G-C3N4/Uio-66-NH2 nanocomposites with enhanced visible light photocatalytic activity for hydrogen evolution and oxidation of amines to imines

Article abstract of DOI:10.1039/c9nj05495b

g-C₃N₄/Uio-66-NH₂ (CNUIO) nanocomposites were prepared by growing an NH₂-mediated zirconium-based metal-organic framework (Uio-66-NH₂) in the presence of g-C₃N₄ nanotubes. CNUIO

Full text of DOI:10.1039/c9nj05495b

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g-C3N4/Uio-66-NH2 nanocomposites with enhanced visible light photocatalytic activity
for hydrogen evolution and oxidation of amines to imines
Shishen Zhang, Kelong Chen, Wen Peng, Jianhua Huang*
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Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, China
*Email: jhhuang@zstu.edu.cn
Abstract
g-C3N4/Uio-66-NH2 (CNUIO) nanocomposites were prepared by growing NH2-
mediated zirconium-based metal-organic framework (Uio-66-NH2) in the presence of g-C3N4
nanotubes. CNUIO exhibited efficient photocatalytic performance for oxidation of amines to
imines and hydrogen production under visible light illumination. The optimal Uio-66-NH2
content in CNUIO was 16.7 wt% with the H2-generation rate of 152.2 μmol·h−1, which is
about 1.7 and 39 times as high as that of g-C3N4 and Uio-66-NH2, respectively. The optimal
conversion rate of benzylamine was 58.9%, which is about 3.4 and 10.5 times as high as that
of g-C3N4 and Uio-66-NH2, respectively. The probable photocatalytic mechanism of CNUIO
composite was investigated. This work provides a new insight that g-C3N4/MOF
photocatalysts have great potential application in hydrogen production and organic synthesis.
Keywords: g-C3N4; Uio-66-NH2; Photocatalyst; Hydrogen evolution; Oxidation of amines
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1. Introduction
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Photocatalytic techniques have attracted increasing attention in solving the problem of
environmental pollution and energy crisis [1-3]. In the past years, many low cost
photocatalysts showing excellent performance have been explored. Graphitic carbon nitride
(g-C3N4), a metal-free semiconductor, has aroused widespread interest for its wide raw
materials, non-toxicity, high stability and microwave absorption property [4-7]. However,
the performance of bulk g-C3N4 is still hindered by the fast recombination of photoexcited
electron-hole pairs [8-9]. To improve the photocatalytic property of g-C3N4, several strategies
have been proposed, including element doping [10-11], heterojunction construction [12-13],
morphology control [14], and metal loading [15]. Among these modifications, combination
of other semiconductors with suitable band gap has become a feasible path to enhance the
photocatalytic activity [16-17].
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Metal-organic frameworks (MOFs), one kind of porous materials constructed by metal
clusters or metal ions and organic ligands, have recently attracted extensive attention because
of the high specific surface area, tunable pore structure and diverse functionalities [18-20].
The porosity and crystallization property endow MOFs with short charge transfer distance,
highly dispersed active sites and rapid utilization of charge carriers. Therefore, some MOFs
are used for organic pollutants degradation [21], carbon dioxide reduction [22], hydrogen
evolution [23] and organic reactions [24] under visible light irradiation. For example,
hydrogen was produced over Zr-containing MOF (Uio-66) under UV light illumination [25].
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However, the single-component MOF suffers from a low performance owing to its limited
light harvesting capacity and poor ability for charge separation. Therefore, it is desirable to
fabricate heterostructures of MOFs and matched semiconductors for improved photocatalytic
property.
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Recently, researchers have realized that the combination of Uio-66 and g-C3N4 can offer
an attractive way to increase the photocatalytic activity [26-30]. Zhang et al. reported that
the degradation rate of RhB over g-C3N4/Uio-66 nanohybrids was nearly 6 and 3.3 times as
high as that of g-C3N4 and Uio-66, respectively [26]. The g-C3N4/Uio-66 composite showed
better photocatalytic activity for methylene blue degradation [27]. Uio-66 incorporated with
g-C3N4 nanosheets exhibited much better photocatalytic performance for carbon dioxide
conversion compared with pure g-C3N4 [28]. The combination of g-C3N4 and Uio-66 could
promote the separation efficiency of photogenerated charges, which showed better
photocatalytic performance for hydrogen production [29-30].
