Nickel-decorated g-C3N4hollow spheres as an efficient photocatalyst for hydrogen evolution and oxidation of amines to imines

Article abstract of DOI:10.1039/d0nj01218a

Photocatalysts composed of earth-abundant elements are highly desired for photocatalytic hydrogen evolution as well as oxidation of amines to imines without the requirement of precious metals. Herein, non-precious Ni metal decorated g-C3N4 hollow spheres

Full text of DOI:10.1039/d0nj01218a

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Nickel-decorated g-C N hollow sphere as an efficient photocatalyst for
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hydrogen evolution and oxidation of amines to imines
Shishen Zhang, Xiaobing Qian, Junqiu Yan, Kelong Chen, Jianhua Huang*
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, China
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Email: jhhuang@zstu.edu.cn
Abstract
Photocatalysts composed of earth-abundant elements are highly desired for
photocatalytic hydrogen evolution as well as oxidation of amines to imines without
requirement of precious metal. Herein, non-precious metal Ni decorated g-C N
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hollow spheres (Ni-CNHS) with different Ni contents were prepared by a simple
calcination strategy followed with photoreduction loading of metal Ni. Under visible
light illumination, Ni-CNHS exhibited efficient photocatalytic performance for
oxidation of amines to imines and hydrogen production from water splitting using
triethanolamine as sacrificial agent. The optimal amount of Ni was 0.41 wt% with a
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H -production rate of 596 μmol·g ·h and the conversion rate of benzylamine of
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61.3%. The probable photocatalytic mechanism of Ni-CNHS was investigated. This
work provides the possibility to construct a great performance non-noble metal
photocatalysts for hydrogen production and organic synthesis.
Keywords: g-C N ; Ni; Photocatalyst; Hydrogen; Oxidation of amines
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1. Introduction
As a novel metal-free semiconductor photocatalyst, graphite carbon nitride
g-C N ) has regarded as a promising material for photocatalytic reaction due to the
(
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excellent optical and electronic properties and suitable bandgap [1-8]. However, the
rapid recombination of photoexcited electron-hole pairs and the low specific surface
area in bulk g-C N result in a low photocatalytic performance. Numerous efforts
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such as element doping [9], heterojunction construction [10], cocatalyst loading
11,12], morphological tuning [13] and functional group grafting [14-15] have been
developed to enhance the photocatalytic performance of g-C N
[
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4.
Cocatalysts are often used to accelerate the separation efficiency of photogenerated
electron-hole pairs and thus enhance the photocatalytic performance of semiconductor.
Among various cocatalysts, noble metals, such as Pt, Au and Pd, frequently exhibit
outstanding performance. However, high price seriously limits their large-scale industrial
application. Among all noble-metal-free cocatalysts, nickel-based cocatalysts have
attracted great attention due to their high activity, stability and low cost [16]. In the past
years, metal Ni [17-22] and several Ni compounds have been proven to be effective
cocatalysts in g-C N -based photocatalytic system [23,24], such as Ni P [25-27],
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Ni(OH) [28], Ni N [29,30], NiB [31], NiO [32], NiS [33,34], NiS [35], and Ni C
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[36]. These important studies revealed significant advantages of Ni-based materials as
cocatalysts of g-C N for efficient hydrogen production.
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Morphology control has been proved to be one of effective methods to overcome
the shortcomings of bulk g-C N through increasing the specific surface area.
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Recently, hollow g-C N nanospheres have attracted more attention due to its unique
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structural features [37,38]. For example, hollow g-C N nanospheres have been
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successfully prepared using SiO particles as templates [13,39,40] or by template-free
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molecular cooperative assembly [41]. Compared with bulk g-C N , hollow g-C N
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nanospheres exhibited enhanced photocatalytic activity for H production, dye
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degradation, and CO reduction under visible light irradiation, because the hollow
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spherical structure usually possessed larger interior and exterior surface area.
Therefore, it is great significant to develop a convenient method to synthesize hollow
g-C N nanospheres.
