Ultrathin HNb3O8 nanosheets with oxygen vacancies for enhanced photocatalytic oxidation of amines under visible light irradiation

Article abstract of DOI:10.1039/c8ta12118d

Ultrathin HNb₃O₈ nanosheets (HNb₃O₈ NSs) with oxygen vacancies were successfully synthesized by a simple bottom-up hydrothermal process. The obtained nanosheets were uniform with a thickness of about 3.02 nm and

Full text of DOI:10.1039/c8ta12118d

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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DOI: 10.1039/C8TA12118D
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ARTICLE
Ultrathin HNb3O8 nanosheets with oxygen vacancies for enhanced
photocatalytic oxidation of amine under visible light irradiation
Junli Chen,a* Hai Wang,a Zhiqiang Zhang,a Lifeng Han,a Yonghui Zhang,a Feilong Gong,a Kefeng Xie,c
Received 00th January 20xx,
Accepted 00th January 20xx
Liancai Xu,a* Wei Song,b* Shide Wua
DOI: 10.1039/x0xx00000x
Ultrathin HNb3O8 nanosheets (HNb3O8 NS) with oxygen vacancies were successfully synthesized by a simple bottom-up
hydrothermal process. The obtained nanosheets were uniform with a thickness of about 3.02 nm and the surface oxygen
vacancies on the HNb3O8 nanosheets were confirmed by electron spin resonance spectroscopy (ESR) and X-ray
photoelectron spectroscopy (XPS). As compared to urchin-like Nb2O5 nanosphere and Nb2O5 nanorods, ultrathin HNb3O8
nanosheets with oxygen vacancies showed the highest activities in the photocatalytic oxidative coupling reaction of
benzylamine under visible light irradiation. The improved photocatalytic activity was attributed to the following reasons: (1)
unique nanosheet structure with several molecular thicknesses that led to the effective separation of the photogenerated
carriers; (2) benzylamine-HNb3O8 surface complex formed on the oxygen vacancies that reduced the bandgap of HNb3O8 NS
and was profitable to harvest visible light; (3) HNb3O8 NS-V could efficiently chemisorb O2 molecules and promote to
activate O2 to reactive oxygen species.
www.rsc.org/
activity.12,28 In order to increase the interaction between
sensitized organic molecules and semiconductor oxides, the
surficial properties of semiconductor oxides could be
1. Introduction
Photocatalytic oxidation of amines to imines has attracted great
attention in recent years due to the versatile application of
imines in fine chemicals and pharmaceuticals.1-3 Many kinds of
photocatalysts, such as organometallic compounds,4 plasmonic
precious metal (Au,5 Ag,6 Pt,7), semiconductor oxides ( i.e.
maneuvered by synthesis and post treatment to increase
surface areas, control morphologies, form surface defects.29-31
Two dimensional (2D) nanosheet materials have attracted
extensive concerns owing to their special electronic structures
and novel physicochemical properties, which show exciting
applications such as photocatalysts, energy storage materials,
sensing platforms.32,33 Among them, niobate and niobium oxide
nanosheets showed excellent enhanced photocatalytic
activities. Two- and three-layer restacked Dion–Jacobson phase
niobate nanosheets prepared by exfoliation of layered
perovskites exhibited enhanced photocatalytic activities of
hydrogen evolution under ultraviolet irradiation (λ > 300 nm).34
Exfoliated ultrathin HNb3O8 nanosheets displayed about 4 times
higher hydrogen evolution activity than that of layered HNb3O8
under ultraviolet irradiation.35 Nakagawa reported that single
molecular sheets of niobate assembled with graphene and
MoS2 showed higher photocatalytic production of hydrogen
from water.36 Liang reported that monolayer HNb3O8 showed
photocatalytic activity of selective oxidation of benzylic alcohols
under visible light response irradiation.37 To the best of our
knowledge, there is no report about ultrathin HNb3O8
nanosheets for the photocatalytic oxidative coupling of primary
amines to the corresponding imines.
TiO2,8-11
Nb2O5,12-14
WO3,15,16
BiVO4,17
TiNb2O7,18
(ZnII/TiIV)LDH,19 BiOBr,20 and BiOCl21), sulfide (WS2,22 CdS23),
C3N4,24 MOF,25 and carbon materials26,27 have shown high
conversion of amine and excellent selectivity for imine.
However, the efficiency and stability of the photocatalyst for
this reaction need to be improved. Specifically, the oxidation of
amine on Nb2O5 has been extensively investigated owing to its
advantageous properties, such as high activity, nontoxicity, and
good chemical stability under light irradiation. Although Nb2O5
cannot adsorb visible light as a wide band gap semiconductor
(about 3.2 eV), it exhibited high activity under visible light
irradiation. The surface complex in situ formed by amine
adsorbed on the surface of Nb2O5, which was the ligand-to-
metal charge transfer (LMCT) mechanism, would conduce to
harvesting visible light and enhancing the photocatalytic
a.College of Materials and Chemical Engineering, Collaborative Innovation Center
of Environmental Pollution Control and Ecological Restoration, Zhengzhou
University of Light Industry, Zhengzhou 450002, P. R. China
*E-mail: chenjunli@zzuli.edu.cn; xulc@zzuli.edu.cn
In this study, we have successfully prepared HNb3O8
nanosheet via a facile hydrothermal method. Compared with
the mostly reported method by exfoliation, this method is
simpler, more maneuverable and easier to obtain a uniform
morphology. The prepared ultrathin HNb3O8 nanosheet was
characterized and applied for the selective catalytic oxidation of
b.School of Chemical Engineering and Light Industry, Guangdong University of
Technology, Guangzhou 510006, China *E-mail: songw@gdut.edu.cn
c. School of Chemical and Biological Engineering, Lanzhou Jiaotong University,
Lanzhou, Gansu 730070, China
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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benzylamine. It was proved that HNb3O8 nanosheet exhibited 250Xi) equipped with a monochromatic Al Kα source (λ = 1486.7
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the enhanced performance in the photocatalytic oxidative eV). Photoluminescence (PL) emissionDOspI:e1c0t.1r0a39o/Cf8TpAo1w21d18eDr
coupling of benzylamine.
samples were acquired using an Edinburgh Instruments
PLSP980 spectrometer under excitation at 300 ~ 330 nm.
