Remedying Defects in Carbon Nitride to Improve both Photooxidation and H2 Generation Efficiencies

Article abstract of DOI:10.1021/acscatal.6b00879

The outstanding visible light response of carbon nitride has aroused intense expectations regarding its photocatalysis, but it is impeded by the inevitable defects. Here, we report on a facile melamine-based defect-remedying strategy and resultant carbon

Full text of DOI:10.1021/acscatal.6b00879

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Article
Remedying Defects in Carbon Nitride to improve
both Photooxidation and H2 Generation Efficiencies
Wenting Wu, Jinqiang Zhang, Weiyu Fan, Zhongtao Li, Lizhuo Wang, Xiaoming
Li, Yang Wang, Ruiqin Wang, Jingtang Zheng, Mingbo Wu, and Haibo Zeng
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00879 • Publication Date (Web): 06 Apr 2016
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ACS Catalysis
Remedying Defects in Carbon Nitride to improve both Photooxidation and H2
Generation Efficiencies
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Wenting Wu, 1,2 Jinqiang Zhang, 1 Weiyu Fan, 1 Zhongtao Li, 1 Lizhuo Wang, 1 Xiaoming Li, 2 Yang Wang, 1
Ruiqin Wang, 1 Jingtang Zheng, 1 Mingbo Wu, *,1 and Haibo Zeng *,2
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1State Key Laboratory of Heavy Oil Processing, School of Chemical Engineering, China University of
Petroleum, Qingdao 266580, China
2Institute of Optoelectronics & Nanomaterials, Herbert Gleiter Institute of Nanoscience, School of Materials
Science and Engineering, Nanjing University of Science and Technology Nanjing, 210094, P. R. China
Corresponding authors:
*E-mail: wumb@upc.edu.cn; zeng.haibo@njust.edu.cn (H.Z.).
ABSTRACT: The outstanding visible light response of carbon nitride has aroused intensive expectation on
its photocatalysis, but it is impeded by the inevitable defects. Here, we report on a facile melamine-based
defect-remedying strategy and resultant carbon nitride high-performance photocatalysts (R-C3N4). Melamine
with amino groups and triazine structure was selected as ‘little patch’ to passivate and remedy various
defects inside carbon nitride. Such remedying effect has been comprehensively proven by Fourier transform
infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), transmission electron microscope
(TEM), scanning electron microscope (SEM), X-ray diffraction (XRD) analyses and ninhydrin test. And
their their effects on photocatalysis were also individually confirmed by chemical method, including cyano
reduction reaction and deamination reaction. Furthermore, the melamine remediation can result in
g-C3N4/mpg-C3N4 junctions, which also favors to the electron transfer and charge separation during the
photocatalytic reaction. In order to explore its broader applications, R-C3N4 was used as photocatalyst for
the photooxidation reaction of 1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate (1,4-DHP) and
simultaneous H2 evolution. The conversion rates of 1,4-DHP and H2 production catalyzed by R-C3N4 were
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enhanced 2 and 6.5 times, respectively. This rational design is beneficial for the conversion of 1,4-DHP
during the preparation of bioactive compounds and clean hydrogen production at the same time.
