Cathodic and anodic photocurrents generation from melem and its derivatives

Article abstract of DOI:10.1039/c5ra02816g

Melem and its derivatives were synthesized from melamine by a two-step heat treatment method between 400 and 650°C in air. It was demonstrated that such polymeric C-N semiconductors possess a photoelectrochemical (PEC) conversion effect with both photocathodic and photoanodic characteristics, which was proposed to be caused by the formation of tri-s-triazine ring. The dimelem synthesized at 450°C exhibited the highest PEC conversion activity. The possible reasons were discussed. The unique bidirectional photocurrent generation makes melem and its derivatives attractive photoelectrochemical materials.

Full text of DOI:10.1039/c5ra02816g

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Cathodic and anodic photocurrents generation
from melem and its derivatives†
Cite this: RSC Adv., 2015, 5, 26675
Xiaoqing Wei, Yu Qiu,‡ Weiyuan Duan and Zhengxin Liu*
Received 13th February 2015
Accepted 9th March 2015
DOI: 10.1039/c5ra02816g
www.rsc.org/advances
Melem and its derivatives were synthesized from melamine by a semiconductors, g-C N possesses a series of properties, such
3
4
ꢀ
two-step heat treatment method between 400 and 650 C in air. It as a low price, a high thermal and chemical stability in air, a
was demonstrated that such polymeric C–N semiconductors possess suitable band edge position, which make it a very promising
10
a
photoelectrochemical (PEC) conversion effect with both material for solar energy systems. Many researchers have
photocathodic and photoanodic characteristics, which was proposed focused on the preparation and characterization of g-C N .
3 4
to be caused by the formation of tri-s-triazine ring. The dimelem Considering the semiconductor property of g-C
synthesized at 450 C exhibited the highest PEC conversion activity. nated from the tri-s-triazine rings in the polymer structure, we
3
N
4
is origi-
ꢀ
The possible reasons were discussed. The unique bidirectional expect the intermediates towards g-C
3 4
N , also containing the
photocurrent generation makes melem and its derivatives attractive tri-s-triazine rings, may possess similar PEC activity. Up to now,
photoelectrochemical materials.
the investigations of PEC activities of intermediates during
condensation of melamine to g-C N have not been reported to
3
4
our knowledge.
In this paper, we prepared melem and its derivatives by
ꢀ
treating melamine in air between 400 and 650 C by a two-step
1
. Introduction
heat treatment method. Photoelectrochemical activities of the
as-prepared products were investigated in a conventional three-
electrode cell. As a result, all of the products showed both
cathodic and anodic photocurrents, indicating that they could
be used as either photocathodic or photoanodic materials.
An energy shortage is one of the most serious challenges faced
by human beings, which has a direct bearing on the sustain-
able development of our society. Among the multitudinous
1
technologies to deal with this challenge, photoelectrochemical
(
PEC) cells are a good choice to convert clean solar energy into
1,2
electrical energy or storable, transportable chemical fuels.
2
. Experimental
During the past 40 years, a variety of n-type metal oxide semi-
conductors have been widely researched as photoanodic
2
.1. Synthesis process
All chemicals used in this research were analytical grade and
BiVO , Zn SnO , etc. However, the p-type semiconductors used as received without further treatment. Melamine was
2 2 3
materials for PEC water oxidation, such as TiO , ZnO, Fe O ,
3
–7
4
2
4
that could be used as photocathodic materials for PEC water purchased from Sinopharm Chemical Reagent Co., Ltd. In a
8
reduction are seldom reported. Recently, it has been reported typical synthetic procedure of melem and its derivatives,
that graphitic carbon nitride (g-C
3 4
N ), a metal-free polymeric melamine (3 g) was put into a crucible with cover. The crucible
semiconductor, can be used as a photocathodic material for was placed in a muffle furnace, which was heated to the pre-
9
ꢀ
chemical fuels. In fact, compared with other polymeric established temperature ranging from 400 to 650 C at a rate
ꢀ
ꢁ1
of 10 C min , and then kept at this temperature for 2 hours.