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Imines and their derivatives are important industrial intermediates, which are
commonly used in the synthesis of fine chemicals, dyes and pharmaceuticals [42,43].
However, traditional synthesis process involves unstable carbonyl compounds and
environmentally unfriendly catalysts [44]. Direct photocatalytic oxidation of amines
to imines can provide an economical and green way. For example, Fe(bpy) /g-C N
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showed high selectivity for photocatalytic oxidation of amines to imines under mild
reaction condition [45]. Xu et al. found highly selective coupling of benzylamines in
air in the presence of core-shell structured CdS@C N [46]. Zhang et al. reported that
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decoration of cyano-groups in g-C N nanosheets could accelerate the generation of
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charge carriers and enhance the photocatalytic performance for oxidation of amines to
imines [47]. Porous layer-structured g-C N also exhibited high activity for the
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oxidative coupling of amines to imines [48]. Although some examples of Ni/g-C N
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nanocomposites for photocatalytic H production have been reported [17-22],
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however, to our knowledge, there is rare report on Ni/g-C N composite with dual
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applications in both hydrogen generation and selective oxidation of amine.
In this work, metal Ni decorated g-C N hollow spheres (Ni-CNHS) were
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synthesized via a simple calcination and subsequent photoreduction process, which
exhibited superior photocatalytic performance and stability towards hydrogen
generation and selective oxidation of amine under visible light illumination. The
loading amount of Ni was optimized. The active species in benzylamine oxidation
process and possible photocatalytic mechanism have been investigated. This work
highlights the potential of non-noble metal co-photocatalyst for organic synthesis and
hydrogen evolution.
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. Experimental section
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.1 Materials
All reagents, including melamine, cyanuric acid, dimethyl sulfoxide (DMSO)
and nickel chloride hexahydrate (NiCl .6H O) were analytical grade and used as
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received without any purification.
.2 Preparation of g-C N hollow sphere
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g-C N hollow spheres (CNHS) were prepared according to literature [41].
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Typically, 0.50 g melamine and 0.51 g cyanuric acid were separately dissolved in 10
mL DMSO. Then, they were mixed, and white precipitates were produced. The white
precipitates were collected by filtration and washing with ethanol, which was dried
and calcined at 550 ℃ for 4 h under nitrogen atmosphere. After naturally cooled down
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to room temperature, yellow powder was obtained.
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.3 Preparation of Ni-g-C N hollow sphere
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0 mg CNHS and a certain amount of NiCl .6H O were put into a Pyrex reaction
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cell (250 mL) containing 100 mL TEOA aqueous solution (10 vol%), which was then
sealed and evacuated to remove the internal air. After irradiation for 1 h by using a
300 W PLS-XE300 Xe lamp, xNi-CNHS catalysts were obtained. The synthesis
process of xNi-CNHS is shown in Fig. 1. The content of Ni was adjusted by
controlling the amount of added NiCl .6H O. The actual mass ratio of Ni in
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xNi-CNHS samples was determined by atomic absorption spectrum (Varian, AA-110),
which are 0.17, 0.30, 0.41, 0.56 and 0.68 wt%, respectively. These samples were then
denoted as 0.17Ni-CNHS, 0.3Ni-CNHS, 0.41Ni-CNHS, 0.56Ni-CNHS and
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.68Ni-CNHS.
Fig.1. Schematic diagram of the synthesis of xNi-CNHS photocatalysts.
2.4 Characterization
The crystallization of samples was analyzed on an X-ray diffractometer
(
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
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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 measured with a SHIMADZU spectrophotometer
(UV-2450). Porous parameters were obtained by nitrogen adsorption-desorption test
at -196 C on an ASAP2020 Micromeritics analyzer. Photoluminescence spectra (PL)
and time-resolved decay curves were recorded on a HORIBA Fluomax-4 fluorescence
spectrometer. The electrochemical tests were characterized on an electrochemical
workstation (CHI660E, China).