KEYWORDS: carbon nitride, defect, remedy, photooxidation, H2 evolution
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INTRODUCTION
Visible light photocatalysis has inspired great interest, offering a sustainable pathway to drive a series of
photooxidation reactions, water splitting and so on. 1-8 Seeking highly active and low-cost photocatalysts has
been a worldwide continuing endeavor. 1-8 Wide bandgap semiconductors such as ZnO and TiO2 have been
explored a lot, but the main issue with them is that photocatalytic response is mainly initiated in near-UV or
UV region and hence to execute the photocatalytic behavior. Thus, the presence of UV light source is
necessary.9-10 The emergence of organic semiconductor, such as polymeric carbon nitride (C3N4), provides a
new chance for the development of the high-performance photocatalysts in the visible light region.11-18
Many efforts have been employed to enhance their photocatalytic activity, such as the metal/non-metal
doping,19,20 the surface heterojunction design,19,21 the texture and morphology engineerings,22-24 etc. The
surface area and photocatalytic ability were improved significantly, however the accompanying disordered
structure caused a large amount of defects in the final product.25,26 And these structure defects extremely
limit electron-hole separation and transport, which are key rate determining steps in the photocatalysis.27
In essence, among various precursors (e.g. cyanamide, dicyandiamide, thiourea and urea), the amino
and cyano groups are not only the crucial functional groups for the chemosynthesis of carbon nitride, but
also the strongest electron donor/acceptor, which play important roles for the electron-hole separation.28 As
the typical groups, they are deserved to be studied in detail. Furthermore, mesoporous carbon nitride
(mpg-C3N4) prepared from cyanamide (only amino and cyano groups) is a good candidate for the defects’
study.29 Conventional method with high calcination temperature could reduce the amount of defects, but the
higher temperature also could decompose C3N4, cut down the product yield.30 The calcination under H2 or
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ACS Catalysis
NH3 is limited for the amino defects.31,32 Thus, it is highly desired to repair various defects and understand
their structure-property relationship, especially for the defects of amino and cyano groups. Due to the
difficulty in the defect characterization, little information is available on the effects of these defects on the
photocatalysis performance of carbon nitride materials.
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Herein, we report a facile and comprehensive defect repair and its robust performance on the
photooxidation and H2 evolution. The melamine with amino groups and triazine structure is selected as
30
functionalized ‘little patch’ to react with various defects
and elaborately remedy them, which were
confirmed by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS),
transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffraction (XRD)
analyses and ninhydrin test. In addition, the ‘little patch’ easily forms nanosheet g-C3N4, which shows higher
CB/VB than that of mpg-C3N4. Besides the fewer defects, the formation of homojunction structure could
improve the electron transfer and charge separation.19,33 In order to explore its broader applications, R-C3N4
was used as photocatalyst for the photooxidation reaction of 1,4-dihydro-2,6-dimethylpyridine-3,5-
dicarboxylate (1,4-DHP) and simultaneous H2 evolution.34 The defect’s effect on photooxidation were
individually confirmed by chemical methods. The conversion rates of 1,4-DHP and H2 evolution catalyzed
by R-C3N4 were enhanced 2 and 6.5 times, respectively. This rational design can be beneficial for the
conversion of 1,4-DHP during the preparation of bioactive compounds,34 and clean hydrogen production at
the same time.
EXPERIMENTAL SECTION
Materials Synthesis. Preparation of mpg-C3N4, g-C3N4 and repaired mpg-C3N4 (R-C3N4): mpg-C3N4 and
g-C3N4 were prepared with method as previously reported.24 R-C3N4 was repaired via a facile
copolymerization method. Cyanamide was dissolved in a 40% dispersion of 12-nm SiO2 particles (Ludox
HS40, Aldrich) in water with stirring at 333 K overnight. Then the resulting transparent mixtures were
completely grinded and mixed with various amount of melamine (25%, 37.5% or 50%). The grinded
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mixtures were then heated at a rate of 2.3 K/min over 4 h to reach a temperature of 823 K and then tempered
at this temperature for another 4 h. The resulting powder was treated with a 4 M NH4HF2 for 24 h to remove
the hard template. The powders were then centrifuged and washed three times with distilled water and twice
with ethanol. Finally the powders were dried at 343 K under vacuum for overnight. For comparison, the
amount of the melamine (25%, 37.5% and 50%) was set as the parameter for ensuring the optimum. When
the ratio of melamine to the mpg-C3N4’s prepolymer comes to 37.5%, R-C3N4 shows least defects and
excellent electron-hole separation and transfer properties. Thus in this work, without special mentioned, the
ratio of melamine to mpg-C3N4’s prepolymer was 37.5% in this work.