Photocurrent measurements were carried out at room
temperature with the CHI 660E electrochemical workstation
(Shanghai Chenhua Instrument Co. Ltd., China) using a home-
made three electrode cell with a quartz optical window
containing 0.5 M Na2SO4 aqueous solution. The working
electrodes were prepared by pasting the slurry of sample
powder and ethanol onto a 1 cm2 (1cm × 1cm) indium−tin oxide
(ITO) glass. Pt electrode and Ag/AgCl electrode were chosen as
the counter and reference electrodes, respectively. A 300 W Xe
lamp (Beijing Prefect Light, Microsolar 300) with a 420 nm cutoff
filter was used as a visible light source to irradiate horizontally
the working electrode.
2. Experimental
2.1. Reagents and materials
NbCl5, ethanol, methanol, tetramethyl ammonium hydroxide,
Na2SO4 and acetone were in analytical grade and used without
purification.
2.2 Preparation of ultrathin HNb3O8 nanosheets
The HNb3O8 nanosheets (denoted as HNb3O8 NS) were
synthesized by a bottom-up hydrothermal method. Typically, 6
mmol of NbCl5 was dissolved into 20 mL of absolute ethanol.
Then, 20 mL of tetramethylammonium hydroxide (TMAOH)
aqueous solution (15 wt%) was added dropwise into the NbCl5
ethanol solution under magnetic stirring by metering pump
with a rate of 0.6 mL/min. After TMAOH aqueous solution was
added completely, the mixed solution turned into clear and
transparent. The mixture was aged for 2 h at room temperature,
and sealed in a 100 mL Teflon-lined stainless-steel autoclave.
Hydrothermal reaction was carried out at 240 °C for 12 h. The
resulted powder was separated, washed in sequence with
acetone, absolute ethanol and deionized water, and dried at
60 °C under vacuum for 8 h (denoted as obtained NS). The
obtained nanosheets (0.2 g) were dispersed into 400 ml 6 M
HNO3 and sonicated for 10 min. Then, the suspension solution
was stirred for 12 h to ion exchange TMA cations to H+. The
achieved powder was centrifuged and washed by ethanol and
deionized water, and dried at 60 °C under vacuum for 8 h
(designated as HNb3O8 NS). For comparison, urchin-like Nb2O5
(denoted as Ur Nb2O5) and Nb2O5 nanorods (denoted as Nb2O5
NR) were prepared according to our previously reported
procedures.38,39
2.4 Photocatalytic selective oxidation of amine
The photocatalytic oxidative coupling of amines under visible
light was carried out in a home-made 50 mL cylindrical glass
container with 300 W Xe lamp (Beijing Perfect, Microsolar 300)
with 420 nm cutoff filter (Fig. S1). Typically, 20 mg catalyst was
dispersed into the mixture which contains 0.25 mmol substrate
and 10 mL acetonitrile. Prior to the irradiation, the mixture was
continuously stirred in the dark for 0.5 h to ensure the
adsorption-desorption equilibrium. The temperature of the
◦
reaction system was controlled to 25 C by circulating water
device. After removal of the catalyst powder by centrifugation,
the upper filtrate was analyzed by Agilent 7890B gas
chromatography to determine the change of reactant
concentration.
2.5 DFT theoretical calculation
The simulations are performed on density functional theory
(DFT) calculations using the Dmol3 code.40,41 The generalized
gradient approximation (GGA) with Perdew-Burke-Ernzerh (PBE)
is adopted for the exchange-correlation term, and the double
numerical plus polarization (DNP) basis set were chosen in all
computational process. Self-consistent field (SCF) calculations
were carried out with a convergence criterion of 10-6 Ha on the
total energy and the electron density. The Brillouin zone is
sampled by 6 × 1 × 3 k-points for all the structures in the
2.3 Characterization
Powder X-ray diffraction (XRD) patterns were recorded by a X-
ray diffractometer (Bruker, D8) using Cu Kα radiation (tube
current: 20 mA, tube voltage: 40 kV). The morphologies were
characterized by scanning electron microscope (SEM, JEOL,
JSM-7001F) and transmission electron microscopy (TEM, JEOL-
2010, 200 kV). The thickness of the prepared nanosheet was
evaluated using a tapping-mode atomic force microscope (AFM,
Nanoscope Multimode IIIa, Veeco Instruments) with Si-tip
cantilever by dropping the suspension on the mica substrate.
Fourier transform infrared (FT-IR) spectra were recorded using
Bruker 70v FT-IR spectrometer with a resolution of 4 cm-1.
Raman spectra were acquired on a confocal laser micro-Raman
spectrometer (HORIBA, Lab RAM HR Evolution) at a wavelength
of 532 nm. Uv-vis diffuse reflectance spectra (DRS) were
collected on a Uv-vis spectrophotometer (Hitachi, U-3900H)
using BaSO4 as a reference. The oxygen vacancy and chemical
states of the samples were analyzed by electron spin-resonance
spectroscopy (ESR, Bruker, Emxplus-10/12) and X-ray
photoelectron spectroscopy (XPS, Thermo Scientific ESCAlab,
geometry optimization calculations, which produces
a
convergence tolerance energy of 1.0 × 10-5 Ha. The densities of
states were computed with 15 k points based on the optimized
structures. Perfect HNb3O8 (denoted as HNb3O8 NS-P) and
HNb3O8 with oxygen vacancies (denoted as HNb3O8 NS-V) layers
were modeled by a 4 × 4 super cell slab, which is periodic in the
x and z direction, whereas separated by a 15 Å vacuum region
in the y direction to avoid the interactions with the adjacent
atomic slabs. No changes were found for total energy when the
vacuum was further increased.
