Single-crystalline melem (C6N10H6) nanorods: A novel stable molecular crystal photocatalyst with modulated charge potentials and dynamics

Article abstract of DOI:10.1039/c9ta02556a

It is still challenging to control the crystallization and morphologies of molecular crystal nanostructures with excellent photocatalytic activities. In this study, single-crystalline melem (C₆N₁₀H₆) nanorods (MNRs) were prepared by a facile thermal polymerization route under a flowing argon atmosphere. Compared with the bulk melem counterpart and g-C₃N₄, the MNRs exhibited substantially improved efficiency and stability towards the photodegradation of diverse organic pollutants including methyl orange, rhodamine B, and methylene blue. Moreover, the average photocatalytic hydrogen evolution rate of MNRs was about 2.6 folds higher than that of bulk melem under the same conditions. The enhanced photocatalytic redox performance of MNRs arises from the single-crystal nature and novel one-dimensional (1D) nanorod morphology, contributing to elevated conduction band potentials and promoting charge transport dynamics. This study not only reports single-crystalline melem 1D nanostructures as novel highly efficient and stable molecular-crystal photocatalysts, but also reveals the important roles of synergetic tuning of charge potentials and dynamics in enhancing the photocatalytic activity of molecular-crystal photocatalysts.

Full text of DOI:10.1039/c9ta02556a

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ISSN 2050-7488
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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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DOI: 10.1039/C9TA02556A
ARTICLE
Single-crystalline Melem (C6N10H6) Nanorods: A Novel Stable
Molecular Crystal Photocatalyst with Modulated Charge
Potentials and Dynamics
Received 00th January 20xx,
Accepted 00th January 20xx
Renbo Lei a, Bingsheng Du a, Xiaofang Lai a, Jing Wu a, Zhihua Zhang b, Shengwei Liu c,*, Rong Wu d,
Xin Li e, Bo Song f,* and Jikang Jian a,*
DOI: 10.1039/x0xx00000x
It is still challenging to control the crystallization and morphologies of molecular crystal nanostructures with excellent
photocatalytic activities. In this study, single-crystalline melem (C6N10H6) nanorods (MNRs) were prepared by a facile thermal
polymerization route under flowing argon atmosphere. Compared with the bulk melem counterpart and g-C3N4, the MNRs
exhibited substantially improved efficiency and stability towards the photodegradation of diverse organic pollutants
including Methyl Orange, Rhodamine B, and Methylene Blue. Moreover, the average photocatalytic hydrogen evolution rate
of MNRs was about 2.6 folds higher than that of bulk melem under the same conditions. The enhanced photocatalytic redox
performance of MNRs arises from the single-crystal nature and novel one-dimensional (1D) nanorod morphology,
contributing to elevated conduction band potentials and promoting charge transport dynamics. This study not only reports
single-crystalline melem 1D nanostructures as novel highly efficient and stable molecular-crystal photocatalysts, but also
reveals the important roles of synergetic tuning of charge potentials and dynamics in enhancing the photocatalytic activity
of molecular-crystal photocatalysts.
photocatalysts.[16] A recent study on fibrous red phosphorus showed
1. Introduction
Photocatalysts play key roles in catalyzing water splitting and
pollutant degradation, two promising technologies for new energy
production and environmental protection.[1–6] Compared to well-
that small size, good crystallization, and uniform morphology can
greatly improve the photocatalytic performance of elemental
photocatalysts.[11]
Melem (2,5,8-triamino-tri-s-triazine, C6N10H6) is a typical
molecular-crystal semiconductor,[17] often formed as an
intermediate during the production of g-C3N4 by the pyrolysis of
various nitrogen-abundant organic precursors such as cyanamide,
dicyandiamide, melamine, urea, and thiourea.[18] In 2014, Chu et al.
reported that melem can be used as a metal-free photocatalyst for
H2 evolution from an aqueous methanol solution,[19] but with a
limited catalytic activity. So far, because of the slow charge transfer
dynamics, only minor achievement was achieved in enhancing the
catalytic performance of melem via constructing heterostructures or
single-atom heterogeneous catalysts,[20,21] while the overall
electrochemical performance still has some room for improvement.
It could be a promising but challenging task to improve the crystal
quality of a nanostructure with well-defined morphologies in further
enhancing the photocatalytic activity of melem.
studied ionic, covalent, and metallic crystal photocatalysts (such as
[7–10]
metal oxides and sulfides)
and elemental photocatalysts (such
as red phosphorus and sulfur),[11,12] molecular-crystal photocatalysts
draw less attention due to low performance and instability.[13]
However, molecular-crystal photocatalysts have some advantages
including
environmentally
friendly,
low
cost,
and
biocompatibility.[14,15] Weak molecular interaction, including
hydrogen bonding or van der Waals forces, is detrimental for charge
transfer, resulting in low performance of molecular crystals
a.School of Physics and Optoelectronic Engineering, Guangdong University of
Technology, Guangzhou 510006, P. R. China
b.
Liaoning Key Materials Laboratory for Railway, School of Materials Science and
Engineering, Dalian Jiao tong University, Dalian 116028, P. R. China
School of Environmental Science and Engineering, Sun Yat-sen University,
c.
In this study, using a simple thermal polymerization of
melamine in a tubular furnace under flowing argon atmosphere,
exceptional crystal quality and uniform nanorod morphology were
simultaneously achieved in the as-synthesized single-crystalline
melem nanorods (labelled as MNRs). In contrast with the bulk melem
(labelled as BM) obtained under otherwise static argon atmosphere,
those MNRs demonstrated significantly enhanced photocatalytic
activity and stability in both pollutant degradation and water splitting.
Moreover, because of low cost, abundance, and special stability, to
Guangzhou 510006, P. R. China
Key Laboratory of Solid-state Physics and Devices, School of Physical Science and
d.