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Measurement and Characterization. The structures and morphologies of the samples were characterized
by X-ray diffraction (XRD) (X'Pert PRO MPD, Holland), scanning electron microscopy (SEM) (Hitachi
S-4800, Japan) and transmission electron microscopy (TEM) (JEM-2100UHR, Japan). The functional
groups in the samples were characterized by Fourier transform infrared spectrometry (FTIR) (Thermo
Nicolet NEXUS670, USA). Further evidence for the composition of the product was inferred from X-ray
photoelectron spectroscopy (XPS), using ESCALAB 250Xi spectrometer equipped with a prereduction
chamber. Elemental analysis was detected by Elementar Vario EL III, which was produced by Elementar
Company of Germany. The UV−Vis diffuse reflection spectra were obtained for the dry-pressed disk
samples using a Scan UV−Vis spectrophotometer (UV−Vis DRS UV-2450, Shimadzu, Japan) equipped
with an integrating sphere assembly, using BaSO4 as reflectance sample. Nitrogen adsorption−desorption
isotherms were obtained on a nitrogen adsorption apparatus (Micromeritics ASAP 2020M) with all samples
degassed at 423 K for 12 h prior to measurements.
Ninhydrin Test.35 The ninhydrin test was performed by adding 1 mL ninhydrin acetone solution (80 mg/mL)
into 2 mL C3N4 suspension (C3N4, 2.0 mg), and then heating the covered mixture in boiling water bath for
15 min. The UV-Vis absorption spectra were measured with a UV-Vis spectrometer directly using the
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ACS Catalysis
solution after heating, the absorption intensity quantitatively indicates the amount of NH2 or NH of the
C3N4.
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Cyano Reduction Reaction.36 NaBH4 (19.0 mg, 0.5 mmol) was dissolved in dry THF (10 mL), and 10 mL
trifluoroacetic acid (TFA) THF solution (0.01 M) was injected into NaBH4 solution drop by drop, then 100.0
mg mpg-C3N4 was added. The suspension was stirred overnight. Finally, the reaction was quenched by
10mL water. The reduced mpg-C3N4 was filtered and washed for four times, and then dried at 60 °C in the
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vacuum oven overnight.
NaBH4
X-C-NH2
X-CN
Preparation of Deaminated C3N4. 100.0 mg mpg-C3N4 was added into 40 mL dilute HCl at 5–10 °C. 10
mL NaNO2 solution (0.05 M) was slowly added into the suspension for 5 h. Then hypophosphorous acid
solution (1.0 g, 50%) was added into the mixture mentioned above. The mixture was stirring overnight.
Finally, the deaminated mpg-C3N4 was filtered and washed for four times, and dried at 60 °C in the vacuum
oven overnight.
Electrochemical Analysis.37 The photocurrent measurement and electrochemical impedance spectroscopy
(EIS) experiments were conducted using an Ametek PARSTAT4000 electrochemistry workstation using a
standard three-electrode cell with a working electrode, a platinum electrode as the counter, and a Ag/AgCl
reference electrode. The working electrodes were prepared as an indium tin oxide glass (ITO) decorated
with catalyst samples. In a typical procedure, 30.0 mg of photocatalyst was firstly dispersed in 2 mL of
acetone containing polyvinylidene fluoride (PVDF) solution (50 ꢀL, 5 wt%), the slurry was then spread onto
the ITO. After air-drying, the electrode was annealed at 573 K for 30 min in air to improve the adhesion.
Na2SO4 (0.5 M) was used as the electrolyte solution. Visible light irradiation was provided by a 300 W
Xe-lamp with a cutoff filter (Kenko L-42). The exposed area under illumination was 1 cm2. For
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electrochemical impedance spectroscopy (EIS) experiments, the perturbation signal was 10 mV, the
frequency ranged from 100 kHz to 10 mHz.
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Photooxidation of 1,4-DHP. The photochemical reaction was performed at room temperature under air
atmosphere in a two neck round bottom flask (25 mL) with irradiation by a 35 W xenon lamp. The aqueous
solution (20 mL) containing 10–4 mol/L of 1,4-DHP and photocatalysts (20.0 mg) was irradiated at λ > 420
nm. This reaction was monitored by UV-Vis spectroscopy and absorption changes were recorded at intervals
of 10 min.