3. Results and discussion
3.1 Characterization of HNb3O8 nanosheets
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As shown in Fig. 1, the XRD pattern of obtained nanosheet
showed diffraction peaks at 2θ of 7.39, 14.07, 17.65, 23.21,
25.54, 27.11, 28.18, 29.79, 30.73, 35.88, 46.77°, which seemed
hardly to be correspond to any known phase associated with
pure niobium oxides. However, the main diffraction peaks of
the sample were similar to the XRD patterns of layered
H3ONb3O8 (JCPDS: 44-0672) and HNb3O8 (PDF no. 37-0833).
After ion-exchanged by HNO3, almost all the diffraction peaks of
HNb3O8 NS slightly red-shifted to high angle, which indicated
that the TMA cations were exchanged by H3O+ or H+.
Furthermore, the intensity of diffraction peaks became lower,
implying that the crystallinity of HNb3O8 NS was lower than that
of obtained nanosheet. The sharp diffraction peak at 2θ of 7.39°
red-shifted to 7.74°, corresponding to the characteristic peak
(020) of layered H3ONb3O8,42 which suggested the interlayer
space shrank from 11.95 to 11.41 Å. The ionic radius of H3O+
(about 2.82 Å) is smaller than that of hydrated forms of TMA+
cations (about 3.67 Å), thus the (020) diffraction peak is red
shifted to the higher angle.43
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DOI: 10.1039/C8TA12118D
Fig. 2 FT-IR (A) and Raman (B) spectra of obtained nanosheet (black line) and HNb3O8 NS
(red line).
assigned to stretching and bending vibration of C-H in CH3 of
TMA+.44 The band at 1485 cm-1 was attributed to the CH3
bending vibration of TMA cations. These peaks nearly
disappeared in the HNb3O8 NS, which indicated that the TMA
cations were substituted by H3O+ or H+. The bands at 3390 and
1636 cm-1 were assigned to the stretching and bending
vibrations of hydroxyl groups. It could be inferred that (CH3)4N+
-
and water intercalated into the Nb3O8 slice could be replaced
by H3O+ or H+ and HNb3O8 NS was synthesized by this simple
bottom-up hydrothermal process. Raman spectra were shown
in Fig. 2B. The peaks in the Raman shift range of 800-1000 cm-1
could be assigned to the stretching mode of the terminal Nb-O
bond striking out into the interlayer.47 The characteristic strong
and broad peaks at 902 and 946 cm-1 for layered compounds
implied that many terminal Nb=O bonds were exposed to the
outer surface.46 However, the broad peak at 888 cm-1 with a
shoulder peak at 957 cm-1 of HNb3O8 NS might be due to the
formation of NbO4−H2O adducts.42 The peaks at 458, 476, 537,
629, 670 cm-1 of the obtained NS were assigned to the
stretching mode of O-Nb-O linkage,47 while there was a broad
Raman band centred at 666 cm-1 for HNb3O8 NS similar with the
Nb2O5·nH2O nanosheet.42 The peaks in the ranges of 200 - 300
cm-1 for the obtained NS were ascribed to the bending mode of
the Nb-O-Nb linkage, while there was a wider peak for HNb3O8
NS. Meanwhile, the Raman band at 147 cm-1 range could be
attributed most likely to the strong interactions of the TMA+
cations with the intercalated water molecules.48 Specially, the
HNb3O8 NS were examined by FT-IR and Raman
spectroscopy in order to gain additional structural information.
FT-IR spectra of the obtained NS and HNb3O8 NS are shown in
Fig. 2A. The IR absorption in the range of 400-1000 cm-1 was
corresponded to the vibration of the Nb3O8- host slice.44 The IR
absorptions at 917 and 826 cm-1 of the obtained NS could be
related to the octahedral vibrations of NbO6 in layered
niobates.45,46 As for HNb3O8 NS, the peak at 917 cm-1 blue-
shifted to 913 cm-1 and its intensity decreased. The absorption
bands around 684 and 628 cm-1 of the obtained NS were
attributed to the characteristic symmetric and antisymmetric
stretch vibration modes of the Nb-O-Nb bridge bond in the
corner-shared NbO6 octahedron, while the absorption band of
HNb3O8 NS shifted to 678 cm-1 and became broad. The
absorption band at 547 cm-1 and weak absorption bands at461
cm-1 for the obtained NS belonged to bending vibrations and
librational modes of NbO6, respectively. Obviously, the peaks of
HNb3O8 NS at this region became weak. The IR weak
absorptions at 3016 and 1416 cm-1 of the obtained NS were
Fig. 3 SEM images of (A) obtained Nanosheet, (B) HNb3O8 NS, (C) AFM images, and
(D) the corresponding roughness profiles for the HNb3O8 nanosheet.
Fig. 1 XRD patterns of obtained Nanosheet and HNb3O8 Nanosheet.
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peak centered at 755 cm-1 should be assigned to the stretching
mode of the C4N+ group in TMA+.49 These two peaks
disappeared in the HNb3O8 NS. Raman spectra also confirmed
that the initial synthesized nanosheet was transformed to
HNb3O8 NS.