Imines and their derivatives are important industrial intermediates, which are commonly
used in the synthesis of fine chemicals, dyes and pharmaceuticals [31-32]. They are
traditionally produced through combination of amines with carbonyl compounds, but the
highly active reactants make the reaction hard to control. The direct oxidation of amines is
an attractive path. However, the oxidation reagents, such as N-tert-butylphenylsulfinimidoyl
chloride, are not environment friendly [33]. Direct oxidation of amines into imines with
molecular oxygen can provide an economical and environment friendly way. Kumar et al.
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reported a Fe(bpy)3/g-C3N4 photocatalyst for coupling of benzylamines using white LED as
light source [34]. Core-shell structured CdS@C3N4 photocatalysts showed high selectivity
for photocatalytic oxidation of amines to imines [35]. Zhang et al. found that decoration of
cyano-groups could accelerate the generation of charge carriers and enhance the
photocatalytic performance of g-C3N4 nanosheets for the oxidation of amines to imines [36].
Although a few examples about photocatalytic oxidation of amines have been reported,
however, to our knowledge, there is no report on g-C3N4/Uio-66-NH2 nanocomposites with
dual applications in photocatalytic reaction including hydrogen generation and selective
oxidation of amine.
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Herein, g-C3N4/Uio-66-NH2 (CNUIO) nanocomposites were obtained via a
solvothermal process, which exhibited superior photocatalytic performance and stability
towards hydrogen generation and selective oxidation of amine under visible light
illumination. The active species in benzylamine oxidation process have been detected. This
work highlights the potential application of g-C3N4/MOF as photocatalyst for organic
synthesis and hydrogen evolution.
2. Experimental method
2.1 Synthesis of g-C3N4 nanotubes
g-C3N4 nanotubes were obtained by calcination using urea and thiourea as precursors.
Typically, 10 g urea, 10 g thiourea and 40 mg Pluronic F127 were fully grinded, mixed and
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then heated at 350 ℃ for 1 h followed by a subsequent calcination at 600 ℃ for 3 h. The
resulted yellow powder was collected.
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2.2 Preparation of CNUIO composite
CNUIO composite was prepared via a solvothermal method that was described in the
previous report with slight modification [37], as shown in Fig. 1. Firstly, the precursors of
MOF, ZrCl4 (0.053 g) and 2-aminoterephthalic acid (ATA, 0.034 g), were dissolved in 50
mL DMF. Then, the as-prepared g-C3N4 nanotubes (0.2 g) were added into the above solution,
followed by continuous stirring for 1 h. After that, the mixture was sealed in a Teflon reactor
and heated at 120 °C for 24 h. The yellow precipitates were centrifuged and washed with
alternating DMF and methanol for several times, which was further soaked in methanol for
3 days (methanol was replaced every 12 h). Finally, the yellow product was separated by
centrifugation, dried at 100 °C under vacuum, and denoted as CNUIO-x, here x is the mass
ratio of g-C3N4 and ZrCl4.
For comparison, Uio-66-NH2 was also prepared under the same condition as described
above without adding g-C3N4
Fig.1. Schematic diagram of the synthesis of CNUIO composites.
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2.3 Characterization
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The crystalline character of samples was analyzed by X-ray diffractometer (XRD, DX-
2700X, Fangyuan, China). Scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) images were obtained on a FE-SEM GEMINI ULTRA 55 Field emission
scanning electron microscope and a JEOL JEM-2100 transmission electron microscope,
respectively. X-ray photoelectron spectra (XPS) were collected on the AXIS Ultra from
Kratos. UV-vis diffuse reflectance spectra (DRS) were recorded on a Shimadzu
spectrophotometer (UV-2450). Porous parameters were obtained by nitrogen adsorption-
desorption tests on an ASAP 2020 Micromeritics analyzer at -196 C. Photoluminescence
spectra (PL) were recorded on a Gidden SENS-9000 fluorescence spectrometer. The
electrochemical tests were performed on an electrochemical workstation (CHI660E,
Chenhua, China). Electron paramagnetic resonance (EPR) signals were recorded by a Bruker
A300 spectrometer at room temperature. The contents of Zr in Uio-66-NH2 and CNUIO-x
were determined using inductively coupled plasma-optical emission spectrometer (ICP-OES,
Agilent 730).