Research Center for New Energy Technology, Shanghai Institute of Microsystem and Aer that, the products were cooled to room temperature with
Information Technology, Chinese Academy of Sciences, 235 Chengbei Road,
furnace naturally. In order to get pure products, the obtained
products were ground and treated for another 2 hours at the
same temperature. Finally, the white or yellow products were
again ground into powder in agate mortar and stored in
different sample bottles. The as-prepared samples were named
as M-x, where x was referred to the treating temperature.
Shanghai, P. R. China. E-mail: z.x.liu@mail.sim.ac.cn; Fax: +86 21 69976902; Tel:
+
†
1
‡
86 21 69976901
Electronic supplementary information (ESI) available: See DOI:
0.1039/c5ra02816g
Current affiliation: Institute of Advanced Photovoltaics, Fujian Jiangxia
University, Fuzhou 350108, China. E-mail: yqiu78@hotmail.com.
This journal is © The Royal Society of Chemistry 2015
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.2. Characterization
Communication
2
The crystalline phase and structure of the products were
analyzed with X-ray diffraction (XRD, Bruker D8) using Cu Ka
irradiation source. The bonding structures were analyzed with a
Fourier transform infrared spectrometer (FTIR, PerkinElmer
Frontier). The optical properties were measured with a
UV/VIS/NIR spectrometer (PerkinElmer, Lamda 950). For this
measurement, the product powders were xed by two quartz
plates, one of which contained a rectangular groove. The diffuse
reection spectra were recorded with a calibrated total-reection
white plate at the rear, while the transmission spectrum was
neglected. The absorption spectrum could be calculated by A ¼ 1
ꢁ
R, where R was the diffuse reectance. The Kubelka–Munk
Fig. 1 XRD patterns of the M-x samples.
2
function F(R) was calculated by F(R) ¼ (1 ꢁ R) /2R. The band gap
n
of the products were deduced by [F(R)hn] ¼ hn ꢁ E
g
, where n was
1/2 for indirect band gap and 2 for direct band gap semi-
With the further increase of polymerization temperature, the
FWHM of (002) peaks becomes narrower and the corresponding
diffraction angle shis towards high-angle direction, implying
the interplanar spacing becomes smaller, slightly. It can be
10
conductor. The BET surface areas of the products were
measured by N absorption at 77 K on an ASAP 2010 instrument.
2
The microstructures of the products were observed with scanning
electron microscope (SEM, Hitachi, S-4700).
The photoelectrochemical activities were investigated using
an electrochemical analyzer (CHI-660D Shanghai Chenhua,
China) with a conventional three-electrode cell, where a plat-
inum wire was used as the counter electrode and an Ag/AgCl
ꢀ
inferred that upon 550 C condensation, the obtained products,
carbon nitrides, contain graphitic-like layered structure, just
like melon.
The structural information of melamine and the prepared
samples are provided by FTIR spectra. In Fig. 2, the spectrum of
melamine observed in our experiment is in agreement with data
(saturated KCl) electrode as the reference electrode. The inci-
dent light was taken from a solar simulator (AM 1.5, 100 mW
14
reported by related literature. The absorption bands at 3470,
ꢁ
2
cm , CEL-S500, Ceaulight, China). The working electrode
photoelectrode) was prepared on Sn-doped In (ITO) glass
substrates. At rst, 10 mg of powder was ground for 5 min in
agate mortar and then mixed with 100 mL of H O. 20 mL of
ꢁ1
3
418, 3328, 3125 cm are attributed to NH
2
stretch vibration,
bend vibra-
(
2 3
O
while the band located at 1649 is assigned to NH
2
ꢁ
1
tion. The bands at 1540, 1469, 1431 cm are assigned to side-
chain asymmetric C–N stretch vibration, side-chain C–N breath
vibration and ring stretch, respectively. The strong band at
2
PEDOT:PSS (Sigama-Aldrich, 2.8 wt%) was added or not for
different photoelectrodes. The mixture was pestled for another
5
ꢁ
1
811 cm
is assigned to the ring-sextant out-of-plane bend
min to get homogeneous slurry. The slurry was then spread
vibration. As for M-400, three major absorption bands are
2
onto 1 cm of ITO glass substrate (length 2.5 cm and width
ꢁ
1
observed at 1602, 1460 and 802 cm , which are the charac-
1
cm) with a glass rod, using adhesive tapes as spaces. Aer air-
15
teristic of the tri-s-triazine ring. This means the formation of
ꢀ
drying, the lm was annealed at 150 C for 12 min in air to
improve the adhesion. The lm thickness measured by optical
microscopy was about 10 mm. The working electrode was
immersed in a 0.1 M KCl aqueous solution, and the glass side
was faced to the incident light.