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Hydrogen Production. The photocatalytic activity of the as-repaired R-C3N4 was also evaluated by the
photocatalytic reduction of 1,4-DHP to produce H2 from acetonitrile solutions .The H2 evolution test was
performed by using a LabSolar-III AG reaction cell (Beijing PerfectLight Company). 1,4-DHP (100.0 mg,
0.4 mmol), 30 mg photocatalyst and (NH4)2PtCl6 (0.5 mg, 0.001 mmol) were dissolved in 40 mL acetonitrile
in a pyrex reactor. The suspension was thoroughly degassed to remove air and irradiated using a 300 W
Xe-lamp (PLS-SEX300/300UV, Beijing PerfectLight Company). A cutoff filter (Kenko L-42) was
employed to achieve visible-light (λ > 420 nm) irradiation. The yield of H2 was determined by using an
on-line gas chromatograph (GC7900, Techcomp) equipped with a column of 5 A molecular sieve and a
thermal conductivity detector, and Ar was used as the carrier gas.
•−
Electron Spin Resonance (ESR) Spectroscopy. For the ESR detection of superoxide anion radical (O2 ),
ESR samples were injected into quartz capillary in darkness and illuminated in the cavity of ESR
spectrometer. Photosensitizers and 5,5-dimethyl-1- pyrroline-N-oxide (DMPO, superoxide anion radical
O2•− scavenger) in air-saturated aqueous solution were stirred in the dark, then the solution was injected into
the quartz capillary. A diode pumped solid state (DPSS) laser (532 nm) was used for the photoirradiation of
the solution in quartz capillary for 100 seconds.
RESULTS AND DISCUSSION
Comprehensive Repairing Strategy for Various Defects.
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Due to the lack of purification steps after synthesis, there are many unreacted amino/cynao groups, and other
relative intermediates are involved into the photocatalysis application. The final products are hard to purify,
and the defects are inevitable in the photocatalysis application. Actually, these various defects could be
repaired through a facile copolymerization with functionalized ‘little patch’ (Figure 1). Melamine is
composed of amino group and triazine structure. The amino group could react with amino/cyano defects and
their relative intermediates to repair these defects.32,38 The triazine structure has a π conjugation, which
benefits for the electron transfer. Thus melamine is a good candidate ‘little patch’ for mpg-C3N4.
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Firstly, the prepolymer of mpg-C3N4 was obtained through cyanamide cladding the silicon dioxide
pellets. Next, melamine as functionalized ‘little patch’ was mixed with the prepolymer of mpg-C3N4 and
polymerized together at high temperature. During the copolymerization process, amino groups of melamine
could repair or passivate defects of mpg-C3N4. Finally, R-C3N4 could be obtained by removing silica
template. In order to ensure the repair optimum, the amount of the melamine (25%, 37.5% and 50%) was set
as the parameter. When the weight ratio of melamine to the total of cyanamide and melamine comes to
37.5%, R-C3N4 shows least defects and excellent electron-hole separation and transfer properties (vide infra).
Thus in this work, without special mentioned, the weight ratio of melamine was 37.5% in the following.
N
defects
C
N
NH
C
N
NH2
C
N
H2N
N
NH2
N
H
H2N
NH2
defects
Imperfect structure
NH2
N
NH2
NH2
N
N
N
N
N
H2N
N
N
H
N
NH2
H2N
N
NH2
NH2
N
NH
C
H2N
N
NH2
N
N
N
+
N
N
N
N
N
N
H
NH2
NH2
little pitch
H2N
NH2
N
perfect structure
not defects
Figure 1. Schematic illustration of repairing defects of mpg-C3N4 to R-C3N4.
Characterization of the Repair for mpg-C3N4.
As shown in the TEM and SEM images (Figure 2 and Figure S1), the as-synthesized R-C3N4 is coated
by the nanosheet g-C3N4. Compared with TEM images of g-C3N4 and mpg-C3N4, R-C3N4 is composed of
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mpg-C3N4 and exfoliated g-C3N4, and this laminar morphology looks like silk veil on the mpg-C3N4 to
smooth and repair the defects of mpg-C3N4. X-ray diffraction (XRD) patterns also confirm the results
mentioned above (Figure 2d). They show obvious signals at 2θ of 27.48° (d=0.326 nm), which originate
from the (002) interlayer diffraction of a carbon nitride graphitic-like structure.39 Compared with g-C3N4, the
(002) diffraction of R-C3N4 is significantly weakened, indicating the nanosheet structure on the surface of
R-C3N4.40,41 Thus, the surface area of R-C3N4 (231.3 m2g–1) is spontaneous larger than others (221.3 m2g–1
and 16.9 m2g–1) (Figure S2 and Table S1).