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2.4 Photocatalytic activity
The photocatalytic performance of CNUIO-x composites was evaluated by hydrogen
production and selective oxidation of amine (> 420 nm). The photocatalytic hydrogen
production was tested on a Labsolar-H2 photocatalytic system (Perfect, China) according to
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our previous work [38]. 50 mg photocatalyst with 1% Pt-loading was added into a Pyrex
reaction cell containing 100 mL TEOA aqueous solution (10 vol%), which was then sealed
and evacuated. A 300W Xenon lamp (PLS-SXE300,  > 420 nm) was used to irradiate the
suspension, which was constantly stirred during the reaction. The generated H2 was
periodically characterized by an on-line GC9800 gas chromatography (Kechuang, China).
The photocatalytic oxidation of amines was performed as follows: 5 mg photocatalyst
with 1% Pt-loading, 0.05 mmol amine and 5 mL CH3CN were added into a quartz tube.
Under continuous stirring, the mixture was illuminated using a 300W Xenon lamp (PLS-
SXE300, > 420 nm) for 5 h under ambient conditions. After the reaction, the photocatalyst
was removed, and the product was analyzed by GC (Shimadzu GC-2014), NMR (Zhongke
Niujin WNMR-I 400) and HRMS (Waters Xevo G2-S).
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3. Results and Discussion
3.1 Characterization of photocatalyst
XRD patterns of the as-prepared composites are shown in Fig. 2. The XRD diffraction
peaks of pure g-C3N4 and Uio-66-NH2 are the same as literature reports [38-39]. The
characteristic peaks of both g-C3N4 and Uio-66-NH2 are observed in allCNUIO-x composites.
The contents of Zr in CNUIO-x and Uio-66-NH2 were measured by ICP-OES, which are in
the range of 2.4-27.9 wt% respectively (Table S1).
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Fig. 2. XRD patterns of pure g-C3N4, Uio-66-NH2 and CNUIO-x composites.
The morphology and microstructure of g-C3N4 and CNUIO-5 were revealed through
SEM and TEM observations. Fig. 3a exhibits a typical SEM image of g-C3N4 nanotubes with
an average diameter of 350 nm. The SEM image of CNUIO-5 (Fig. 3b) displays that Uio-66-
NH2 particles, with a size of 30-60 nm, are embedded on the surface of g-C3N4 nanotubes.
Fig. 3c and 3d display TEM images of a typical CNUIO-5 nanotube under different
magnifications. Clearly, Uio-66-NH2 nanoparticles are intimately adhering on the surface of
g-C3N4 nanotube, in agreement with SEM observation.
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Fig. 3. SEM image of pure g-C3N4 (a) and CNUIO-5(b), TEM images of CNUIO-5 (c, d).
XPS measurement can provide the information on chemical environment of CNUIO-5
nanocomposite. The C 1s regional spectrum (Fig. 4a) depicts two peaks at 284.8 and 287.2
eV. The latter peak is related to the sp2-bonded carbon (C-C=N) in g-C3N4 [40], and that at
284.8 eV is ascribed to the C atom in ATA ligand. For the N 1s regional spectrum (Fig. 4b),
four peaks were located at 397.9, 399.1, 400.4, and 403.7 eV, representing N species in the
C=NC, N-(C)3, C-N-H and positive charge localization in heptazine rings, respectively [41].
Two peaks at 532.8 and 531.5 eV in the O 1s regional spectrum (Fig. 4c) are assigned to the
C=O bonds in ATA ligand and Zr-O bonds in Uio-66-NH2, respectively [42]. Two signals
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for Zr 3d5/2 and Zr 3d3/2 at 182.9 eV and 185.2 eV are observed in the Zr 3d regional spectrum
(Fig. 4d), implying the presence of Zr4+ [43].
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Fig. 4. Region XPS spectra of C 1s (a), N 1s (b), O 1s (c), and Zr 3d (d) of CNUIO-5.
Pore structure and specific surface area of CNUIO-5 were investigated by N2 adsorption-
desorption measurement. For comparison, g-C3N4 and Uio-66-NH2 were also analyzed. As
displayed in Fig. 5, g-C3N4 has a typical Ⅳ isotherm that is characteristic of mesoporous
structure [41]. Uio-66-NH2 possesses a typical microporous structure with type I isotherm
[43]. CNUIO-5 exhibits hybrid type I/IV isotherm, revealing the existence of micropores and
mesopores. Detailed pore parameters of g-C3N4, CNUIO-x and Uio-66-NH2 are given in
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Table 1. These unique pore characteristics make photocatalyst with more active sites, which
are helpful for the improvement of photocatalytic performance [44].