ꢀ
melem when the condensation temperature reaches 400 C,
which is in accordance with our XRD result. At about 450 C, the
three main absorption bands located at 1602, 1460 and
ꢀ
3. Result and discussion
The structures of the M-x samples condensed at different
temperatures are rst evaluated by XRD. From Fig. 1, it can be
ꢀ
seen that the material obtained at 400 C, namely M-400 can be
1
1,12
attributed to melem.
When the polymerization temperature
ꢀ
reaches 500 C, two typical peaks for graphitic-like layered
structures are observed, which are located around 13.0 and
7.4 , respectively. It has been reported that the peak around
7.4 is a characteristic interplanar stacking peak of aromatic
systems, which is indexed as the (002) peak, with the corre-
sponding d-spacing to be 0.326 nm. The peak around 13.0 is
related to an in-plane structural packing motif, corresponding
ꢀ
ꢀ
ꢀ
2
2
13
ꢀ
13
to a distance d ¼ 0.678 nm. M-500 can be assigned to be melon,
12,13
which is also in agreement with the following FTIR analysis.
Fig. 2 FTIR spectra of melamine and the M-x samples.
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Fig. 3 Possible reaction pathway for the formation of melem and its derivatives by thermal condensation of melamine.
ꢁ
1
8
cm
02 cm remain, while a few new peaks at 1403, 1327, 1252 of polymerization degree. An approximate linear equation may
ꢁ
1
16
(n(C–N) of aromatic secondary and tertiary amines ) be concluded as E (t) ¼ 3.4228–0.0014t, where t is in degrees
g
occur. This means that the structure of melem is changed and a Celsius. “Tail-up” phenomenon between 450 and 600 nm in the
new kind of material forms by the connection of two adjacent absorption spectra occurs perhaps due to the defects formed
ꢀ
13,19
melem molecules, which can be assigned to dimelem, an during the heating processes, especially at T $ 600 C.
intermediate from melem to melon as shown in Fig. 3. As for
13
I–V curves of photocurrent response at different electrodes
M-500, melon, the observed spectrum in Fig. 2 is in accordance are presented in Fig. 5. The dimelem photoelectrode shows
15
with the spectrum reported in the literature. In the case of either cathodic or anodic photocurrents at different range of
M-550, no obvious differences can be observed, compared with electrode potential, and the onset potential is about 0.3 V. This
melon. Further increasing the temperature, the intensities of phenomenon is quite different from that of typical bulk semi-
ꢁ
1
bands at 1318, 1232, 1209 cm
corresponding to trigonal conductors, where either anodic or cathodic photocurrent is
C–N(–C)–C are enhanced, which indicates the formation of a
17
more condensed carbon nitride polymer. It should not be
ꢁ1
ignored that the weak absorption band around 3167 cm duo
to n(N–H) suggests the residual of little hydrogen in the
products.
The optical properties of the synthesized samples are
investigated by the UV-vis DRS. A typical absorption pattern of a
semiconductor is observed for all M-x samples in Fig. 4a.
Obviously, the absorption edges of the samples exhibit the red
shi with the increase of condensation temperature, which is in
accordance with the gradual change in colour from white to
dark yellow. It has been reported that the optical properties of
the products are dominated by the tri-s-triazine nucleus, from
12,15
melem to carbon nitride polymers.
The optical band gap of
the samples are calculated according to literatures as indirect
13,15,18
band gap materials.
The results are shown in Fig. 4b. Eg Fig. 5 Current–potential curves of (a) ITO, (b) ITO/M-450 (dimelem),
(c) ITO/PEDOT:PSS/M-450 in 0.1 M KCl.
decreases with the increase of temperature, due to the increase
Fig. 4 (a) UV-vis spectra and (b) optical band gap of the M-x samples.
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Fig. 6 Generation mechanism of photocurrent (a) anodic photocurrent; (b) cathodic photocurrent.