Technology, Xinjiang University, Urumqi 830046, P. R. China
e. College of Forestry and Landscape Architecture, Key Laboratory of Energy Plants
Resource and Utilization, Ministry of Agriculture,South China Agricultural
University, Guangzhou 510642, P. R. China
f. National Key Laboratory of Science and Technology on Advanced Composites in
Special Environments, Harbin Institute of Technology, Harbin 150080, P. R. China
*E-mail addresses: liushw6@mail.sysu.edu.cn (S. Liu); songbo@hit.edu.cn(B. Song);
jianjikang@gdut.edu.cn (J. Jian)
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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the best of our knowledge, MNRs are one of the best molecular- using NETZSCH STA 409PC equipment at a heating rate of 10 C/min
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DOI: 10.1039/C9TA02556A
crystal photocatalysts. Impressively, under visible-light irradiation, under argon atmosphere in an open system. BET surface area was
96.3% of 20 mg/LRhodamine B (RhB) aqueous solution was degraded measured using a Micromeritics ASAP 2020 equipment. The
over MNRs within 30 min, and the photocatalytic hydrogen defective states in melem samples were identified by electron
−
1
generation rate over MNRs reached 3.73 μmol h . Detailed paramagnetic resonance (EPR) using a Bruker A300 spectrometer,
structural and defect characterizations show that the synergistic operating with 9.4453 GHz at 100 K. The photoelectrochemical (PEC)
effects of both low-dimensional morphologies and single-crystal properties of samples were examined in a conventional three-
nature are critical for modulating charge potentials and dynamics, electrode system using an electrochemical workstation (CHI 660E,
thus achieving a high photocatalytic activity. This study not only Chenhua, China). The counter and reference electrodes were a Pt foil
provides a unique opportunity to enrich our knowledge about electrode and an Ag/AgCl (saturated KCl) electrode, respectively. The
molecular-crystal photocatalysts, but also paves a new pathway for working electrode was prepared on FTO glass substrates. 30 mg
improving the photocatalytic activity of melem and melem-related samples were dissolved in 3 mL ethanol, and 10 L naphthol was
compounds.
added. After ultrasonic dispersion for 15 min, a mixture of 20 L was
distributed onto the FTO substrate (active area 1.5 cm  1 cm) and
then dried for 4 h. The thickness of the prepared working electrode
was about 4 m. A 0.1 M Na2SO4 electrolyte was used for PEC
measurements. The electrochemical impedance spectroscopy (EIS)
was performed in dark or under AM 1.5G illumination in the
frequency range from 0.01 to 105 Hz, with an alternating current
amplitude of 10 mV, at a bias of 1.23 VRHE. The arc in the EIS spectra
was fitted using a simplified Randles equivalent circuit that consisted
of a resistance (Rs, electrical conductivity of electrodes), a charge-
transfer resistance (Rct, interface electrocatalytic reaction between
electrode and electrolyte), and a constant-phase element (CPE), and
the fitting parameters were estimated using the Levenberg–
Marquardt minimization procedure. The Mott–Schottky (M–S) plots
were recorded in the impedance–potential mode.
2. Experimental Section
2.1. Chemicals and Reagents
All the purchased reagents were directly used without further
purification. Melamine (99%) was purchased from Alfa Aesar. RhB
(analytical reagent), Methyl Orange (MO, 96% purity), and
Methylene Blue (MB, analytical grade) were purchased from Aladdin.
Triethanolamine (TEOA) was of analytical grade supplied by
Guangzhou chemical reagent factory.
2.2. Synthesis of photocatalysts
All the samples were synthesized in a horizontal tube furnace.
Analytical-grade melamine (~ 2 g) was loaded in a ceramic boat and
placed in the central region of tube furnace, which was then sealed
and evacuated using a vacuum pump followed by flushing under
high-purity argon (99.999%) flow at 50 standard-state cubic
centimeter per minute (sccm) to remove the residual oxygen.
Annealing was performed in the tube furnace at desired
temperatures from 300 C to 550 C (300 C, 350 C, 400 C, 450 C,
500 C, and 550 C; among them, 450 C for MNRs and 550 C for g-
C3N4)[4] for 2 h at a heating rate of 10 C min−1 under an argon flow
of 50 sccm. In addition, the reference bulk melem sample was also
synthesized in the tubular furnace with two closed ends and filled
with 0.03 MPa Ar at 450 C for 2 h.
2.4. Measurements of photocatalytic activities towards
degradation of pollutants
The photodegradation activities of as-synthesized samples were
evaluated by examining the degradation rates of MO, RhB, and MB.
A 300-W Xe arc lamp (CEL-HXF300, CEAULight, China) with 14A
operating current was used as the light source. For photocatalytic
experiments, the as-prepared samples (25 mg) were added into the
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1
aqueous solution (50 mL, 10–20 mg L ). Prior to irradiation, each
suspension containing the photocatalyst was stirred for 60 min in
dark to achieve an adsorption equilibrium between the
photocatalyst and dyes. The suspension was subjected to vigorous
magnetic stirring to suspend particles in dye solutions. Next, the
suspensions (5 mL) were added to centrifuge tubes and centrifuged
for 5 min at 3500 rpm to eliminate solid particles. The clear
supernatant was then used to measure changes in dye
concentrations, where solid particles and clear supernatant were
poured back into the original solution. The dye concentrations were
monitored at maximum absorbance wavelengths of 465 nm, 554 nm,
and 600 nm. Percentage degradation was recorded as C/C0, where C
is the absorbance of dye solutions at certain irradiation time intervals
and C0 is the absorbance of initial dye solutions. For comparison, the
blank solution was also tested under the same conditions.
2.3. Characterization
X-ray diffraction (XRD) patterns of the samples were obtained
using a Philips X’Pert PRO diffractometer operating at 3 kW with a Cu
Kα radiation source and scanning rate of 5 min−1 in two theta range
of 5–80. The calculated XRD data of MNRs were obtained using
Dicvol software, and the crystallographic information (CIF) of MNRs
[17]
was acquired from literature.