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DOI: 10.1039/C8TA12118D
SEM images of the obtained NS and HNb3O8 NS are shown
in Fig. 3A–B. It can be observed that the obtained NS with
microflower morphology was composed of two dimensional
interleaving nanosheets. After treated by HNO3, the 3D
hierarchical structure remained. Fig. 3C shows AFM images of
layered HNb3O8 nanosheet. It revealed that the 2D nanosheets
were spread out on the surface of mica plate with a lateral scale
about several hundred nanometres. From the roughness profile
shown in Fig. 3D, the thickness range of HNb3O8 nanosheet was
from 2.82 to 3.23 nm. As it was reported that the single layered
HNb3O8 slab along b axis is about 0.9 ~ 1.05 nm,42 we speculate
that our HNb3O8 NS might be composed of three layers of single
HNb3O8 slabs.
In addition, the HNb3O8 NS was further investigated by TEM
(Fig. 4). The low-magnification TEM images (Fig. 4A) manifested
that the typical morphology was microflower composed of well-
defined aggregated nanosheets. The side-view TEM image of
HNb3O8 NS (Fig. 4B) showed that the distance between the two
single sheets was estimated to be 1.03 nm, which was slightly
less than that evaluated by XRD (2θ = 7.74°, d = 1.14 nm).
Moreover, Fig. 4B also showed that the layer number of HNb3O8
nanosheet was 2-5, which was in consistent with the results of
AFM. The high-resolution TEM(HRTEM) images of HNb3O8 NS
(Fig. 4C and Fig. S2C, D) showed that the lattice fringes were
discontinuous and chaotic, implying structural distortion, which
might arise from the occurrence of defects that was always
produced with unsaturated Nb and O atoms. The corresponding
selected area electron diffraction pattern (SEAD) displayed a
Fig. 5 (A) Room-temperature ESR spectra of HNb3O8 NS, Ur Nb2O5 and Nb2O5 NR. (B), (C)
High-resolution Nb 3d and O 1s XPS spectra of HNb3O8 NS, Ur Nb2O5 and Nb2O5 NR. (D)
UV-vis spectra of HNb3O8 NS, Ur Nb2O5 and Nb2O5 NR and the corresponding
benzylamine-adsorbed niobium oxide catalysts.
series of dispersive diffraction rings which indicated the
polycrystalline characteristics of the HNb3O8 NS (Fig. 4D and Fig.
S2B).
Room-temperature electron spin resonance (ESR)
spectroscopy and X-ray photoelectron spectroscopy (XPS) are
both conducted to identify the oxygen vacancy and chemical
states of the samples (Fig. 5). As compared with Ur Nb2O5 and
Nb2O5 NR, HNb3O8 NS exhibited an intense ESR signal at g =
2.003 which could be assigned to the envelope of the hyperfine
structure of Nb4+ species.50 The intensity of ESR signal of HNb3O8
NS was higher than those of Ur Nb2O5 and Nb2O5 NR, which
revealed that the density of oxygen vacancies in the HNb3O8 NS
was much higher than that in the Ur Nb2O5 and Nb2O5 NR.51 This
is not in accordance with the HNb3O8 nanosheets prepared by
exfoliation process.35 As shown in Fig. 5B, the binding energies
of Ur Nb2O5 and Nb2O5 NR centered at 207.1 and 209.9 eV,
could be ascribed to Nb 3d5/2 and Nb 3d3/2, respectively, and the
spin orbit separation was 2.8 eV, which suggested the totally
existence of Nb5+.38,51 Whereas for HNb3O8 NS, besides the
characteristic peaks at 209.9 eV and 207.1 eV corresponding to
the Nb5+ ions, the peaks located at 209.2 eV and 206.3 eV were
also presented and well corresponded to the Nb4+ ions. The
percentage of Nb4+ in HNb3O8 NS was about 19.9%, which
indicated HNb3O8 NS possessed abundant oxygen vacancy sites.
The binding energies of O1s could be deconvoluted into two
peaks (shown in Fig. 5C). The peaks at 530.3 eV originated from
the characteristic lattice oxygen of niobium oxide, while the
peaks at 531.68 eV arose from surface adsorbed oxygen or
oxygen vacancies.51 The percentage of surface adsorbed oxygen
or oxygen vacancies were 14.7%, 15.3% and 27.8% for Nb2O5
NR, Ur Nb2O5 and HNb3O8 NS, respectively. This indicated that
the surface of HNb3O8 NS had more oxygen vacancies than
those of Nb2O5 NR and Ur Nb2O5. These results are in agreement
with the HRTEM results.
Uv-vis spectra of HNb3O8 NS, Ur Nb2O5 and Nb2O5 NR and
the corresponding benzylamine-adsorbed niobium oxide
Fig. 4 (A) Typical TEM images of HNb3O8 microfowers, (B) Side view, (C) HRTEM
image, (D) SAED image of HNb3O8 nanosheet.
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catalysts are displayed in Fig. 5D. The bandgaps of HNb3O8 NS,
Ur Nb2O5 and Nb2O5 NR were 3.40, 3.20 and 3.19 eV,
respectively, calculated by the Tauc method, which implied that
all the three samples did not exhibit any visible-light absorption.
However, Uv-vis spectra of the benzylamine-adsorbed Ur Nb2O5
and Nb2O5 NR were slightly red-shifted to visible-light region,
while Uv-vis spectrum of the benzylamine-adsorbed HNb3O8 NS
was greatly shifted to red light region (λ = 650 nm). The
differential spectra before and after the adsorption of
benzylamine and wavelength-dependent apparent quantum
yield (AQY) values towards N-benzylidenebenzylamine
evolution over the three catalysts are shown in Fig. S3.