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b
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200 nm
R-C N 37.5%
3 4
R-C N 25%
3 4
d
c
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3 4
g-C N
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2
θ
Figure 2. TEM images of (a) R-C3N4, (b) mpg-C3N4 and (c) g-C3N4. (d) XRD patterns of mpg-C3N4, R-C3N4 and g-C3N4.
Cyano defects exist in imperfect structure which can be seen in Figure 1. Fourier transform infrared
spectrometry (FT-IR) spectra was undertaken to determine the repair of cyano defects. FT-IR spectra of the
as-prepared C3N4 from cyanamide shows different intensities around 2150 cm−1 (Figure 3a). This peak is
attributed to cyano terminal groups C≡N (major defects). Peak of cyano can be clearly observed in
mpg-C3N4. As the introduction of melamine into R-C3N4, the peak at 2150 cm−1 almost disappears, implying
that the cyano defects of R-C3N4 have been successfully repaired.
Amino groups in the imperfect structure is also another typical defects (Figure 1). X-ray photoelectron
spectroscopy (XPS) was employed to accurately measure the chemical bonding of the samples and amino
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groups. Herein, these amino groups include amino defects and amino groups from perfect triazine structure
(not amino defects). Figure 3b and 3c show the typical peaks of C1s and N1s in the XPS spectra of the
as-prepared C3N4 which confirm the decrease of amino groups mentioned above. For the 288.0 eV peak for
C–N bonds in the C1s spectra, it could be further deconvoluted into 287.9 eV (C1) and 288.4 eV (C2),
which are attributed to the sp2 hybridized carbon bonded to N inside the triazine rings (C=N) and the sp2
hybridized carbon bonded to the –NH2 group (=C–NH2), respectively. To some extent, the ratio of C2 to C1
indicates that the degree of depolymerization and the amount of amino groups. mpg-C3N4 shows the highest
ratio of C2 to C1 (1.19), followed by g-C3N4 (1.04). After repaired with melamine, R-C3N4 shows the lowest
ratio of C2 to C1 (0.82), indicating that R-C3N4 shows higher polymerization degree and less amino groups.
Similar results were also observed through N1s spectra. The N1s spectra can be deconvoluted into ≈ 398.7
eV (N1), 399.5 eV (N2) and 400.7 eV (N3), which correspond to sp2 N atoms involved in triazine rings
(C=N), N atoms in H–N–(C)2 and C–NH2, respectively. The ratio of the sum between N2 and N3 to N1
indicates that the degree of the depolymerization. mpg–C3N4 shows the highest ratio (1.46), followed by
g–C3N4 (1.30), R–C3N4 has the lowest ratio (1.17). These are consistent with the results obtained from the
C1s spectra and elemental analysis (Table S2).
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N 1s
C1s
C1
N1
N2
N3
4
R-C
N
c
a
b
3
4
mpg-C N
3 4
C2
R-C N
3
g-C N
3 4
g-C N
3
4
R-C N
3 4
mpg-C N
3 4
mpg-C N
3
4
290 288 286 284 282
402 400 398 396
2800 2400 2000
Wavenumber/cm
-1
Binding energy/ eV
Binding energy/ eV
Figure 3. (a) FT-IR spectra of mpg-C3N4 and R-C3N4. (b,c) High resolution XPS spectra of C1s and N1s compared with R-C3N4 ,
mpg-C3N4 and g-C3N4.
In order to further identify the amino defects, ninhydrin test was employed to measure the
concentration of amino defects on the surface of these samples. Ninhydrin (2,2-dihydroxyindane-1,3-dione)
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could only react with amino defects without the interference of other NH2/NH groups in the perfect triazine
structure.42 Subsequently it produces ‘Ruhemann's purple’, which can be detected at 570 nm of UV-Vis
absorption spectra.35 The absorption intensity quantitatively indicates the amount of the amino defects in
carbon nitride. As can be seen from Figure 4a, the intensity of R-C3N4 is only two thirds of mpg-C3N4,
which is consistent with XPS results. Therefore, the amino defects were also well repaired through this
simple copolymerization method.
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Wavelength / nm
Figure 4. (a) Optical absorption spectra of the solutions from the “ninhydrin tests”, (b) Conversion of photooxidation 1,4-DHP
using different samples based on the same procedure.