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2.5 Photocatalytic activity
The photocatalytic performance of xNi-CNHS was evaluated by hydrogen
production and selective oxidation of amine under visible light irradiation provided by
a 300 W Xenon lamp ( > 420 nm).
The photocatalytic hydrogen production was tested on a Perfect Labsolar-H2
water splitting system according to our previous work [49]. Typically, 50 mg
photocatalyst was added into a Pyrex reaction cell containing 100 mL of TEOA
aqueous solution (10 vol%), which was then sealed and evacuated. Afterwards, the
suspension was irradiated under constant stirring. The generated H was periodically
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measured on line using a GC9800 gas chromatography (Kechuang, China).
The photocatalytic oxidation of amines was performed at room temperature.
Typically, 5 mg photocatalyst and 0.05 mmol amine were dispersed into 5 mL
CH CN in a 10 mL quartz tube, which was then illuminated under continuous stirring
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supernatant was examined by GC (Shimadzu GC-2014), NMR (Zhongke Niujin
WNMR-I 400) and HRMS (Waters Xevo G2-S) measurements.
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. Results and discussion
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3.1 Structural and morphology characterizations
The phase structure of as-prepared xNi-CNHS was firstly analyzed by XRD
measurements. CNHS and xNi-CNHS samples exhibit a strong diffraction peak
located at 27.5° and a small peak at 13.2° corresponding to (002) and (100) plane of
g-C N (Fig. 2), originating from the interlayer stacking of aromatic structure and an
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in-plane structural packing motifs of the continuous heptazine network, respectively
20]. However, no peaks of metal Ni were detected in all xNi-CNHS samples. This
[
may be due to the low loading of Ni on CNHS.
Fig. 2. XRD patterns of CNHS and xNi-CNHS composites with different Ni contents.
The morphology and microstructure of CNHS and 0.41Ni-CNHS were examined
by SEM and TEM observation. Fig. 3a and 3b show typical SEM images of CNHS,
which reveal its uniform hollow structure with a diameter of 1-3 μm. The
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.41Ni-CNHS possesses similar morphology and size (see the TEM image in Fig. 3c).
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From the HRTEM image shown in Fig. 3d, we observe several lattice fringes on the
surface of 0.41Ni-CNHS with a lattice spacing of 0.205 nm, which is consistent with
the (111) plane of metal Ni [19].
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Fig. 3. (a, b) SEM images of CNHS, (c) TEM image and (d) HRTEM image of 0.41Ni-CNHS.
The composition of 0.41Ni-CNHS was further analyzed by XPS measurement. The
XPS survey spectrum shows that the 0.41Ni-CNHS mainly contains C, N, O, and Ni
elements (Fig. 4a). The peak of O 1s comes from the surface-adsorbed H O and
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CO on 0.41Ni-CNHS [49]. In the C 1s spectrum (Fig. 4b), two peaks at 284.5 eV
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and 287.9 eV can be assigned to the sp -bonded C=C species and the N-C=N bond in
g-C N structure, respectively [17, 18]. Fig. 4c presents the N 1s spectrum, where the
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peaks at 398.5, 399.7, 401.0, and 404.1 eV can be attributed to the N species in
C=N-C, N-(C) and C-N-H groups and positive charge localization in heptazine rings,
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peak at 852.3 eV (Ni 2p ), together with that at 870.1 eV (Ni 2p ), correspond
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Fig. 4. Survey XPS spectrum (a), regional spectrum of C 1s (b), N 1s (c) and Ni 2p (d) of
.41Ni-CNHS.
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The specific surface area (SBET) and pore structure of CNHS and xNi-CNHS were
determined by performing N adsorption-desorption measurement. All samples
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exhibit type IV isotherm with a H3-type hysteresis loop, indicating the existence of
mesopores and macropores [28]. The N adsorption-desorption isotherms of CNHS
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and 0.41Ni-CNHS are shown in Fig. 5. They both have a wide distribution in the pore
size with the range of 2 to 100 nm (Inset in Fig. 5). The corresponding SBET and pore
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volume of CNHS and xNi-CNHS samples are listed in Table 1. Clearly ，the SBET
value gradually decreases with the loading of Ni.
