Li, F. Zhang, Spectrochim. Acta A Mol. Biomol.
Spectrosc. 2010, 75, 1329-1332.
[15] A. Schwarzer, T. Saplinova, E. Kroke, Coord. Chem. Rev. 2013, 257,
2032-2062.
[30] L. Stagi, D. Chiriu, A. Ardu, C. Cannas, C. M. Carbonaro, P. C. Ricci, J.
Appl. Phys. 2015, 118, 125502(1-7).
[16] B. Jurgens, E. Irran, J. Senker, P. Kroll, H. Muller, W. Schnick, J. Am.
Chem. Soc. 2003, 125, 10288-10300.
[31] G. Liu, P. Niu, C. Sun, S. C. Smith, Z. Chen, G. Q. Lu, H.-M. Cheng, J.
Am. Chem. Soc. 2010, 132, 11642-11648.
[17] D. Hollmann, M. Karnahl, S. Tschierlei, K. Kailasam, M. Schneider, J.
Radnik, K. Grabow, U. Bentrup, H. Junge, M. Beller, S. Lochbrunner, A.
Thomas, A. Brueckner, Chem. Mater. 2014, 26, 1727-1733.
[18] S. Chu, C. Wang, J. Feng, Y. Wang, Z. Zou, Inter. J. Hydrogen Energy
2014, 39, 13519-13526.
[32] S. C. Yan, Z. S. Li, Z. G. Zou, Langmuir 2009, 25, 10397-10401.
[33] H. Zhang, S. Li, R. Lu, A. Yu, ACS Appl. Mater. Interfaces 2015, 7,
21868-21874.
[34] Y. Chen, B. Wang, S. Lin, Y. Zhang, X. Wang, J. Phys. Chem. C 2014,
118, 29981-29989.
[19] H. B. Zheng, W. Chen, H. Gao, Y. Y. Wang, H. Y. Guo, S. Q. Guo, Z. L.
Tang, J. Y. Zhang, J. Mater. Chem. C 2017, 5, 10746-10753.
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Entry for the Table of Contents
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Jing Wen， Ruiyu Li， Rong Lu,* and
The photophysical properties of
monomer melem were reinvestigated.
The photocatalytic hydrogen evolution
of melem data indicates that monomer
melem has negligible photoinduced
water splitting activity. The current
studies will help to understand the
Anchi Yu*
Page No. – Page No.
Photophysics and Photocatalysis of
Melem：A Spectroscopic
Reinvestigation
photocatalytic
mechanism
of
heptazine-based materials.
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