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Table 1. Specific surface area (SBET) and pore characteristics of the as-prepared
samples
Sample
SBET
(m2.g-1)
62.5
Pore volume
(cm3.g-1)
0.352
Average pore size
(nm)
25.20
4.06
g-C3N4
CNUIO-1
CNUIO-3
CNUIO-5
CNUIO-7
CNUIO-9
Uio-66-NH2
519.0
254.1
142.4
108.3
99.9
0.527
0.461
7.26
0.393
9.89
0.507
18.74
21.54
1.72
0.538
924.2
0.397
Fig. 5. N2 adsorption–desorption isotherms of g-C3N4, CNUIO-5 and Uio-66-NH2.
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3.2 Photocatalytic activity
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The photocatalytic H2-evolution performance was tested using triethanolamine (TEOA) as
electron donor sacrificial agent. As shown in Fig. 6a, g-C3N4 and Uio-66-NH2 generate H2 at
a rate of 90.1 and 3.9 μmol.h-1, respectively. After coupling of g-C3N4 and Uio-66-NH2, the
H2-evolution rate markedly changes. With an increasing loading of Uio-66-NH2, the H2-
evolution rate of CNUIO increases first and then decreases. The optimal proportion of Uio-
66-NH2 in CNUIO-5 is 16.7 wt% with a rate of 152.2 μmol·h−1 for H2-evolution, which is
about 1.7 and 39 times as high as that of g-C3N4 and Uio-66-NH2, respectively. A further
addition of Uio-66-NH2 results in the decrease of H2-evolution rate. It may be due to the
heavy coverage of active sites by Uio-66-NH2 on g-C3N4 surface, and the photocatalytic
reaction thus slows down. Therefore, a suitable loading amount of Uio-66-NH2 is needed to
acquire the optimal H2 production rate over CNUIO nanocomposites. In addition, CNUIO-5
without Pt loading shows a low H2-evolution rate of 4.6 μmol.h-1, this is because Pt with a
low Fermi level can suppress electron-hole pair recombination and significantly improve the
photocatalytic performance of CNUIO [45-46].
The H2 production rates of CNUIO-5 and some reported g-C3N4 compounds are listed
in Table S2. Clearly, CNUIO-5 exhibits higher activity compared with Uio-66/g-C3N4 [29]
and Nax-C3N4/Pt@Uio-66 [30] because Uio-66-NH2 has better visible light absorption than
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Uio-66. CNUIO-5 also shows better performance than g-C3N4-Co2P-0.1 mM K2HPO4 [47],
CN-AB-CuS [48], g-C3N4-Cu3P [49], H-g-C3N4 [50], HM-CN [51], g-C3N4-0.5%Ni-
1.0%NiS [52], g-C3N4-0.5%CB-1.0% [53] and M-CN [54]. This may be resulted from the Pt
cocatalyst and the intimate contact between Uio-66-NH2 and tubular g-C3N4 in our CNUIO-
5. In the above articles [47-49, 52, 53], transition metal sulfides and transition metal
phosphides were used as cocatalysts.
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Fig. 6. (a) Rates of H2 production over g-C3N4, Uio-66-NH2 and CNUIO-x composites, (b) Repeating
cycles of H2 production over CNUIO-5 under visible light (λ> 420 nm) irradiation.
The stability and reusability are very crucial for the practical application of
photocatalysts. As shown in Fig. 6b, no obvious reduction in H2 generation is observed after
five continuous cycles for a total of 25 h. There is no obvious difference between the fresh
and used CNUIO-5 analyzed by XRD measurement (Fig. 7). The ICP result indicates that
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the content of Zr was 4.8 wt% after five cycles, close to the fresh CNUIO-5 (5.0 wt%).
Besides, the similar morphology of fresh and used CNUIO-5 was observed in SEM images.
The SEM image (Fig. S1) shows that Uio-66-NH2 particles are still embedded on the surface
of g-C3N4 nanotubes after five continuous cycles. Thus, it reveals that our CNUIO-5
composite can be used as a stable photocatalyst for H2 generation under visible light.