The surface morphologies were
observed using a scanning electron microscope (SEM, S-3400N-II).
High-angle annular dark field (HAADF) imaging in a dedicated
scanning transmission electron microscope (STEM) was performed
using a FEI TEM microscope (Talos F200S). Selected-area electron
diffraction (SAED) was performed using a transmission electron
microscope (TEM, Jeol, JEM-3200FS). Fourier transform infrared (FT-
2.5. Measurements of photocatalytic activity towards hydrogen
evolution reaction
IR) spectra were recorded using a Nicolet CCR-1 spectrometer in the
range of 4000–400 cm−1 at resolution of 4 cm . X-ray photoelectron
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1
Photocatalytic water splitting was carried out in a 100-mL Pyrex
round-bottom flask. The openings were sealed with a silicone rubber
septum at ambient temperature and atmospheric pressure. A 300-W
Xenon lamp was used as the light source, and visible-light irradiation
was achieved by attaching a 400-nm cut-off filter. The lamp was
positioned at 19 cm away from the reactor, and the focused intensity
spectra (XPS) were recorded using a Thermo EscaLab 250Xi using Al
Kα (hν = 1486.6 eV) as the excitation source. UV–visible spectra were
obtained using
a
Shimadzu UV-3600 plus UV–VIS–NIR
spectrophotometer. Chemical elemental analysis (EA) was
performed using a Perkin Elmer Series II 2400. Thermogravimetric
(TG) and differential scanning calorimetry (DSC) were carried out
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on the flask was ca. 160 mWcm . In a typical photocatalytic reaction,
powder sample (50 mg) was dispersed in an aqueous solution (80 mL)
containing TEOA (10 vol%) as the sacrificial electron donor and 1 wt%
Pt as the co-catalyst. Pt was photodeposited on the catalyst using
H2PtCl6 dissolved in the reaction mixture. Before irradiation, the
reaction mixture was dispersed in an ultrasonic bath, and nitrogen
was bubbled through the reaction mixture for 30 min to remove
oxygen ensuring that the reaction system was under anaerobic
conditions. Next, the solution was irradiated under full arc light (λ >
300 nm) for 30 min to facilitate the deposition of Pt, and nitrogen
was bubbled for 30 min. The hydrogen evolved was analyzed using a
gas chromatograph (GC-790, TCD, with N2 as the carrier) for 1-h
illumination under visible light. The reaction was carried out for 4 h.
2.6. Theoretical calculations
Density functional theory (DFT) calculations were performed
using the Vienna Ab-Initio Simulation Package with Heyd–Scuseria–
Ernzerhof functional.[22,23] A cutoff energy of 500 eV was used for
plane-wave expansion.[24,25] The Brillouin zone was sampled using a
Gamma-centered 3 × 3 × 2 k-mesh. A van der Waals correction based
on Grimme’s scheme[26] was adopted to better describe the
nonbonding forces. An isolated C3N5 unit was modeled using a 20 ×
21 × 22 Å rectangular cell to prevent unphysical interaction between
periodic images. The geometry was optimized until all residual forces
are below 0.02 eV/Å.
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DOI: 10.1039/C9TA02556A
Fig. 1 (a) Low-magnification and (b) high-magnification SEM images
of MNRs. (c) TEM image of the as-synthesized MNRs. (d) SAED
pattern of an MNR. (e) HAADF image (top) of a single MNR and the
corresponding EDS elemental mapping of C (red) and N (green).
Fig. S2a shows that melamine powder, i.e., the precursor of
thermal polymerization, was composed of micro-sized particles with
irregular shapes. After the pyrolysis at a temperature of 450 C for
10 min (Fig. S2b), partial melamine particles were converted into
nanorods, and then a large scale of nanorods were obtained when
the pyrolysis time was prolonged to 120 min (Fig. S2c). As the
temperature was increased to 550 C, g-C3N4 microparticles were
obtained (Fig. S3e). The SEM images (Fig. S3) show that only at the
synthesis temperatures within a narrow range of 440–460 C,
nanorods with uniform morphology can be obtained. In addition, it
was found that the flowing Ar atmosphere plays another key role in
determining the final morphologies of samples; otherwise, only bulk
samples can be synthesized (Fig. S3f), consistent with previous
reports.[19] To further characterize the crystal and chemical
structures of the as-synthesized samples, XRD and Fourier transform
infrared spectroscopy (FT-IR) were used. Fig. 2a shows both the XRD
pattern of the as-synthesized MNRs and the calculated Bragg
reflection peaks for melem based on the crystalline structure
reported in Ref. 17. The experimental and calculated data are also
summarized in Table S1. Apparently, the observed XRD peaks are in
good agreement with the calculated Bragg reflections, which can be
well indexed as monoclinic melem. [9] The elemental analysis shows
that the ratios of C/N in MNRs and BM are 0.606 and 0.607,
respectively, slightly higher than the ideal ratio in melem (C/N = 0.6),
indicating a N-deficiency in the as-synthesized samples. The XRD
patterns of other reference samples including BM, g-C3N4, and
melamine are shown in Fig. S4. The sample obtained at 550 C was
identified as g-C3N4 owing to diffraction peaks at 13.1 and 27.4,
characteristics of in-plane trigonal N linkage of tri-s-triazine motifs
and typical stacking structure of melon layers in (002) direction.[28,29]
3. Results and Discussion
The samples were synthesized by the thermal polymerization of
melamine in a tubular furnace (see details in Experimental Section of
Electronic Supplementary Information). The synthetic route of two
typical melem samples (MNRs and BM) is schematically shown in Fig.