Obviously, the adsorption intensity of the benzylamine-
adsorbed HNb3O8 NS was higher than that of HNb3O8 NS, which
indicated that the surface complex in situ formed by
benzylamine adsorption was conducive to harvesting visible
light and enhancing the photocatalytic reaction. In addition,
HNb3O8 NS exhibited a higher AQY value of 6.57% at 420 nm,
which was 8.01, 2.83 and 3.00 times of Ur Nb2O5, Nb2O5 NR,
Nb2O5 reported in the literatures (Table S1),12,14 respectively.
Since HNb3O8 NS possessed larger fraction of unsaturated
surface metal sites (verified by ESR and XPS results), the amount
of adsorbed benzylamine on the HNb3O8 NS was higher than
that of the reported Nb2O5,13 which would lead to HNb3O8 NS
possessed higher photocatalytic activity for amine oxidation.
To verify the oxygen vacancies and the charge carrier
dynamics (separation, trap, transfer and recombination), room-
temperature steady state and time-resolved photolumin-
escence (PL) spectroscopies were conducted. As shown in Fig.
6A and Fig. S4, all the spectra displayed a series of emission
peaks located at ~410, ~419, ~450, ~468, ~473, ~481, ~492,
~540 and ~620 nm. On the whole, the shapes of the PL spectra
looked similar for three different samples though their
morphologies were widely divergent. More intriguingly, HNb3O8
NS possessed higher PL intensity compared with Ur Nb2O5 and
Nb2O5 NR, suggesting their higher concentration of oxygen
vacancies and defects. The defect concentration increased the
number of charge recombination centers, which might hamper
the photoactivity.52 On the other hand, oxygen vacancies and
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DOI: 10.1039/C8TA12118D
Fig. 6 (A) PL spectra of HNb3O8 NS, Ur Nb2O5 and Nb2O5 NR. (B) The corresponding
decay curves. (C) Transient photocurrent responses and (D) Electrochemical
impedance spectroscopy (EIS) of HNb3O8 NS, Ur Nb2O5 and Nb2O5 NR.
HNb3O8 NS, Ur Nb2O5 and Nb2O5 NR were illustrated in Fig. 6b
to get more information on the influences of oxygen vacancies
on the lifetimes of charge carriers. The PL peak decay spectra of
Ur Nb2O5 and Nb2O5 NR were fitted well to a single-exponential
function (I = A1 e-t/τ1), whereas the PL peak decay spectrum of
HNb3O8 NS was fitted more reasonably by a double-exponential
model (I = A1 e-t/τ1 + A2 e-t/τ2), revealing two radiative lifetimes
(τ1 and τ2) with different corresponding magnitudes (A1 and A2).
The calculated results were summarized in Table S2. The fast
decay (τ1) was originated from the radiation emission of direct
interband exciton recombination, while the slower decay (τ2)
was generated from the indirect recombination of trapped
electrons with VB holes.56,57 Since HNb3O8 NS had two emissive
states with τ1 = 0.94 ns and τ2 = 5.13 ns, the calculated average
lifetime of HNb3O8 NS was 1.30 ns, which was greater than that
of Ur Nb2O5 (0.85 ns) and Nb2O5 NR (0.76 ns), respectively,
indicating that oxygen vacancies on the surface of HNb3O8 NS as
trapping sites for the photoinduced electron acceptor
prominently prolonged the lifetime of charge carriers.
Therefore, HNb3O8 NS would exhibit higher photocatalytic
performance than other two samples.
defects also contribute to activate oxygen molecules and
53,54
benzylamine. Similar to PL spectra of TiO2
and ZnO55, the
Transient photocurrent measurements and electrochemical
impedance spectroscopy (EIS) were further carried out to verify
the separation of photoinduced charge carrier. Fig. 6C showed
that the photocurrent of HNb3O8 NS was 2.2 and 5.3 times of Ur
Nb2O5 and Nb2O5 NR, respectively, in the presence of
benzylamine. However, the photocurrents of the three samples
were almost similar without benzylamine in the electrolyte (not
shown). Photocurrent generated from the transmission of
photoinduced electrons to the surface of ITO, meanwhile
benzylamine (hole acceptor) in the electrolyte accepted the
photogenerated holes. Due to the ultrathin nanosheet
structure, the photo-induced carriers were engendered on the
surface of the nanosheet to directly enter into the reaction,
which enormously avoided recombination of photo-electron
and photo-holes during the electron transmission. In contrast,
above phenomenon often unavoidably happened for the
traditional bulk material. Thus incident photon-to-electron
peaks centered at 410 nm (3.02 eV) and 419 nm (2.96 eV) were
ascribed to the strong excitonic emission. The blue emissions at
~450 (2.76 eV), ~468 (2.65 eV), ~473 (2.62 eV), ~481 (2.58 eV),
and ~492 (2.53 eV) nm were attributed to the radiative
recombination of electrons from the local defect level of
shallow donor centers with holes at the valence band (VB). The
broad green emissions at 540 nm (2.29 eV) corresponded to the
radiative transition from the shallow donor levels to the oxygen
vacancy related trap states. In addition, HNb3O8 NS showed the
higher intensity of the peak at 540 nm than those of Ur Nb2O5
and Nb2O5 NR, implying that there were abundant oxygen
vacancy sites in the surface of HNb3O8 NS. The peak at 620 nm
was not reported earlier, though it was likely to be related to an
intrinsic defect. According to the above analysis, the simplified
band diagram with different levels involved in the emission was
displayed in Fig. S5. Additionally, time-resolved PL spectra of
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Table 1. Photocatalytic oxidation of various benzylic amines to imines over HNb O8 NS
View Articl3e Online
conversion efficiency is enhanced as a result of the nanosheet
structure. That is to say, a higher photocurrent density often
means a higher ability in separating the photo-generated
electrons and holes. Fig. 6D showed that The charge transfer
resistances (Rct) of HNb3O8 NS, Ur Nb2O5 and Nb2O5 NR were
60.3, 63.5, and 71.2 Ω, respectively. As is known, the smaller
charge transfer resistance, the higher the charge transfer
efficiency.58 From the results of transient photocurrent
response and EIS spectra, we could speculate that the as-
prepared HNb3O8 nanosheet could improve the activity of
photocatalytic reaction through the more efficient separation
of the photo-induced electrons and photo-induced holes.