Optical and Electrical Properties of R-C3N4
It should be noted that the repair of mpg-C3N4 with melamine is not merely a simple physical mixing
process. The surface of R-C3N4 repaired by nanosheet g-C3N4 became smooth, which could be confirmed by
the relatively weaker absorption intensity of R-C3N4 (Figure S3). Meanwhile, it could produce close isotype
homojunction, providing an electron-hole separate transfer route in the isotype homojunctions of g-C3N4 and
mpg-C3N4.19,43 The band structures of g-C3N4 and mpg-C3N4 were examined by valence-band (VB) XPS
and band gap energies (Figure 5). Based on VB XPS, the VB maxima of g-C3N4 and mpg-C3N4 are 1.65 and
1.98 V versus NHE, respectively. The VB potential of mpg-C3N4 is 0.33 V more positive than that of g-C3N4,
which can drive the migration of photoinduced holes (h+) from mpg-C3N4 to g-C3N4. Combined with the
band gap energies, the CB potentials of g-C3N4 and mpg-C3N4 are calculated to be −0.93 and −0.56 V
respectively, indicating that the CB potential of g-C3N4 is 0.37 V more negative than that of mpg-C3N4. The
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Figure 5. (a)(b) VB (valence band) XPS of mpg-C3N4 and g-C3N4. (c) UV–vis diffuse reflection spectra of mpg-C3N4, R-C3N4
and g-C3N4. (d) electron transfer mechanism in g-C3N4/mpg-C3N4 homojunction.
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CB offset between g-C3N4 and mpg-C3N4 can drive the migration of photogenerated electrons (e–) from
g-C3N4 to mpg-C3N4. Thus electrons are redistributed on mpg-C3N4 side of the junction, and the holes are
located on the opposite side (g-C3N4), which greatly reduce the energy-wasteful electron-hole recombination
(Figure 5d).
mpg-C N
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Figure 6. (a) ESR spectra of g-C3N4, mpg-C3N4 and R-C3N4. (b) Photoresponses of g-C3N4, mpg-C3N4 and R-C3N4. (c) Nyquist
plots for g-C3N4, mpg-C3N4 and R-C3N4 electrodes under visible light irradiation (λ > 420 nm, [Na2SO4] =0.5 M).
To analyze the generation of unpaired electron and hole in the photocatalyst, as-prepared carbon nitride
were measured by ESR spectroscopy at room temperature, as shown in Figure 6a. The tensor parameter (g)
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is 2.0020, which could be attributed to the unpaired electrons on π-conjugated carbon nitride aromatic
rings.21 Compared with g-C3N4 and mpg-C3N4, R-C3N4 shows a much stronger signal for the unpaired
electrons, indicating that repair with functionalized ‘little patch’ has improved the efficiency of electron-hole
separation in the photocatalyst. Furthermore, Figure 6b studied the transient photocurrent responses via three
on−off cycles of g-C3N4, mpg-C3N4 and R-C3N4 electrodes, which directly correlate with the separation
efficiency of photogenerated electron and hole. It can be clearly seen that these samples behave photocurrent
with good reproducibility under visible light, indicating that the carbon nitride electrodes are stable and the
photoresponses can be reversible. The visible irradiated photocurrent density of R-C3N4 is nearly 4.0 times
that of mpg-C3N4. Electrochemical impedance spectrosscopy (EIS) was also studied as shown in Figure 6c.
R-C3N4 shows smaller arc radius in the EIS Nyquist plot, suggesting the less interface electron resistance.
Therefore, the repair of mpg-C3N4 with functionalized ‘little patch’ has enhanced photoinduced
electron-hole separation and conductivity of carbon nitride, which also benefit electron transfer in both the
photoxidation reaction and H2 generation.