S1 (ESI). The SEM image in Fig. 1a and enlarged SEM image in Fig. 1b
indicate that high-density nanorods with uniform morphology were
synthesized. The TEM image (Fig. 1c) clearly shows that one-
dimensional (1D) morphology was obtained with diameters ranging
from 50 nm to 150 nm. The SAED of an individual nanorod shown
in Fig. 1d can be indexed to a known melem crystal structure with
monoclinic unit,[17] confirming the single-crystalline nature of the as-
synthesized samples. To the best of our knowledge, this is the first
study to successfully obtain the single-crystalline 1D nanostructure
of carbon nitrides, except cubic C3N4.[27] Note that it failed to obtain
the lattice-resolved HRTEM image of MNRs because their crystalline
lattice could not stand the electron beam bombardment of HRTEM
test. The HAADF image and the corresponding elemental EDS
mapping images of C and N (Fig. 1e) and SEM-EDS elemental mapping
images (Fig. S2) clearly indicate that not only the individual nanorods
but also the total samples are composed of C and N elements with a
homogeneous distribution (Fig. 1e), confirming the nature of carbon
nitrides.
Fig. 2b shows the FT-IR spectra of MNRs, BM, g-C3N4, and melamine.
The absorption peaks from 3000 cm− to 3600 cm are associated
−
1
1
with the stretching mode of N–H bonds.[30] The peaks from 1200 cm−
1
to 1640 cm−1 can be attributed to the stretching vibration of C–N and
C=N heterocycles. The sharp peak at 809 cm− corresponds to s-
1
triazine ring vibrations. [31,32] The absorption peak at 1095 cm−1 (Fig.
2b) was only observed in MNRs and BM, the characteristic peak of
melem.[17] Next, XPS was used to identify the chemical environments
of samples as shown in Figs. 2c and S5. Only C, N, and trace amounts
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of O can be detected (Fig. S5). The C1s signals can be fitted into four
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To understand the effects of size and shape onVtiehweAbrtaicnledO-nglainpe
DOI: 10.1039/C9TA02556A
components with binding energies of 284.6 eV, 285.1 eV, 287.8 eV, and other charge behavior of melem, DFT calculation was performed.
and 288.3 eV (left half in Fig. 2c), attributed to graphitic carbon sp2C– Fig. 3a shows the crystal structure of melem; the corresponding
C, sp2C=N, sp3C–N, and C–O, respectively.[33-35] The main peak electronic structures for both MNRs and BM were calculated. Fig. 3b
located at 284.6 eV was exclusively assigned to carbon atoms (C–C shows the density of states for BM and an isolated melem unit.
bonding) in pure carbon environment, such as graphitic or Clearly, the orbitals of isolated melem unit are more localized. The
amorphous carbons.[36] The C–O peak at 288.3 eV can be attributed band gap (Eg) of the former (BM) was calculated to be 4.29 eV,
to surface absorption and oxidation.[37] The N1s peak (right half in Fig. whereas the Eg of the latter is slightly larger (4.52 eV) due to the
2c) can be deconvolved into three peaks with binding energies of quantum confinement effect.[45] To verify the theoretical calculation,
398.4 eV, 399.0 eV, and 401.0 eV, well corresponding to the N Fig. 3c shows the UV–visible diffuse reflectance spectra of samples.
atoms sp2-bonded to two carbon atoms (C–N=C), N atoms bonded to The respective absorption thresholds of melamine, BM, MNRs, and
three sp2 carbon atoms (N–[C]3), and amino functions carrying g-C3N4 were determined to be 265, 425, 429, and 445 nm,
hydrogen (C–N–H), respectively.[38,39] The emergence of N–[C]3 respectively. Then, the optical Eg can be estimated from plots of
undoubtedly indicated the formation of tri-s-triazine units (C6N7, (Ahν)2 versus (hν) based on the optical absorption data shown in Fig.
basic part of melem molecule) through the thermal S6, where A is the absorbance, ν is the frequency of light, and h is
polycondensation of melamine. Generally, the thermal stability of Planck’s constant (6.62610−34 Js), respectively.[46,47] As shown in Fig.
molecular crystals is not very good due to weak interaction,[40–42] S6, the estimated optical Eg was 4.67, 2.91, 2.89, and 2.79 eV,
restricting their applications. The thermogravimetric (TG) analyses of corresponding to melamine, BM, MNRs, and g-C3N4, respectively. In
melamine, BM, MNRs, and g-C3N4 were carried out with a heating addition, the Eg for other samples as a function of pyrolyzing
rate of 10 C/min under argon flow as shown in Fig. 2d. For temperature is also shown in Fig. S7. Obviously, with the increase in
melamine, one sharp weight loss peak occurred in the range of pyrolysis temperature, the Eg gradually decreased due to the loss of
263.4–344.6 C. The weight loss was only 11.6% below 344.6 C, and nitrogen atoms. The Mott–Schottky plots of both MNRs and BM
approached nearly 100% above 400 C, indicating that the melamine exhibited positive slopes at various frequencies,
a typical
completely decomposed during the pyrolysis. In contrast, significant characteristic of n-type semiconductors (Fig. S8). In addition, the flat
improvements were observed in the thermal stability of melem and band potentials derived from the x-intercept in the Mott–Schottky
g-C3N4 samples, and no apparent weight loss occurred for both BM plots of BM and MNRs were −0.64 V and −0.88 V vs. the saturated
and MNRs below 500 C. The DSC curve of MNRs shows three Ag/AgCl reference electrode, respectively. Considering that the
exothermic peaks located at 500.7 C, 525.5 C, and 580.9 C, conduction band (CB) potential for n-type semiconductors is more
attributed to the deamination effect of melem transforming into g- negative by 0.2 V compared to that of flat band potential,[48] the CB
C3N4.[43,44] In addition, the DSC curve of MNRs also shows two potentials of BM and MNRs were corrected to be −0.84 V and −1.08
exothermic peaks after 700 C, assigned to denitrification. The TG V vs. Ag/AgCl, respectively. The obtained potentials vs. Ag/AgCl were
results clearly indicate that the thermal stability of melem can be at converted to the normal hydrogen electrode (NHE) scale according
least comparable with that of g-C3N4, also refreshing the to equation ENHE = E + 0.197, where ENHE is the potential vs. NHE and
conventional insight on molecular crystal.