DOI: 10.1039/C8TA12118D
3.2 Catalytic Performance
To verify the photocatalytic performances of the prepared
catalysts, the oxidative coupling of amines was performed. The
blank reaction was conducted in the absence of the catalyst for
12 h under visible light irradiation. Nevertheless, no product
was detected. Similarly, to demonstrate the effect of visible
light on reaction, control experiments were carried out in the
dark with other conditions identical. The control reaction was
testified to be very slow and few products were obtained in the
presence of photocatalysts (Table S3). But in the presence of
visible light under identical conditions, excellent conversion to
N-benzylidenebenzylamine was observed. Fig. 7A showed the
time-dependent catalytic conversion over HNb3O8 NS, Ur Nb2O5
and Nb2O5 NR, respectively. Apparently HNb3O8 NS showed
excellent photocatalytic activity for the oxidative coupling
reactions of benzylamine under visible light irradiation
compared to Ur Nb2O5 and Nb2O5 NR. The conversions of
benzylamine were monotonically increasing with reaction time
for the three samples. They exhibited a typical zero-order
kinetic reaction character (shown in Fig. 7B). The apparent
reaction rate constant (k) for the oxidation of benzylamine
were4.45 × 10-3 mol L-1 h-1 for HNb3O8 NS, which was 1.14 and
Reaction conditions: HNb3O8 NS (20 mg), benzylamine (0.25 mmol), acetonitrile
(10 mL), in air, Reaction time 6 h, Temperature 25 °C.
1.97 times of Ur Nb2O5 (3.89 × 10-3 mol L-1 h-1) and Nb2O5 NR
(2.66 × 10-3 mol L-1 h-1), respectively. As shown in Table S4, the
normalized rate constant of HNb3O8 NS was still the highest in
the three samples. This demonstrated that the morphology of
niobium oxide had a great influence on the photocatalytic
performance in the selective oxidation of benzylamine.
The conversions of benzylamine and selectivity to N-
benzylidenebenzylamine on the catalysts of layered HNb3O8 NS,
Nb2O5 NR and Ur Nb2O5 are shown in Fig. 7C. All the
photocatalysts showed very high selectivities to N-
benzylidenebenzylamine, which were 98.9%, 98.8% and 97.5%
for HNb3O8 NS, Nb2O5 NR and Ur Nb2O5, respectively. HNb3O8
NS exhibited the highest conversion of benzylamine (95.0%)
while the other counterparts showed 88.8% and 60.3%
benzylamine conversion for Ur Nb2O5 and Nb2O5 NR,
respectively. Fig. 7D showed the recyclability of HNb3O8 NS. In
the five recycled experiments, there was no obvious change in
photocatalytic efficiency. The product yield was comparable to
the fresh sample. Hence, it is reasonable to believe that the as-
prepared HNb3O8 NS is perfectly stable and recyclable.
The catalytic activity of HNb3O8 NS was further inquired into
various amine derivatives with electron-donating groups
(CH3O– and CH3) and electron- withdrawing groups (–F and –Cl).
The substituents on the benzene ring had a less influence on the
reaction conversion (89.4% - 96.5%) (Table 1), nevertheless, the
selectivity to the corresponding imines was affected by
different substituents. When the substituents were CH3O– and
–Cl, the selectivity to the corresponding imines was 75.0% and
78.2%, respectively. It was reported that during the oxidation
process, benzylamine (or benzyl alcohol) was adsorbed on the
uncoordinated Nb atom near the surface complex by the
adsorption band formed by lone pairs of N (or O) filled the
vacant 3d orbital of Nb atom.12,37 Therefore, we speculated that
when the substituent was CH3O–, there were two adsorption
models, that is, CH3O– or –NH2 might be adsorbed on the
uncoordinated Nb atom. The results indirectly manifested that
the surface complex formed by amine adsorbed on the surface
of HNb3O8 NS by LMCT mechanism promoted the photocatalytic
activity of benzylamine oxidation under visible light.
Fig. 7 Catalytic performances in selective oxidation of benzylamine. (A) Time-
dependent catalytic conversion. (B) The kinetic rate plots and calculated kinetic
rate constants. (C) Performances of HNb3O8 NS, Ur Nb2O5 and Nb2O5 NR for the
selective oxidation of benzylamine under visible light. (D) Recyclability and
stability of HNb3O8 NS in the benzylamine oxidation reaction.
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View Article Online
DOI: 10.1039/C8TA12118D
Fig. 9 In situ FT-IR spectra of benzylamine adsorbed on HNb3O8 NS (A) and after
irradiation under a dry air atmosphere at different time (B). Comparison of FT-IR
spectra of adsorption and irradiation at different time (C).
ring(νC-C, 1499 and 1525 cm-1) could not be clearly distinguished,
the peak centered at 1645 cm-1 in the region of 1500 – 1750 cm-
1 was rather broad.59 The weak and broad peaks in the region of
1280 – 1400 cm-1 and 1100 – 1200 cm-1 also increased in
intensity with adsorption time, which might be assigned to CH2
bending vibrations (νCH2) and C–N stretching vibration (δC–N).59
Obviously, the result obtained by the in-situ FT-IR spectra
indicated that benzylamine was adsorbed on the surface of
HNb3O8 NS in the form of molecules.