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Fig. 5. The N adsorption–desorption isotherm and pore size distribution (inset) of CNHS and
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Table 1. SBET and pore volume of the as-prepared samples
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Sample
CNHS
SBET (m .g )
107.0
101.8
89.6
Pore volume (cm .g )
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0.17Ni-CNHS
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.41Ni-CNHS
0.56Ni-CNHS
.68Ni-CNHS
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The UV-vis DRS of CNHS and xNi-CNHS are presented in Fig. 6a. The
absorption edge of CNHS locates at about 435 nm. After loading metal Ni,
xNi-CNHSs display a greatly enhanced light-harvesting ability across the whole
optical spectrum. After careful fitting the Tauc plots of 0.41Ni-CNHS and CNHS,
we find they roughly have the same bandgap energy of 2.82 eV (Fig. 6b), which
shows that the loading of Ni does not change the energy band structure of CNHS.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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0
1
2
3
4
5
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8
9
0
1
2
3
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9
0
1
2
3
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8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 6. (a) UV-vis DRS of CNHS and xNi-CNHSs; (b) Tauc plots of CNHS and 0.41Ni-CNHS.
3.2 Photocatalytic activity
The photocatalytic activity of xNi-CNHS samples was first evaluated by H2
production from water splitting using TEOA as sacrificial agent. The rates of H2
generation over different samples are presented in Fig. 7a. Pure CNHS produces trace
amount of H . After Ni loading, H is smoothly generated. The H -production rate of
2
2
2
-1
-1
0.17Ni-CNHS is 216 μmol·g ·h . With an increasing amount of Ni loading, the
H -production rate markedly rises. And 0.41Ni-CNHS exhibits the highest rate of 596
2
-1
-1
μmol·g ·h . However, it decreases when excess Ni is loaded. The effect of
metal-loading amount on the H -evolution rate has been investigated by several
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groups. For example, Chen et al. considered that the existence of an optimal Pt
loading amount for H evolution originated from the contradiction between the
2
increased electrons trapped by Pt and the reduced active sites for methanol
adsorption with increasing the Pt-loading amount [53]. Some authors thought
that with successive loading, metal particles may act as recombination centers of
charge carriers, and less semiconductor surface may be available for
photoabsorption [54-57]. Therefore, the reaction rate is decreased. Moreover,
some groups indicated that the effects of the intrinsic electronic properties, the
particle size and dispersion of metal cocatalysts should be also taken into account
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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7
8
9
0
1
2
3
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5
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8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
[
56-60]. In the present work, excessive loaded Ni may act as shielding for
photoabsorption of semiconductor surface and recombination centers of
photogenerated electrons and holes [54-57], which is supported by the transient
photocurrent response and photoluminescence intensity of xNi-CNHS
composites presented in section 3.3. The photocatalytic activity is thus decreased.
However, no clear explanation is yet known, and further study is needed.
The H2 production rates of 0.41Ni-CNHS and some reported Ni
compounds/g-C N photocatalysts are listed in Table S1. 0.41Ni-CNHS exhibits
3
4
higher activity compared with Ni/g-C N [20-22], Ni P/g-C N [25-27],
3
4
2
3
4
Ni(OH) /g-C N [28], Ni N/g-C N [29,30], NiB/g-C N [31], NiO/g-C N [32],
2
3
4
3
3
4
3
4
3
4
NiS /g-C N [33], Ni C/g-C N [36] and 0.5%Ni-1.0%NiS/g-C N [61]. This may be
2
3
4
3
3
4
3
4
resulted from the hollow spherical structure and its large SBET. However, our
.41Ni-CNHS exhibits poorer performance than Ni-C N composites, this may be due
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to the chemically bonded nickel cocatalyst through Ni-S-C-N chemical bonds [17],
doping of S and Ni [18] and high nickel loading [19] in g-C N lattice.