E is the measured potential vs. Ag/AgCl.[49,50] Therefore, the CB
potentials of BM and MNRs were equivalent to −0.643 V and −0.883
V vs. NHE, respectively. In combination with the value of Eg, the
valence band (VB) potentials of BM and MNRs were thus calculated
to be 2.274 V and 2.01 V vs. NHE, respectively. Therefore, the band
structures for BM and MNRs are illustrated in Fig. 3d. Obviously, the
CB potential of MNRs (−0.88 V) is elevated, relative to BM (−0.64 V),
indicating the stronger reduction ability of CB electrons of MNRs.
Moreover, it should be mentioned that a much smaller slope was
acquired in the MNRs from the Mott–Schottky curve, manifesting a
larger free carrier density and higher electrical conductivity for MNRs,
i.e., MNRs has a much higher collection efficiency of photoinduced
electrons than BM.[51]
Fig. 2 (a) XRD pattern of as-synthesized MNRs. The Bragg reflection
peaks for melem shown at the bottom were calculated according to
the reported crystalline structure in Ref. 17. (b) FT-IR spectra of g-
C3N4, BM, MNRs, and melamine. (c) XPS spectra of MNRs, left: C1s
and right: N1s. (d) TG curves of melamine, BM, MNRs, and g-C3N4
under Ar atmosphere. The red dotted line represents the DSC curve
of MNRs under Ar atmosphere.
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and the optimized amount of Pt NPs was 1 wt% when the hydrogen
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generation rate reached 3.73 μmol h . Within the 4 h continuous
photocatalytic water splitting processes, the yield of hydrogen
steadily increased linearly, further confirming the stability of MNR
photocatalysts. Moreover, the hydrogen production rate of MNRs
(3.73 μmol/h) was 2.6 times higher than that of BM (1.45 μmol/h),
highlighting the advantages of 1D nanorod morphology in enhancing
the photocatalytic reduction performance. Fig. S12 shows the
photocatalytic hydrogen evolution of MNRs and g-C3N4. The result
indicates that the photocatalytic hydrogen evolution activity of
MNRs is lower than that of g-C3N4 and is lower than that of g-C3N4
reported in literature.[4] Note that it has been demonstrated that g-
C3N4 with a layered structure was an excellent support with a high
efficiency for loading Pt nanoparticles applied in catalytic
reductions.[53] The highly efficient loading of Pt nanoparticles on the
prepared g-C3N4 with plate-like shape is probably responsible for its
high photocatalytic hydrogen evolution activity.
Fig. 3 (a) An illustration of melem crystal structure. (b) Density of
states for BM (black) and an isolated melem unit (red). The Fermi
level is set to be zero. (c) UV–vis absorption spectra of samples. (d)
Band structure of alignments for BM and MNRs.
Then, the photocatalytic oxidation activity of MNRs was
demonstrated by the photodegradation of several dye pollutants in
water, including MO, RhB, and MB, in comparison with the reference
samples (Figs. 4a and b). As expected, MNRs exhibited the highest
photodegradation activity towards all the three dyes. Specifically,
96.3% of 20 mg/LRhB aqueous solution was degraded within 30 min;
93.3% of 20 mg/L MO was degraded within 60 min, and 76.3% of 10
mg/L MB was degraded within 40 min. Compared with the reported
g-C3N4 in photocatalytic dye degradation,[52] the MNRs exhibited a
higher catalytic performance. In this study, the MNRs achieved a
76.3% degradation of MB within 40 min, a much higher rate than
~25% degradation reported in literature.[52] Interestingly, the as-
synthesized MNRs maintained almost the same catalytic activity in a
Fig. 4 (a) Time-dependent photocatalytic degradation of RhB, MO,
and MB on photocatalysts under full arc-light irradiation. (b) Cycling
low-temperature environment (0–3 C) (Fig. S9), and it could thus test of photocatalytic degradation of RhB on BM, MNRs, and g-C3N4.
(c) Time course of H2 evolution from a 10 vol% aqueous TEOA
solution with 1 wt% Pt-loaded MNRs and BM under visible-light
irradiation. (d) The effect of Pt-loading amount on photocatalytic H2
production using MNRs under visible-light irradiation.
effectively service in broad temperature ranges. Moreover, the
photocatalytic RhB degradation activity over MNRs for five cycles
was comparable without obvious deactivation (Fig. 4b), indicating its
excellent cycling performance. To reveal the structural and
morphological information of MNRs after the cycling test, the XRD
patterns and SEM images of MNRs after each test for the
photocatalytic degradation of RhB are shown in Figs. S10 and S11,
respectively. All the XRD patterns (Fig. S10) can be indexed as
monoclinic melem, indicating no change in their crystal structure.
The SEM images (Fig. S11) show that the nanorod morphology of
sample was well preserved after five cycling tests. Notably, the as-
prepared g-C3N4 exhibited a higher activity than BM, similar to the
results reported by Xiong et al.,[43] but lower performance than that
of MNRs, indicating the significance of single-crystal nature in
combination with the unique 1D morphology in enhancing the
The abovementioned photocatalytic results indicate the
superior photocatalytic performance of single-crystalline MNRs
towards both the oxidation of organic dyes and reduction of water.