Fig. 8 (A) Photocatalytic selective oxidation of benzylamine over HNb3O8 NS with
different radical scavengers under visible light irradiation for 4 h. AO = ammonium
oxalate, IPA = isopropyl alcohol and BQ = benzoquinone. (B) Room-temperature
ESR spectra of the HNb3O8 NS, Ur Nb2O5 and Nb2O5 NR.
3.3 Mechanism Investigation
To get more insight into the role of photogenerated radical
species in photocatalytic oxidation of benzylamine, the
scavenger measurements were performed on HNb3O8 NS
catalyst, where K2S2O8, ammonium oxalate (AO), benzoquinone
(BQ) and isopropyl alcohol were added into the photocatalysis
system to selectively eliminate photogenerated electrons,
After dry air was introduced into the IR cell and irradiation
with Xe lamp for various period of time, the changes in FT-IR
spectra were recorded during the irradiation process as shown
in Fig. 9B. The comparison of adsorption and irradiation IR
spectra was displayed in Fig. 9C. After irradiated 3 min under
visible light, the bands at 1651, 1413 cm-1 formed and were
•-
holes, O2 radicals, and hydroxyl radicals (•OH), respectively.5
The results are illustrated in Fig. 8A. Obviously, the additions of
AO and K2S2O8 both led to significant decreases in the activities
for the photocatalytic oxidation of benzylamine, which clearly
demonstrated that both photogenerated holes and electrons
were essential drivers for the photocatalytic reactions. In the
attributed to νC=O and ẟ
CHO, respectively.12,59 This means that
benzaldehyde was formed firstly during the benzylamine
oxidation. The bands at 1544, 1510, 1489 cm-1 belonging to C–
C stretching vibrations of aromatic rings increased in intensity
•-
different reactive oxygen species, O2 radicals showed
significant suppression of benzylamine oxidation, whereas
hydroxyl radicals (•OH) executed negligible influences on the
•-
conversion of benzylamine. Therefore, O2 radicals could be
determined to be the dominating reactive oxygen species in the
photocatalytic molecular oxygen activation of HNb3O8 NS.
•-
Upon identifying the generation of O2 radicals on the
surface of HNb3O8 NS with oxygen vacancies, ESR spin-trapped
technique with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a
spin-trapping agent was carried out. As displayed in Fig. 8B, the
four characteristic peaks of DMPO-O2 adduct were evident in
the mixed CH3CN dispersion with 2 mg of HNb3O8 NS and
[DMPO] at 2 × 10-5 M under visible light irradiation for 20 s,
verifying the generation of O2 .
•- 16 Meanwhile, no ESR signal was
detected over Ur Nb2O5 and Nb2O5 NR at the same condition,
•-
demonstrating that the amount of O2 radicals was
inappreciable over Ur Nb2O5 and Nb2O5 NR. Hence, the
existence of surface oxygen vacancies was the vital factor to
activate molecular oxygen to O2•- radicals over HNb3O8 NS under
visible light irradiation.
In order to investigate the mechanism of benzylamine
oxidation, the surface adsorption of the reactant was studied by
in situ FT-IR spectroscopy. After exposing to vaporous
benzylamine at 95 Pa, the intensities of bands at 3515, 3354,
and 1645 cm-1 increased with time on HNb3O8 NS. The bands at
3515 and 3354 cm-1 could be attributed to the stretching
vibrations of hydroxyl groups and N-H respectively.12 Due to the
fact that the OH bending vibration, NH2 bending vibration (1620
and 1583 cm-1) and C-C stretching vibrations of an aromatic
Fig. 10 The optimized adsorption configurations of benzylamine on HNb3O8 NS-P
(A) and HNb3O8 NS-V (B). Atoms are color labeled: Nb (cyan), H (white), O (red), C
(grey) and N (dark blue). PDOS plots of benzylamine on HNb3O8 NS-P and HNb3O8
NS-V (C), benzylamine (D), N atom (E), and Nb atom (F). Top, HNb3O8 NS-P; down,
HNb3O8 NS-V. The Fermi level is set to zero, as shown by the dashed line.
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vacancy and HNb3O8 NS-P (Fig. 11 and Fig. S6D). Fig. 11A
View Article Online
DOI: 10.1039/C8TA12118D
showed that O2 molecule was physisorbed on the surface of the
HNb3O8 NS-P surface with the slightly elongated O-O bond
length of 1.27 Å (from 1.22 Å to 1.27 Å, Table S5). The calculated
Eads of O2 was merely -0.47 eV and O2 was negatively charged by
0.30 |e| according to the Mulliken charge analysis. In sharp
contrast, molecular oxygen could be intensively chemisorbed
on the surface oxygen vacancies of HNb3O8 NS-V through an
end-on configuration with a significant elongation of O-O bond
length of 1.39 Å (Fig. 11B and Table S5), promoting the
•- 63
generation of O2
.
Meanwhile, the chemisorbed O2 could
receive 1.16 |e| from the HNb3O8 NS-V. The highly enhanced
adsorption energy (Eads) of -4.08 eV (about 8.7 times for that of
HNb3O8 NS-P) indicated that the O2 chemisorption was more
favorable on the defect surface of HNb3O8 NS-V, which
promoted efficient O2 activation during the photocatalytic
process. The PDOS plots of O2 adsorbed on HNb3O8 NS-P
displayed the divergence of spin-up and spin-down peaks,
which demonstrated that a non-zero magnetic dipole moment
was generated by the two single electrons of the π2p orbital with
Fig. 11 The optimized adsorption configurations of O2 on HNb3O8 NS-P (A) and HNb3O8
NS-V (B). PDOS plots of O2 on HNb3O8 NS-P (C) and HNb3O8 NS-V (D). Top, O2 molecule;
down, O2 on HNb3O8 NS. The Fermi level is set to zero, as shown by the dashed line.
with further prolonging the irradiation time to 17 min,60,61
which might imply that the N-benzylidenebenzylamine was
produced on the surface of HNb3O8 NS.
a
ferromagnetic spin configuration, hence O2 was just
physisorbed on the surface of
To gain insights into the formation of surface complex
between benzylamine and the HNb3O8 NS with oxygen vacancy,
we investigated the adsorption of benzylamine on HNb3O8 NS
with oxygen vacancy (HNb3O8 NS-V) and perfect HNb3O8 NS
(HNb3O8 NS-P) using the density function theory (Fig. 10 and Fig.