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Fig. 7. XRD patterns of CNUIO-5 before and after five cycles of photocatalytic experiments.
To further investigate the photocatalytic performance of CNUIO-x samples, the
selective oxidation of amines to corresponding imines was carried out. As shown in Table 2,
g-C3N4 and Uio-66-NH2 both show relatively poor photocatalytic activity with the
benzylamine conversion rate of 17.4% and 5.6%, respectively. The photocatalytic activity
enhances gradually by increasing the Uio-66-NH2 content in CNUIO-x composites. With the
optimal amount of CNUIO-5, the conversion of benzylamine is 58.9% with 99% selectivity,
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which is about 3.4 and 10.5 times as high as that of g-C3N4 and Uio-66-NH2, respectively.
However, with further loading of Uio-66-NH2 in CNUIO composites, the conversion shows
a declined tendency. Such phenomenon has also been observed for H2 evolution as described
above. It may be due to the excess Uio-66-NH2 on the surface of g-C3N4, which would cover
the active sites, and thus decreases the photocatalytic conversion [29]. As a comparison,
CNUIO-5 without Pt loading shows a benzylamine transformation of 20.6%, this
demonstrates that Pt metal acts as an effective cocatalyst to improve the photocatalytic
performance. A blank experiment was conducted in the dark or without adding catalyst, and
no imine was detected, suggesting that the selective oxidation of benzylamine originates from
the photocatalytic activity of CNUIO-x composites driven by visible light.
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Seen from Table 1, SBET increases in the following order: g-C3N4 < CNUIO-9 < CNUIO-
7 < CNUIO-5 < CNUIO-3 < CNUIO-1 < Uio-66-NH2. And the photocatalytic activity for H2
generation and benzylamine conversion also increases in the following order: g-C3N4 <
CNUIO-9 < CNUIO-7 < CNUIO-5, indicating a positive correlation between the
photocatalytic activity and SBET. However, the photocatalytic activity drops significantly in
CNUIO-3 and CNUIO-1. Details are presented in Fig. S2. It may be due to the excess Uio-
66-NH2 on the surface of g-C3N4, which would cover the active sites, and the photocatalytic
conversion is thus decreased.
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Table 2. Photocatalytic selective oxidation of benzylamine. Reaction condition: 0.05 mmol benzylamine,
6
7
5 mg photocatalyst, 5 mL CH3CN, irradiation for 5 h.
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9
catalyst
N
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NH2
visible light, air, RT
Entry
Catalyst
Conv. (%)
Sel. (%)
1
2
g-C3N4
Uio-66-NH2
CNUIO-1
CNUIO-3
CNUIO-5
CNUIO-7
CNUIO-9
-
17.4
5.6
>99
>99
>99
>99
>99
>99
>99
-
3
44.0
46.3
58.9
36.2
31.7
0
4
5
6
7
8a
9b
CNUIO-5
0
-
a Without catalyst; b In the dark.
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Table 3. Photocatalytic selective oxidation of various amines over CNUIO-5. Reaction condition: 0.05
6
7
mmol amine, 5 mg CNUIO-5, 5 mL CH3CN, irradiation for 5 h.
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9
catalyst
visible light, air, RT
N
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NH2
R
R
R
Conv.
(%)
Sel.
(%)
Entry
Substrate
Product
NH2
N
1
2
3
4
5
6
58.9
34.3
27.2
31.4
71.9
63.9
>99
>99
>99
>99
98
NH2
NH2
NH2
N
N
N
N
F
F
F
Cl
Br
Cl
Cl
Br
Br
NH2
MeO
MeO
OMe
NH2
N
N
99
NH2
7
8
51.5
29.6
>99
>99
NH2
N
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The photocatalytic activity of CNUIO-5 was further explored for various benzylamine
derivatives under optimal reaction condition, see Table 3. Benzylamines with different
substituent groups (Table 3, entries 2-8) were converted into imines with high selectivity
(≥98%). The resulted imines were analyzed by NMR and HRMS measurements (Fig. S3-
S26). However, the conversion rate is dependent on the substituent group on benzene ring
caused by the electronic effect. Amines with electron-donating substituent group show higher
conversion rate than those with electron-withdrawing substituent group. Benzylamines with
methyl substituting at different positions show the conversion rate in the order of orth- <
meta- < para-substituted (Table 3, entries 6-8), resulting from the steric hindrance [55-56].