Usually, several advantages of 1D nanostructures together account
for the enhanced photocatalytic performance. For example, in most
cases, 1D nanostructures have a higher specific surface area and
more active adsorption and reaction sites.[54] However, in this case,
the measured Brunauer–Emmett–Teller (BET) surface areas (SBET) of
MNRs and BM are 4.87 m2/g and 7.02 m2/g, respectively, indicating
that some factors other than the specific surface area play key roles
in enhancing the photocatalytic performance. It is well known that,
photocatalytic oxidative degradation performance. Furthermore, the in addition to the adsorption, activation, and reaction process, the
photocatalytic reaction efficiency usually depends on two
intercorrelated photophysical processes, i.e., charge generation and
charge transport.[ 55,56] Because of the less adsorption and reaction
sites as a result of lower SBET in MNRs, the enhanced photocatalytic
performance of MNRs can be attributed to the promoted
photocatalytic reduction activity of MNRs was evaluated in terms of
photocatalytic hydrogen production by water splitting under visible-
light irradiation (Figs. 4c and d). The introduction of a suitable
amount of Pt nanoparticles (NPs) as the cocatalyst is crucial in
optimizing the photocatalytic hydrogen production activity of MNRs,
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photophysical process.[57–59] First, the charge generation process is
closely related to the band structure of semiconductor photocatalyst,
determining the light absorption and charge potentials. Usually, a 1D
nanostructure has a wider Eg (Fig. 3b) and narrower light absorption
range.[60] However, in this study, the as-synthesized MNRs exhibited
a wider light absorption range and red shift in absorption edge
compared with BM, possibly originating from the self-doping or
intrinsic defects.[61,62] Generally, a higher visible-light absorption
capacity usually leads to a higher charge generation rate.[63]
Moreover, the band structure shown in Fig. 3d indicates that the
redox potential of CB electrons in MNRs was much stronger than that
in BM. This change is helpful in both activating oxygen for
photocatalytic oxidative reactions and splitting water to produce
hydrogen. Second, the charge transport process strongly depends on
both the crystal structure and crystal quality (degree of crystallinity).
A higher degree of crystallinity means less bulk defects and less
recombination centers, significantly facilitating fast charge transport
towards the surface of photocatalyst.[64] Generally, a molecular
crystal usually has a lower charge transport ability.[65,66] To further
understand the advantages of MNRs in improving the charge
transport dynamics, photoelectrochemical measurements (transient
photocurrent and EIS analyses) were performed. Fig. 5a shows the
transient photocurrent responses of BM and MNRs recorded for
several on–off cycles of light irradiation. Obviously, compared with
BM, MNRs showed an obvious enhancement in the photocurrent
response under UV–visible-light irradiation, indicating a higher
separation efficiency of charge carriers. The enhanced charge
transfer efficiency was also confirmed by EIS analyses (Fig. 5b). The
Nyquist plots can be interpreted in terms of the equivalent circuit
shown in the inset in Fig. 5b. In the equivalent circuit, Rs is the
solution resistance, CPE is the constant-phase element for the
electrolyte/electrode interface, and Rct is the charge-transfer
resistance across the electrode/electrolyte interface. Efficient charge
transfer at the interface between photoelectrode and electrolyte
effectively could hinder the charge recombination, resulting in a
facile charge transport through the samples. Thus, the as-obtained
MNRs with a lower Rct have a higher charge transfer efficiency. It has
been demonstrated that the features of 1D nanostructure can
significantly enhance the charge separation and transfer processes.
This can be attributed to the synergistic effect of a high surface-to-
volume ratio, efficient charge separation and transfer, and a short
lateral charge transport length.[67] Moreover, based on low-
temperature EPR results (Fig. 5c), it was found that in contrast with
BM, a lower defect concentration in MNRs helps to reduce charge
recombination. Combining the above analyses, the superior
photocatalytic performance of as-prepared MNRs can be ascribed to
the synergetic optimization of charge potentials and dynamics. The
modulation of charge potentials originates from the subtle tuning of
band structure of as-prepared MNRs, inducing both widen visible-
light absorption and elevated CB potential simultaneously.
Furthermore, the novel 1D nanostructures and low defects in the
MNRs lead to the promoted charge transport dynamics, thus
achieving a significant promotion in photocatalytic performance in
single-crystalline MNRs. Thus, a possible photocarrier separation
mechanism is proposed in Fig. 5d.
View Article Online
DOI: 10.1039/C9TA02556A
Fig. 5 (a) Transient photocurrent response, (b) EIS, (c) low-
temperature (100 K) EPR measurements for MNRs and reference
samples, and (d) schematic illustration of photocarrier separation in
MNRs.
4. Conclusion
Herein, single-crystalline MNRs were synthesized through direct
pyrolysis of melamine under flowing argon atmosphere. Compared
to their bulk counterparts BM, the as-obtained 1D molecular crystals
exhibited improved photocatalytic activities towards both the
degradation of organic pollutants and hydrogen generation. The high
degree of crystallinity and 1D morphology of MNRs achieved the
synergetic optimization of charge potentials and dynamics. These
were critical factors in enhancing the photocatalytic performance,
indicating that the design of a single-crystalline 1D nanostructure is
a feasible strategy to create high-performance molecular-crystal
photocatalysts. Overall, these findings not only provided a reliable
route for controllable synthesis of the first single-crystalline 1D
carbon nitride molecular-crystal nanostructures, but also shed new
light on the synergetic roles of crystallinity and anisotropy in tuning
photophysical processes and photocatalytic performances of
molecular-crystal photocatalysts for their applications in
environmental remediation and solar fuel generation.
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
This work was financially supported by the National Scientific
Foundation of China (Grant Nos. 51672051, 51572209, 51672057,
51722205 and 51872341), and Science and Technology Program of
Guangzhou, China (201707010251), Fundamental Research Funds
for the Central Universities (Grant No. HIT.BRETIV.201801), Natural
Science Foundation of Heilongjiang Province (Grant No. E2018032),
and Program for New Century Excellent Talents in University (NCET-
13-0174).