22
observed at n-type and p-type semiconductors, respectively. intrinsic g-C N semiconductor. On the basis, a preliminary
3
4
Generally, the conduction type of semiconductors can be generation mechanism of photocurrent is proposed, as shown
6,7
identied by this method. To clarify the generation mecha- in Fig. 6.
nism of photocurrent at dimelem photoelectrode, the I–V curve The bidirectional photocurrents start to appear aer
ꢀ
of ITO is also measured and shown in Fig. 5. It can be seen that the melem structure is formed at 400 C. As for melem and
the only anodic photocurrent formed at ITO electrode is quite its derivatives, this PEC activity may be originated from the
ꢁ
small, which results from the photocatalyzed oxidation of Cl
tri-s-triazine rings due to its band structure. The photocurrent
20
by holes accumulated at the surface of ITO. Therefore, the reaches the maximum for dimelem, which probably relates to
formation of both cathodic and anodic photocurrents at the unique nanowire shape of dimelem as shown in Fig. S3.†
dimelem electrode is mainly due to the existence of the organic The nanowire morphology promotes the transport and separa-
semiconductor, dimelem. It is also observed that the cathodic tion of photogenerated carriers by shortening the transport
and anodic photocurrents both increase dramatically when distance, resulting in a higher PEC activity, as shown in
23
PEDOT:PSS, a hole-transport material, is added during the Fig. S4.† Besides, the photocurrents are prompt, steady and
process of preparing electrode. It is considered that PEDOT:PSS reproducible during light on/off for cycles, as shown in Fig. S5.†
not only enhances the adhesion between ITO glass and dime- The BET surface area is not the major factor according to our
lem lm, but also increases the separation efficiency of photo- results. Secondly, dimelem is the intermediate product between
generated excitons by the formation of donor–acceptor single tri-s-triazine rings and conjugated tri-s-triazine rings. In
10
construction.
fact at this intermediate condensation temperature, a mixture
In order to compare the PEC activities of the as-synthesized of single and conjugated tri-s-triazine rings can be formed,
samples, M-x, the current–potential curves of corresponding which may form heterojunctions in situ due to the Fermi level
electrodes are measured and shown in Fig. S1.† Interestingly, all difference, thus improve the charge separation. Such hetero-
the samples show cathodic and anodic photocurrents, espe- junctions might exist for all the intermediate phases during the
cially for melem and dimelem. On the other hand, M-450, transition of melamine to the ideal graphitic carbon nitride
namely dimelem, produces the highest photocurrent. Then using the current method. When the heating temperature is too
with the further increase of temperature, the photocurrent low or too high, one component will dominate, and therefore,
produced by corresponding sample decreases gradually as we the number of heterojunctions decreases, resulting in the
can see from Fig. S2.† It should be pointed out that the BET decreasing photocurrents.
2
ꢁ1
2
ꢁ1
surface area increases from 3.9 m g to 9.8 m g with the
increase of temperature as shown in Table S1.†
4
. Conclusions
It has been reported that the wavefunction of the valence
band for polymeric melon is a combination of the HOMO levels
of the melem monomer and the conduction band may be
assigned to the LUMO of the melem monomer. The HOMO of
In this study, melem and its derivatives with different photo
response were synthesized by treating melamine between 400
and 650 C by a two-step heating method. They all show
cathodic and anodic photocurrents, indicating the potential as
photocathodic or photoanodic materials. The PEC activities of
melem and its derivatives may be originated from tri-s-triazine
rings. The bidirectional photocurrents generation of melem
and its derivatives are considered to be different from that of
typical n-type or p-type bulk semiconductors. In particular,
ꢀ
melem is derived from heterocyclic nitrogen p orbitals, and
z
1
9
LUMO mainly consists of carbon p orbitals. The redox
z
+
ꢁ
potentials of H /H
edge positions of melem and its derivatives.
been proven that the difference between the Fermi level and the
valence band of g-C
is (1.5 ꢂ 0.3) eV, which is a proof for an
2 2
and Cl /Cl couples lie between the band-
19,21
Besides, it has
3 4
N
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ꢀ
dimelem obtained at 450 C contains the highest PEC activity
due to promoted transport and separation efficiency of photo-
generated carriers.
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