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Application of R-C3N4 on the Photooxidation and H2 Evolution from 1,4-DHP
To confirm the photocatalytic abilities of mpg-C3N4 after repair, the photooxidation of 1,4-DHP was carried
out, which was irradiated under air atmosphere instead of pure oxygen atmosphere. As shown in Figure S4,
upon irradiating the mixed solution of photocatalyst and 1,4-DHP with a Xenon lamp, the absorption of
1,4-DHP at 374 nm decreased and the absorption of the product (pyridine derivatives, 1b) at 280 nm
increased, which were demonstrated by the changes of UV curves. After reaction photocatalized by R-C3N4,
the product 1b was obtained by simple filtration without further purification. Figure 7a and Figure 7b show
that the efficiency of photocatalytic oxidation in R-C3N4 is nearly 2 times that of mpg-C3N4 and almost 5
times that of g-C3N4. Either no R-C3N4 or no light was used for blank experiments, there was no obvious
decrease of the absorption at 374 nm, indicating no conversion of 1,4-DHP. To further underline the
necessity of excellent interconnection for R-C3N4, the physically-mixed sample of mpg-C3N4 and g-C3N4
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was also tested for photocatalytic activity, and no significant enhancement could be observed (Figure S4).
Figure 7c shows that the durability of R-C3N4 for photocatalytic oxidation 1,4-DHP. After three consecutive
operations, the efficiency remains stable without noticeable deactivation.
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Time/min
Figure 7. (a,b) Comparison with the conversion of 1,4-DHP among R-C3N4, g-C3N4 and mpg-C3N4. (c) Durability of R-C3N4 in
photooxidation of 1,4-DHP. (d) Amount of H2 produced by R-C3N4 and mpg-C3N4.
In order to further enhance the atom economy, R-C3N4 were applied to hydrogen evolution with
1,4-DHP and (NH4)2PtCl6 in absence of oxygen. As shown in Figure 7d, R-C3N4 exhibits superior H2
evolution (2700 ꢀmol/g). Meanwhile, the turnover number (TON, over a platinum cocatalyst) of R-C3N4 is
2400, 6.5 times faster than that of mpg-C3N4 (TON: 368, Table S1). No H2 evolution was detected in the
system of g-C3N4 because of the low photocatalytic performance. The R-C3N4 performance of H2 evolution
is better than that of the photooxidation (2 times). It further indicates the importance of the defect repair.
Mechanism of Photooxidation and H2 Evolution
The mechanism of the photooxidation for 1,4-DHP is shown in Figure 8a. The electron-hole separation
and transfer are crucial for the photocatalysis. In order to confirm the mechanism, electron spin resonance
(ESR) spectroscopy was employed with the 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as scavenger for
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•−
O2 . There was no signal for the mixture of 1,4-DHP and DMPO under dark (Figure S5a). When they are
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obvious electron transfer of 1,4-DHP→C3N4→O2.
Figure 8b shows the mechanism of photocatalysis hydrogen evolution from 1,4-DHP, which is similar
to photooxidation mechanism, especially for the electron transfer of 1,4-DHP→C3N4. The electron transfer
of 1,4-DHP→C3N4 from ESR spectrum show that the generation speed of hole in the VB with 1,4-DHP is
slower than that without 1,4-DHP (Figure 8c 8d), indicating the electron transfer from 1,4-DHP to the CB of
R-C3N4 after irradiation. Subsequently, electron transfer from the CB of R-C3N4 to Pt salt. TEM images and
0min
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Magnetic field(G)
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Magnetic field(G)
Figure 8. (a) Mechanism for the photooxidation of 1,4-DHP with R-C3N4 in the presence of O2. (b) Mechanism for the H2
evolution from 1,4-DHP in the absence of O2. (c) ESR spectra of R-C3N4 with 1,4-DHP. (d) ESR spectra of R-C3N4 without
1,4-DHP.
XPS spectra of the R-C3N4/Pt after H2 evolved reaction provide valuable information on the catalytically
active species (Figure S6). High-resolution XPS spectra of Pt 4f5/2 and Pt 4f7/2 from R-C3N4/(NH4)2PtCl6
exhibit binding energies of 75.2 and 72 eV, respectively (Figure S6a), which is characteristic of Pt0,
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suggesting that electron transfer from 1,4-DHP to R-C3N4 and (NH4)2PtCl6 results in the reduction of
(NH4)2PtCl6 to Pt0.44-46 In order to confirm the existence of Pt0 in the photocatalytic H2 evolution reaction,
high-resolution TEM (HRTEM) images of R-C3N4/(NH4)2PtCl6 sample after H2 evolution reaction were
measured as shown in Figure S6b. R-C3N4/Pt exhibits well-resolved lattice fringes with interplanar spacing
of 0.227 nm, which is believed to the (111) diffraction facet of pure Pt0.47 At the same time, 1,4-DHP
transformed into highly reactive 1,4-DHP cation radical, which is preferable to release H+ and produce the
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final aromatization product 1b, which were confirmed by the H NMR spectra, the HRMS (Figure S7, S8)
and HPLC spectra (Figure S9).