S6). The optimized adsorption geometry of benzylamine on
HNb3O8 NS with oxygen vacancy (Fig. 10B, Fig. S6B) showed that
the nitrogen is deeply inserted in the oxygen vacancy of
defective HNb3O8 NS by the simultaneous formation of a -N-Nb-
coordinate bond (2.27 Å, Table S5) and an -N-H-O- hydrogen
bond (1.81 Å) where the O atom was the adjacent crystal lattice
oxygen of HNb3O8 NS-V.12,37,62 However, for HNb3O8 NS-P,
benzylamine was adsorbed by hydrogen bond formed through
the bonding of N atoms of benzylamine to surface hydroxyl of
HNb3O8 NS-P (Fig. 10A).16 The adsorption energy was estimated
at about -2.59 eV for the HNb3O8 NS-V and 2.1 times of that for
the HNb3O8 NS-P (-1.23 eV) (Table S5). The strong interactions
between benzylamine and the oxygen vacancy of defective
HNb3O8 NS led to an upward shift of p orbitals of benzylamine
about 0.21 eV (Fig. 10D) and a downward shift of Nb 3d orbitals
about 0.06 eV (Fig. 10F), which induced a bandgap narrowing of
approximately 0.15 eV (Fig. 10C and Fig. S7). Compared with
PDOS projected on N atoms adsorbed on the HNb3O8 NS-V and
HNb3O8 NS-P (Fig. 10E), we could observe that the electronic
density of p orbital projected on N atoms adsorbed on the
HNb3O8 NS-V decreased dramatically at the valence-band
maximum, which indicated that lone pair electrons of N atom
were transferred to the adjacent Nb atom. This also could be
confirmed by the electronic density of d orbital projected on Nb
atoms increased and widened (Fig. 10F). These results clearly
exhibited that the surface complex of benzylamine-HNb3O8 NS-
V would reduce the bandgap of HNb3O8 NS and be profitable to
harvest visible light.
Fig. 12 (A) Reaction scheme for the photocatalytic oxidation of benzylamine to N-
benzylidenebenzylamine on HNb3O8 NS with oxygen vacancy. (B) Proposed
mechanism for selective aerobic oxidation of benzylamine to imine over HNb3O8
NS under visible light irradiation.
To further demystify O2 activation by surface oxygen
vacancies, we theoretically simulated the adsorption
configurations of O2 molecule on HNb3O8 NS with oxygen
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HNb3O8 NS-P through the hydrogen bond between O2 and bandgap of HNb3O8 NS and was induced to harvesVtievwisAirbtilcelelOignhlinte,
DOI: 10.1039/C8TA12118D
surficial hydroxyl (Fig. 11C). In contrast, the PDOS plots of O2 but also could efficiently chemisorb O2 molecules and promote
adsorbed on HNb3O8 NS-V showed symmetric spin-up and spin- to activate O2 to reactive oxygen species. Furthermore, unique
down peaks without magnetic moment (Fig. 11D), which was nanosheet structure with several molecular thicknesses led to
•- 63,64
chemisorbed O2
.
The new-formed orbitals in the valence the effective separation of the photogenerated carriers. These
band were filled by electrons, whereas there were no electrons findings deepen our knowledge about the roles of oxygen
left in the conduction band when the O2 molecule was vacancies on the surface of ultrathin transition metal oxide
adsorbed.65,66 Therefor, HNb3O8 NS-V could efficiently nanosheet, also shed light on exploring defective wide-bandgap
chemisorb O2 molecules and promote to activate O2 to reactive semiconductors for the photocatalytic organic synthesis.
oxygen species.
Based on the above analysis and the previous
reports,8,13,18,21,37 we proposed a possible mechanism to explain
the photocatalytic oxidation of benzylamine on HNb3O8
nanosheets with oxygen vacancies (shown in Fig. 12). Firstly,
benzylamine was adsorbed on the oxygen vacancy sites of
HNb3O8 NS, forming a surface complex (Step 1). Secondly, the
surface charge transfer complex adsorbed visible light, and
charge carriers (h+/e-) were generated. The adsorbed
benzylamine was oxidized by the photogenerated hole (h+) to
form the relevant nitrogen-centered radical (intermediate A),
meanwhile the abstracted H+ would react with the adjacent
Lewis base site and form a surficial hydroxyl. Thirdly, molecular
oxygen was activated by the photogenerated electron (e-) to
produce superoxide radical anion (O2•-) on the same oxygen
vacancy site. At the same time, an intramolecular hydrogen
transfer process was introduced from -CH2 to nitrogen-centered
radical of intermediate A, which would result in a more stable
carbon-centred radical of intermediate B was formed. Fourthly,
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors are grateful to the financial support from the
National Natural Science Foundation of China (NSFC. 2160309
and 2177166), Leading talents in scientific and technological
innovation (ZYQR201810017), Key Program of Henan Province
for Science and Technology (16A150047 and 122300410240),
The School Doctor Foundation of Zhengzhou University of Light
Industry (Grant No. 2012BSJJ014).
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