In order to detect the active species in the photocatalytic benzylamine oxidation, control
experiments were performed by adding different radical scavengers into the photoreaction
system. As shown in Fig. 8a, the conversion rate decreases rapidly from 58.9% to 10.0% after
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-
-


adding p-benzoquinone (capturing O2 ), indicating that O2 is the major active species. A
moderate decrease in the conversion rate is observed when K2S2O8 (electron scavenger) or
ammonium oxalate (hole scavenger) is used as quenching agent,
showing that
photogenerated holes and electrons are both involved in photocatalytic benzylamine
oxidation. EPR experiment further confirms the reactive oxygen species. As can be seen from
-
Fig. 8b, the characteristic peak of 5,5’-dimethyl-1-pyrroline N-oxide (DMPO)O2 appears
-
after the exposure of CNUIO-5 to visible light, indicating the formation of plenty O2 species
during the photocatalytic process [35]. This result is in agreement with the control
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experiments presented above. In addition, it was found that H2O2 was generated during the
reaction of benzylamine oxidation, and the amount of H2O2 could be detected by iodimetry
[57]. Our experiments indicate that the ability of CNUIO-5 to produce H2O2 is greater than
that of g-C3N4 and Uio-66-NH2 (Fig. 8c).
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Fig. 8. (a) Quenching experiments by introducing different radical scavengers in photocatalytic
benzylamine oxidation over CNUIO-5. (b) EPR spectra of CNUIO-5 in the dark and exposure to visible
light. (c) The amount of H2O2 generated by Uio-66-NH2, g-C3N4 and CNUIO-5.
3.3 Proposed mechanism for photocatalytic reaction
PL spectroscopy is an effective method to investigate the separation efficiency of
photoexcited electrons and holes [58-59]. PL spectra of our photocatalysts are shown in Fig.
9. Pure g-C3N4 presents a strong PL emission peak around 490 nm. Obviously, there is a
notable decrease in the PL intensity upon decorating with Uio-66-NH2, indicating that the
recombination of electrons and holes is efficiently hampered in CNUIO composites. The
rapid transfer of charges reflects the intimate contact between Uio-66-NH2 and g-C3N4, which
is consistent with TEM observation, and thereby promoting the separation of photoexcited
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electrons and holes. However, the PL peak shows an ascendant tendency for excess Uio-66-
NH2. This agrees with the photocatalytic activity of the corresponding CNUIO-x composites.
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Fig. 9. PL spectra of pure g-C3N4 and CNUIO-x composites with an excitation wavelength of 350 nm.
The optical property of CNUIO-x composites was investigated by UV-vis DRS
measurement and plotted in Fig. 10. g-C3N4 presents an intense absorption band at 450 nm
corresponding to π-π* transition of conjugated tri-s-triazine units [60]. Uio-66-NH2 exhibits
a sharp absorption edge at 470 nm, which results from the interaction between ATA linkers
and Zr-O clusters [61]. These spectra indicated that both g-C3N4 and Uio-66-NH2 are
responsive to visible light. According to the Tauc plot obtained from DRS (Fig. S27), the
band gap of Uio-66-NH2 and g-C3N4 is 2.48 and 2.75 eV, respectively. CNUIO-x composites
exhibit a blended absorption with a slight red shift compared with pure g-C3N4. The
semiconductive behavior and the flat band potentials (Efb) of Uio-66-NH2 and g-C3N4 were
studied by electrochemical measurement. Fig. S28 presents the Mott-Schottky curves of Uio-
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66-NH2 and g-C3N4. The positive slope suggests their n-type semiconductive behavior.
Generally, Efb in n-type semiconductor is near the conduction band (CB). Therefore, the edge
potential of CB in Uio-66-NH2 and g-C3N4 are estimated to be -0.79 and -1.14 eV vs Ag/AgCl
(namely -0.59 and -0.94 eV vs NHE), respectively. Furthermore, the valence band (VB) edge
potential of Uio-66-NH2 and g-C3N4 are calculated to be 1.89 and 1.81 eV vs NHE through
the formula: Eg = EVB - ECB. Such an energy-band structure in Uio-66-NH2 and g-C3N4 makes
CNUIO-x theoretically feasible for photocatalytic hydrogen production (0 V vs. NHE) and
oxidation of benzylamine (+0.76 V vs. NHE).