3
4
To evaluate the stability of 0.41Ni-CNHS, cycling photocatalytic reaction was
performed under visible light irradiation for a total of 25 hours. The reaction system
1
1
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1
1
1
1
1
1
1
2
2
2
2
2
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2
2
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3
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0
1
2
3
4
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9
0
1
2
3
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9
0
1
2
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9
0
1
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3
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
was evacuated after every 5 h. The results are presented in Fig. 7b. Clearly, H is
2
continuously produced without obvious decrease after five cycles, reflecting that
0
.41Ni-CNHS is a promising stable photocatalyst towards water splitting.
Fig. 7. (a) Rates of H -production over xNi-CNHS composites, (b) Stability of 0.41Ni-CNHS for
2
H -production.
2
The photocatalytic performance of xNi-CNHS samples was further investigated
by the selective oxidation of amines to imines using benzylamine as the model
substrate. As shown in Table 2, the pure CHNS shows relatively low conversion rate
of 25.1%. It is gradually enhanced with Ni loading in CNHS. The optimal loading
amount of Ni (0.41Ni-CNHS) is 0.41 wt% and the corresponding optimal conversion
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rate is 61.3% with 99% selectivity, which is about 2.4 times as high as those of
CNHS. However, the conversion rate shows a declining tendency with further
increasing of Ni loading in xNi-CNHS composites. Such phenomenon is also found
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
for H evolution as described above. No imine was obtained without adding
2
photocatalyst or in the dark, demonstrating that the selective oxidation of benzylamine
is induced by xNi-CNHS composites under visible light.
Table 2. Photocatalytic selective oxidation of benzylamine. Reaction condition: 0.05 mmol
benzylamine, 5 mg photocatalyst, 5 mL CH CN, irradiation for 5 h.
3
catalyst
N
NH2
visible light, air, RT
Entry
Catalyst
CNHS
Conv. (%)
25.1
51.3
56.1
61.3
58.8
56.7
0
Sel. (%)
>99
>99
>99
99
1
2
0.17Ni-CNHS
0.3Ni-CNHS
0.41Ni-CNHS
0.56Ni-CNHS
0.68Ni-CNHS
-
3
4
5
>99
99
6
7a
8b
-
0.41Ni-CNHS
0
-
a
b
Without catalyst; In the dark.
Table 3. Photocatalytic selective oxidation of various amines over 0.41Ni-CNHS. Reaction
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condition: 0.05mmol amine, 5 mg 0.41Ni-CNHS, 5 mL CH CN, irradiation for 5 h.
3
catalyst
visible light, air, RT
N
NH2
R
R
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
R
Conv.
%)
Sel.
(%)
Entry
Substrate
Product
(
NH2
N
N
1
2
3
4
5
6
61.3
30.4
31.1
35.7
56.3
47.5
99
>99
>99
99
NH2
F
F
F
NH2
NH2
N
Cl
Br
Cl
Cl
N
Br
Br
NH2
N
>98
99
MeO
MeO
OMe
NH2
N
N
NH2
7
8
40.2
28.6
>99
>99
NH2
N
The photocatalytic activity of 0.41Ni-CNHS was further explored for various
benzylamine derivatives under the same reaction condition (Table 3). All kinds of
benzylamines can be efficiently converted into their corresponding imines (selectivity
≥
98%), which were determined by NMR and HRMS measurements (Fig. S1-S24).
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The electronic effect of the reaction was checked. Obviously, the amines bearing
electron-donating groups on the benzene ring reacted better than those with
electron-withdrawing groups. Moreover, the reaction with sterically encumbered
benzylamine with orth-methyl group afforded the desired product in less conversion
rate [62-63].
0
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9
0
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0
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7
8
9
0
Fig. 8. (a) Control experiments by introducing different scavengers in photocatalytic benzylamine
oxidation over 0.41Ni-CNHS. (b) The amount of H O generated by 0.41Ni-CNHS and CNHS.