Assessing the Defect Effects on the Photooxidation
In order to confirm and understand the structure-property relationship of defects, the cyano groups of
mpg-C3N4 were reduced by NaBH4, consequently converted into amino defects (see methods).36 To verify
this conversion, ninhydrin test was employed to measure the concentration of amino defects on the surface
of these samples. As can be seen in Figure 4a, the reduced mpg-C3N4 shows almost twice absorption
intensity at 570 nm of mpg-C3N4, indicating the cyano→amino conversion. Elemental analysis shows that
the reduced mpg-C3N4 has more H content (3.88 % H, Table S2), which agrees with the result of ninhydrin
test. Then the photooxidation of 1,4-DHP was used to measure the photocatalytic ability of the modified
mpg-C3N4. When the conversion of 1,4-DHP came to 99.9 %, the reaction time was 50 mins, which is 30
mins shorter than that of mpg-C3N4 (Figure 4b). It indicates that the cyano defects greatly reduce the
photocatalytic performance.
In addition, the amino defects of mpg-C3N4 were removed by the deamination reaction (see methods).
The absorption intensity of ninhydrin test using deaminated mpg-C3N4 is almost 2/3 of mpg-C3N4, and the H
content comes to 3.16, indicating that some amino defects were indeed removed. The photooxidation ability
of 1,4-DHP by deaminated mpg-C3N4 was slightly improved, the reaction time came to 70 mins as the
product yield came to 99.9%. Compared with amino defects, the cyano defects more greatly reduce the
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photocatalytic ability of mpg-C3N4. Moreover, it indicates that the improvement of photocatalysis ability
mainly benefits from the defect repair.
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CONCLUSION
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In summary, through a facile and comprehensive copolymerization, the functionalized ‘little patch’
(melamine) successfully remedies various defects of mesoporous carbon nitride, especially for the cyano and
amino defects. When the weight ratio of melamine comes to 37.5%, R-C3N4 shows least defects and
excellent electron-hole separation and transfer properties, which were confirmed by the ESR, photocurrent
and conductivity measurements. R-C3N4 in the photoxidative aromatization of 1,4-DHP and H2 evolution
from 1,4-DHP not only meet the demand of sustainable chemistry, but also offer another approach for H2
production. H2 evolution by R-C3N4 comes to 2700 ꢀmol/g, and the turnover number is 2400, 6.5 times
faster than that of mpg-C3N4. Under air atmosphere, the yield of the product comes to 99.5 % after 40 mins,
which is almost 2 times higher than that of mpg-C3N4 and almost 5 times higher than that of g-C3N4. The
effect of various defects on photooxidation was confirmed by chemical method. This repair strategy offers
new opportunities for developing the photocatalytic performance of carbon nitride.
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Supporting Information
Additional experimental data including the SEM images, Element analysis, XRD patterns, N2
adsorption−desorption isotherms, UV–Vis diffuse reflection spectra, VB (valence band) XPS, ESR spectra
and UV-Vis absorption spectra of photooxidation of 1,4-DHP of prepared materials. This material is
available free of charge via the Internet at http://pubs.acs.org.
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final
version of the manuscript.
Acknowledgement
This work was financially supported by the National Basic Research Program of China
(2014CB931700), NSFC (61222403, 21302224, 51303212, 51372277, 51172285 and 51303202), China
Postdoctoral Science Foundation (2014M560590 and 2015T80758), Shandong Provincial Natural Science
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Foundation (ZR2013BQ028, ZR2013EMQ013), Project of Science and Technology Program for Basic
Research of Qingdao (14–2-4-47-jch), the Fundamental Research Funds for Central Universities
(15CX05010A) and the State Key Laboratory of Fine Chemicals (KF1203).
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