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Fig.10. UV-vis DRS of g-C3N4, Uio-66-NH2 and CNUIO-x composites.
It is known that the transfer and accumulation of photogenerated charge carriers play
important roles in the photocatalytic reaction [62]. In order to reveal the charge separation
and transfer between g-C3N4 and Uio-66-NH2, the transient photocurrent responses of g-C3N4,
Uio-66-NH2 and CNUIO-5 were measured. The I-t curves with four cycles of on/off
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irradiation (λ > 420 nm) are shown in Fig. 11a. The photocurrent density of CNUIO-5 is 1.6
μA·cm-2, which is significantly higher than that of g-C3N4 (1.0 μA·cm-2) and Uio-66-NH2
(0.4 μA·cm-2). The enhanced transient photocurrent density revealed the efficient charge
transfer between Uio-66-NH2 and g-C3N4. In addition, the electrochemical impedance
spectroscopy (EIS) of Nyquist diagram (Fig. 11b) clearly shows that the arc radii of CNUIO-
5 is smaller than that of g-C3N4 and Uio-66-NH2, which means higher efficiency in photo-
generated charge separation and electron transfer in CNUIO-5 [26].
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Fig. 11. (a) Transient photocurrent response and (b) EIS profile of g-C3N4, Uio-66-NH2 and CNUIO-5.
Based on the above discussion, we propose the mechanism for the photocatalytic
redox reaction over CNUIO composites, which is shown in Fig. 12. Under visible light
irradiation, electrons are excited from VB to CB of g-C3N4 and Uio-66-NH2. The
photoexcited electrons in the CB of g-C3N4 move to the CB of Uio-66-NH2, which convert
H+ to H2, and the photogenerated holes are consumed by TEOA (Fig. 12a). While in the
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photocatalytic benzylamine oxidation reaction (Fig. 12b), the photoexcited electrons activate
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8
-

molecular oxygen to generate O2 (-0.33 eV vs NHE). And the photoexcited holes are
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consumed by benzylamine to form benzylammonium cation radicals, which combine with
-
O2 to form Ph-CH=NH intermediate and H2O2. Finally, the Ph-CH=NH intermediate reacts
with benzylamine to yield the corresponding N-benzylidene benzylamine [63-66].
Fig. 12. (a) Schematic illustration of photocatalytic mechanism for H2 evolution and (b) oxidation of
amine over CNUIO composite under visible light irradiation.
In order to elucidate the mechanism of photocatalytic benzylamine oxidation reaction, the
controlled HRMS experiments were conducted to detect any intermediates produced during
benzylamine oxidation. To our delight, the Ph-CH=NH intermediate is detected by HRMS,
indicating that Ph-CH=NH is a key intermediate in benzylamine transformation process (Fig.
S29).
4. Conclusion
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In summary, CNUIO nanocomposites were successfully constructed by embedding Uio-
66-NH2 on g-C3N4 nanotubes via solvothermal strategy. These nanocomposites are efficient
photocatalysts for hydrogen generation and selective oxidation of amines (λ> 420 nm). The
loading ratio of Uio-66-NH2 in CNUIO showed a great influence on the photocatalytic
activity. CNUIO-5 composite (containing 16.7 wt% of Uio-66-NH2) exhibited the best
photocatalytic performance. The H2-evolution rate over CNUIO-5 was 152.2 μmol·h−1,
which is 1.7 and 39 times as high as that of g-C3N4 and Uio-66-NH2, respectively.
Furthermore, cyclic photocatalytic experiments proved the stability of CNUIO-5 for H2
evolution. The conversion rate of benzylamine over CNUIO-5 composite was 58.9%, which
is 3.4 and 10.5 times as high as that of g-C3N4 and Uio-66-NH2, respectively. EPR and
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-
quenching experiment showed that O2 was main active species in oxidation of amines. PL
spectra, transient photocurrent responses and EIS Nyquist plots indicated more effective
separation of electron-hole pairs in CNUIO-x composites compared with g-C3N4 and Uio-
66-NH2. Our work displays the potential application of g-C3N4/MOFs as photocatalyst for
hydrogen production and organic synthesis.
Conflicts of interest
There are no conflicts to declare.
Acknowledgement
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Funding was provided by the National Natural Science Foundation of China under Grant No.
21574117.
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