2
2
In order to study the mechanism of photocatalytic benzylamine oxidation,
control experiments were carried out to determine the active species in benzylamine
oxidation process. Fig. 8a shows that the conversion rate of benzylamine drops
-
quickly from 61.3% to 12.1% after adding p-benzoquinone (capturing O ) into the
2
-
photoreaction system, indicating that O is the main active species. However, when
2
K S O (electron scavenger) or ammonium oxalate (hole scavenger) is added, the
2
2
8
conversion rate exhibits moderate reduction. It can be inferred that both
photogenerated holes and electrons are also involved in benzylamine oxidation. In
addition, H O was detected based on iodimetry [64]. As shown in Fig. 8b,
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0.41Ni-CNHS produces more H O than CNHS does.
2
2
3.3 Possible mechanism for photocatalytic reaction
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9
0
1
2
3
4
5
6
7
8
9
0
Based on the above characterization and photocatalytic activity, the mechanism
for the photocatalytic redox reaction over Ni-CNHS composite is proposed (Fig. 9).
Under visible light irradiation, CNHS generates holes in the valence band (VB) and
electrons in the conduction band (CB). The excited electrons in CB will transfer to
+
metal Ni and thereby reducing H to H (Fig. 9a). And the VB holes are consumed by
2
TEOA. While in benzylamine oxidation (Fig. 9b), the photoexcited electrons reduce
-
O2 to O . Meanwhile, the photoexcited holes oxidize benzylamine to
2
-
benzylammonium cation radicals, which react with O to generate Ph-CH=NH
2
intermediate and H O . Finally, Ph-CH=NH intermediate combines with benzylamine
2
2
to yield the corresponding N-benzylidenebenzylamine [65-69].
Fig. 9. Schematic illustration of photocatalytic mechanism for (a) H evolution and (b) oxidation
2
of amine over Ni-CNHS composite under visible light irradiation.
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In order to support the mechanism of photocatalytic benzylamine oxidation
reaction, we conducted HRMS experiment to characterize intermediate produced by
benzylamine after irradiation 3 h. Besides the start material (benzylamine) and
product (N-benzylidenebenzylamine), Ph-CH=NH intermediate was detected (Fig.
0
1
2
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9
0
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9
0
1
2
3
4
5
6
7
8
9
0
1
0), indicating that Ph-CH=NH is a key intermediate during photocatalytic
benzylamine oxidation.
Fig. 10. HRMS spectrum of benzylamine (a), phenylmethanimine (b) and
N-benzylidenebenzylamine (c).
Photoluminescence (PL) spectrum is usually adopted to study the recombination
of photogenerated electron-hole pairs. Therefore, CNHS and xNi-CNHS composites
were further characterized by performing PL measurements under 350 nm excitation
wavelength. Seen from Fig. 11a, they both possess a broad luminescence band in the
region of 400-600 nm, corresponding to the recombination of electron-hole pairs. The
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maximum quenching is observed for 0.41Ni-CNHS, implying the maximum
reduction in the recombination rate of electron-hole pairs. The PL intensity
increases in the following order: 0.41Ni-CNHS < 0.56Ni-CNHS < 0.3Ni-CNHS <
0.68Ni-CNHS < 0.17Ni-CNHS < CNHS, which is in good agreement with the
photocatalytic activities of these samples. We speculate the intimate contact
between Ni and CNHS is favorable for the rapid transfer of charges, thereby
promoting the separation of photoexcited electrons and holes [23].
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
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3
3
3
3
3
3
3
4
4
4
4
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4
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4
4
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5
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5
5
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5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 11. (a) Steady state PL spectra of CNHS and xNi-CNHS photocatalysts and (b) time-resolved
transient PL decay of CNHS and 0.41Ni-CNHS.
Time-resolved transient fluorescence lifetimes of CNHS and 0.41Ni-CNHS were
measured to further reveal the transfer efficiency of photoexcited electrons and holes.