References
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Journal of Materials Chemistry A
Please do not adjust margins
Journal Name
ARTICLE
1
2
Z.G. Zou, J.H. Ye, K. Sayama and H. Arakawa, Nature, 2001, 33 R.B. Lei, J.K. Jian, Z.H. Zhang, B. Song VainewdArtRic.le OWnlinue,
414, 625-627.
Nanotechnology, 2016, 27, 395603.
Q. Wang, T. Hisatomi, Q.X. Jia, H. Tokudome, M. Zhong, C.Z. 34 C. Li, X.G. Yang, B.J. Yang, Y. Yan and YD.TO. IQ: 1i0a.n10, 3M9/aCt9eTrA. 0C2h5e5m6A.
Wang, Z.H. Pan, T. Takata, M. Nakabayashi, N. Shibata, Y.B. Li,
Phys., 2007, 103, 427-432.
I.D. Sharp, A. Kudo, T. Yamada and K. Domen, Nat. Mater., 35 P. Srinivasu, A. Vinu, S. Hishita, T. Sasaki, K. Ariga and T. Mori,
2016, 15, 611-615. Micropor. Mesopor. Mat., 2008, 108, 340-344.
J.X. Low, J.G. Yu, M. Jaroniec, S. Wageh and A.A. Ghamdi, Adv. 36 J.S. Zhang, M.W. Zhang, G.G. Zhang and X.C. Wang, ACS Catal.,
Mater., 2017, 29, 1601694. 2012, , 940-948.
3
4
2
X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. 37 Y.H. Cheng, X.L. Qiao, J.G. Chen, Y.P. Wu, C.S. Xie, S.B. Muo,
Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009,
8
,
Y.B. Sun and B.K. Tay, Appl. Phys. A., 2002, 74, 225-231.
38 X.C. Wang, S. Blechert and M. Antonietti, ACS Catal., 2012,
1596-1606.
39 J.S. Zhang, F.S. Guo and X.C. Wang, Adv. Funct. Mater.,
2013, 23, 3008-3014.
76-80.
2,
5
6
7
8
9
J.R. Ran, J. Zhang, J.G. Yu, M. Jaroniec and S.Z. Qiao, Chem. Soc.
Rev., 2014, 43, 7787-7812.
Q. Li, X. Li, S. Wageh, A.A. Al-Ghamdi and J.G. Yu, Adv. Energy
Mater., 2015,
5
, 1500010.
40 W. Liu, K. Zhu, S.J. Teat, G. Dey, Z.Q. Shen, L. Wang, D.M.
O’Carroll and J. Li, J. Am. Chem. Soc., 2017, 139, 9281-9290.
41 M. Irie, T. Fukaminato, K. Matsuda and S. Kobatake, Chem.
Rev., 2014, 114, 12174-12277.
K.X. Zhang, H. Su, H.H. Wang, J.J. Zhang, S.Y. Zhao, W.W. Lei,
X. Wei, X.H. Li and J.S. Chen, Adv. Sci., 2018, 5, 1800062.
Q.J. Xiang, J.G. Yu and M. Jaroniec, Chem. Soc. Rev., 2012, 41
782-796.
42 K. Naito and A. Miura, J. Phys. Chem., 1993, 97, 6240-6248.
J. Wang, Z. Yang, X. Gao, W. Yao, W. Wei, X. Chen, R. Zong and 43 Y.W. Yuan, L.L. Zhang, J. Xing, M.I.B. Utama, X. Lu, K.Z. Du, Y.M.
Y. Zhu, Appl. Catal. B: Environ., 2017, 217, 169-180.
Li, X. Hu, S.J. Wang, A. Genc, R. D. Borkowski, J. Arbiol and Q.
H. Xiong, Nanoscale, 2015, , 12343-12350.
44 S. Tonda, S. Kumar, S. Kandula and V. Shanker, J. Mater. Chem.
A., 2014, , 6772-6780.
45 Q.H. Wang, K. Kalantar-Zadeh, A. Kis, J.N. Coleman and M.S.
Strano, Nat. Nanotechnol., 2012, , 699-712.
10 Y. Hu, X. Gao, L. Yu, Y. Wang, J. Ning, S. Xu and X.W. Lou,
Angew. Chem. Int. Ed., 2013, 52, 5746-5749.
7
11 Z.F. Hu, L.Y. Yuan, Z.F. Liu, Z.R. Shen and J.C. Yu, Angew. Chem.
Int. Ed., 2016, 55, 9580-9585.
12 G. Liu, P. Niu, C.H. Sun, S.C. Smith, Z.G. Chen, G.Q.(Max) Lu
2
7
and H. M. Cheng, J. Am. Chem. Soc., 2010, 132, 11642-11648. 46 J. Liu, Y. Liu, N. Y. Liu, Y.Z. Han, X. Zhang, H. Huang, Y. Lifshitz,
13 T. Kim, M.K. Al-Muhanna, S.D. Al-Suwaidan, R.O. Al-Kaysi and
C.J. Bardeen, Angew. Chem. Int. Ed., 2013, 125, 7027-7031.
14 A. Nithya, K. Jothivenkatachalam, S. Prabhu and K. Jeganathan,
Mater. Sci. Forum, 2014, 781, 79-94.
15 G. Zhang, C. Kim and W. Chio, Catal. Today, 2017, 281, 109-
116.
16 D. Braga, L. Maini, M. Polito and F. Grepioni, Struct. Bond.,
2004, 111, 1-32.
17 B. Jürgens, E. Irran, J. Senker, P. Kroll, H. Müller and W. Schnick,
J. Am. Chem. Soc., 2003, 125, 10288-10300.
18 A. Thomas, A.Fischer, F. Goettmann, M. Antonietti, J.O. Müller,
S.T. Lee, J. Zhong and Z.H. Kang, Science, 2015, 347, 970-974.