As shown in Fig. 11b, the average lifetime  is obtained according to reference
av
2
2
[36]:  = (A  +A  )/(A  +A  ), where  and  are the emission lifetimes,
av
1 1
2 2
1 1
2 2
1
2
and A and A are the corresponding amplitudes, respectively. The average PL
1
2
lifetime of CNHS and 0.41Ni-CNHS is 4.56 and 3.38 ns, respectively. The fast
component ( ) can be attributed to the radiative recombination process of CB
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electrons and VB holes. The slow component ( ) is believed to be the radiative
2
recombination of long-lived electron-hole pairs in CNHS [70]. The shorter PL
lifetime reveals the faster electron transfer from CNHS to Ni, thus enhancing the
photocatalytic performance of CNHS.
0
1
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1
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8
9
0
To gain deeper insight into the charge transfer between CNHS and Ni, the
transient photocurrent responses of CNHS and xNi-CNHS composites were
measured. The I-t curves with four cycles of on/off irradiation (λ > 420 nm) are
shown in Fig. 12a. The photocurrent density of 0.41Ni-CNHS is 1.2 μA·cm-2,
-2
significantly higher than that of CNHS (0.2 μA·cm ) and other xNi-CNHS. The
enhanced transient photocurrent density could be attributed to the fact that the loaded
Ni could effectively accelerate charge transfer through Ni-CNHS interface [19]. In
addition, the electrochemical impedance spectroscopy (EIS) of Nyquist diagram (Fig.
1
2b) clearly shows that the arc radii of 0.41Ni-CNHS is smaller than that of CNHS,
which means higher efficiency in photogenerated charge separation and electron
transfer in 0.41Ni-CNHS [49]. These results are consistent with the above PL
measurement.
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Fig. 12. (a) Transient photocurrent response and (b) EIS profile of CNHS and 0.41Ni-CNHS.
Fig. S25 presents the plots of the normalized photocatalytic activity, SBET
,
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
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0
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0
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0
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0
1
2
3
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9
0
optical absorption, photocurrent density, and PL intensity of xNi-CNHS/CNHS
versus Ni amount. With an increasing amount of Ni loading, the H -production
2
rate and conversion rate of benzylamine both first rise and then decline, and
0
.41Ni-CNHS exhibits the best activity. They are in good agreement with the
variation of photocurrent density, optical absorption in visible light region and
quenched PL intensity. However, a continuous decrease in the SBET value with
increasing Ni amount is observed. Therefore, it is reasonable to conclude that the
visible light absorption and the separation of photogenerated electrons and holes
play key roles in the photocatalytic performance of the xNi-CNHS composites.
4. Conclusions
In this paper, xNi-CNHS photocatalysts were prepared by calcination of
melamine-cyanuric acid supramolecular aggregates followed by loading Ni using
photodeposition method. The diameter of CNHS was 1-3 μm observed from SEM and
TEM images. The loading of metal Ni on CNHS was confirmed by HRTEM and XPS
measurements, and the loaded amount of Ni was determined by AAS. The optimal
0
.41Ni-CNHS composite exhibited the best and stable photocatalytic performance
-1
-1
with the H -evolution rate of 596 μmol·g ·h . The conversion rate of benzylamine
2
over 0.41Ni-CNHS was 61.3%, which was about 2.4 times as high as those of CNHS.
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DOI: 10.1039/D0NJ01218A
Based on experiments for active species capture in benzylamine oxidation, and PL,
transient photocurrent and EIS measurements, we proposed the photocatalytic
mechanism of benzylamine oxidation and H generation in the presence of Ni-CNHS.
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This work provides a novel insight for designing non-noble-metal photocatalyst for
H -evolution and organic synthesis.
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Conflicts of interest
There are no conflicts to declare.
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Nickel-decorated g-C N hollow sphere as an efficient photocatalyst for hydrogen
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evolution and oxidation of amines to imines