47 Y.F. Li, R.X. Jin, Y. Xing, J.Q. Li, S.Y. Song, X.C. Li, M. Li and R.C.
Jin, Adv. Energy. Mater., 2016, 6, 1601273.
48 C.M. Li, G. Chen, J.X. Sun, H.J. Dong, Y. Wang and C. Lv, Appl.
Catal. B-Environ., 2014, 160, 383-389.
49 C. Ri, S. Kim, K. Ju, H. Ryo, C. Mun and U. Kim, RSC Adv., 2018,
8, 5433-5440.
50 H. Li, Y.D. Shao, Y.T. Su, Y.H. Gao and X.W. Wang, Chem.
Mater., 2016, 28, 1155-1164.
51 A. Ghobadi, T.G.U. Ghobadi, F. Karadas and E. Ozbay, Sci. Rep.,
2018, 8, 16322.
R. Schögl and J.M. Carlsson, J. Mater. Chem., 2008, 18, 4893- 52 S.Z. Liu, H.Q. Sun, K. O’Donnell, H.M. Ang, M. O. Tade and S.B.
4908.
19 S. Chu, C.C. Wang, J.Y. Feng, Y. Wang and Z.G. Zou, Int. J. 53 X.F. Chen, L.G. Zhang, B. Zhang, X.C. Guo and X.D. Mu, Sci.
Hydrogen. Energy, 2014, 39, 13519-13526. Rep., 2016, , 28558.
20 X.D. Wang, T. Yan, X.H. Liu, P.G. Ji, M. Sun, D. Wei, L.G. Yan 54 L.Q. Jing, W. Zhou, G.H. Tian and H.G. Fu, Chem. Soc. Rev.,
and B. Du, Appl. Surf. Sci., 2016, 387, 513-520. 2013, 42, 9509-9549.
Wang, J. colloid Interf. Sci., 2016, 464, 10-17.
6
21 Z.P. Chen, E. Vorobyeva, S. Mitchell, E. Fako, N. López, S.M. 55 S.C. Yan, Z.S. Li and Z.G. Zou, Langmuir, 2009, 25, 10397-
Collins, R. K. Leary, P.A. Midgley, R. Hauert and J. Pérez-
Ramírez, Natl. Sci. Rev., 2018, , 642-652.
10401.
5
56 X.J. Bai. R.L. Zong, C.X. Li, D. Liu, Y.F. Liu and Y.F. Zhu, Appl.
Catal. B-Environ., 2014, 147, 82-91.
22 G. Kresse and J. Furthmüller, Phys. Rev. B, 1996, 54, 11169-
11186.
57 X.B. Chen, S.H. Shen, L.J. Guo and S.S. Mao, Chem. Rev., 2010,
110, 6503-6570.
58 T. Hisatomi, J. Kubota and K. Domen, Chem. Soc. Rev., 2014,
43, 7520-7535.
59 H.L. Wang, L.S. Zhang, Z.G. Chen, J.Q. Hu, S.J. Li, Z.H. Wang, J.S.
Liu and X.C. Wang, Chem. Soc. Rev., 2014, 43, 5234-5244.
23 J. Heyd, G.E. Scuseria and M. Ernzerhof, J. Chem. Phys., 2003,
118, 8207-8215.
24 P.E. Blöchl, Phys. Rev. B, 1994, 50, 17953-17979.
25 G. Kresse and D. Joubert, Phys. Rev. B, 1999, 59, 1758-1775.
26 S. Grimme, J. Comput. Chem., 2006, 27, 1787-1799.
27 L.L. Guan, H. Li, X.J. Liu, Y. Zhao, Z.Q. Xu, J. Sun, Z.F. Ying, J. D. 60 Y.L. Yu and L.Y. Cao, Opt. Express, 2012, 20, 13847-13856.
Wu and N. Xu, CrystEngComm, 2015, 17, 512-515.
28 J.S. Zhang, M.W. Zhang, C. Yang and X.C. Wang, Adv. Mater.,
2014, 26, 4121-4126.
61 J. Fu and S.E. Skrabalak, Angew. chem. Int. ed., 2017, 56,
14169-14173.
62 Y.J. Cui, Z.X. Ding, X.Z. Fu and X.C. Wang, Angew. chem. Int.
ed., 2012, 124, 11984-11988.
29 W.N. Wang, W.J. An, B. Ramalingam, S. Mukherjee, D.M.
Niedzwiedzki, S. Gangopadhyay and P. Biswas, J. Am. Chem. 63 L. Sun, M.J. Yang, J.F. Huang, D.S. Yu, W. Hong and X.D. Chen,
Soc., 2012, 134, 11276-11281.
30 B.V. Lotsch, M.D. blinger, J. Sehnert, L. Seyfarth, J. Senker, O. 64 M. Zhou, P.J. Yang, R.S. Yuan, A.M. Asiri, M. Wakeel and X.C.
Oeckler and W. Schnick, Chem. Eur. J., 2007, 13, 4969-4980. Wang, ChemSusChem, 2017, 10, 4451-4456.
31 S. Martha, A. Nashim and K.M. Parida, J. Mater. Chem. A., 65 Z.Q. Li, V. Podzorov, N. Sai, M.C. Martin, M.E. Gershenson, M.
2013, , 7816-7824. Di Ventra and D.N. Basov, Phys. Rev. Lett., 2007, 99, 016403.
32 P. Wu, J.R. Wang, J. Zhao, L. J. Guo and F. E. Osterloh, J. Mater. 66 V. Coropceanu, J. Cornil, D.A. da Silva Filho, Y. Olivier, R. Silbey
Chem. A., 2014, , 20338-20344. and J.L. Brédas, Chem. Rev., 2007, 107, 926-952.
Adv. Funct. Mater., 2016, 26, 4943-4950.
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67 M. Wu, J. Zhang, B.B. He, H. W. Wang, R. Wang and Y.S. Gong,
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Appl. Catal. B-Environ., 2019, 241, 159-166.
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