Separated redox site strategies for engineering highly efficient photocatalysts: a pagoda-like In2O3/CuO heteroepitaxial structure coated with a N-doped C layer

Article abstract of DOI:10.1039/d0ta10831f

In order to prepare efficient photocatalysts, it is necessary to consider not only the generation, separation and migration of photo-generated carriers, but also the effective activation of reactants and the reasonable distribution of redox sites. In this

Full text of DOI:10.1039/d0ta10831f

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Materials Chemistry A
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Separated redox site strategies for engineering
highly efficient photocatalysts: a pagoda-like
In2O3/CuO heteroepitaxial structure coated with
a N-doped C layer†
Cite this: J. Mater. Chem. A, 2021, 9,
4310
*
*
Yun Yang, Liming Sun, Wenwen Zhan, Xiaojun Wang
and Xiguang Han
In order to prepare efficient photocatalysts, it is necessary to consider not only the generation, separation
and migration of photo-generated carriers, but also the effective activation of reactants and the reasonable
distribution of redox sites. In this article, we utilized a MOF-on-MOF heterostructure with epitaxial
relationship as the precursor to synthesize N-doped C encapsulated pagoda-like CuO–In2O3 (In2O3/
CuO@N-C) heteroepitaxial micro-rods. In the obtained In2O3/CuO@N-C hierarchical pagoda-like micro-
rods, the loose lamellar structure enhanced the light-harvesting capability, the formed CuO–In2O3 p–n
heterojunction improved the separation efficiency of photo-generated carriers, and the coated N-doped
carbon layer facilitated the transfer of photo-generated carriers and the activation of reactants. More
importantly, the directional separation of CuO (external nanosheets) and In2O3 (central axis) acted as
oxidation and reduction sites to react with 2-phenyl-1,2,3,4-tetrahydroisoquinoline and indole,
respectively, which effectively avoided the inverse reaction of cross-dehydrogenative coupling (CDC).
Therefore, the obtained pagoda-like In2O3/CuO@N-C heteroepitaxial micro-rods exhibited excellent
photocatalytic activity for the CDC reaction. The present work provides a new design strategy for the
comprehensive construction of a high efficiency photocatalytic material system.
Received 6th November 2020
Accepted 6th January 2021
DOI: 10.1039/d0ta10831f
rsc.li/materials-a
inverse reactions, it is considered to be the most important but
complicated process in photocatalysis. Therefore, the efficient
Introduction
separation and migration of photo-generated carriers and the
thermodynamic requirements are not enough to achieve a photo-
catalytic reaction. The effective activation of reactants and the
reasonable distribution of redox sites should also be consid-
ered,10–13 which is very important for the construction of an effi-
cient photocatalyst system.
With the increasing demand for energy and the increasing
shortage of energy, the semiconductor photocatalysis tech-
nology, which can convert green and sustainable solar energy
into chemical energy, has been widely explored.1–5 It is common
knowledge that both reductive and oxidative processes involving
photo-generated electrons and holes, respectively, occur in a pho-
tocatalytic reaction. The effective separation of photo-generated
electrons and holes in photocatalysts is the rst step to promote
a photocatalytic reaction. On the other hand, the interfacial redox
reaction, which is usually the direct output of photocatalysis, is
considered as the last step of photocatalysis.6 Because this inter-
facial redox reaction contains a series of radical chain reactions,
which are oen thought to be initiated from the activation of the
reactants by photo-generated carriers,7–9 and their corresponding
Recently, the synthesis of carbon-coated semiconductor
metal oxide heterostructures by using metal–organic frame-
works (MOFs) as precursors has become one of the important
means to construct efficient photocatalysts.14–29 A photocatalyst
prepared by this method has a compact heterojunction inter-
face and a uniformly coated carbon layer, which can ensure the
effective separation and smooth migration of photo-generated
electrons and holes. In addition, the coated carbon layer can
also enhance the adsorption and activation of some reactants,30
especially some organic reactants containing six-membered
carbon rings, which is of great signicance to expand the
application of inorganic photocatalysts in the eld of organic
reactions. However, the two metal oxide particles in these het-
erojunction photocatalysts are usually mixed indiscriminately,
which makes the redox sites of photocatalysis also randomly
distributed. The short distance between the randomly distrib-
uted redox sites can lead to the inverse reaction of
Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials,
Department of Chemistry, School of Chemistry and Chemical Engineering, Jiangsu
Normal University, Xuzhou, 221116, P. R. China. E-mail: xghan@jsnu.edu.cn
† Electronic supplementary information (ESI) available: Experimental details,
powder XRD, EDX mapping, FT-IR spectra and TGA curves of MIL-68-In and
MIL-68-In@Cu-BDC precursors, the corresponding characterization of reference
samples and In2O3/CuO@N-C aer the catalytic reaction, contrast experiments
for the catalyzed CDC reaction and Mulliken charge population. See DOI:
10.1039/d0ta10831f
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photocatalysis and is not conducive to the interfacial redox
reaction.31–33
If utilizing a MOF-on-MOF heterostructure with epitaxial
relationship as the precursor, we can not only retain the above
advantages in the obtained metal oxide heterojunction, but also
directly separate the two kinds of metal oxide particles, so as to
realize the effective spatial separation of redox sites in the
photocatalysis and inhibit the inverse reaction. Based on this
design strategy, in this work, Cu-BDC nano-sheets were epitax-
ially grown on MIL-68-In micro-rods to form the MIL-68-In@Cu-
BDC heterostructure, which was calcined as a precursor to
obtain a N-doped C encapsulated CuO–In2O3 heteroepitaxial
structure (In2O3/CuO@N-C) with the morphology of hierar-
chical pagoda-like micro-rods. The In2O3/CuO@N-C hierar-
chical pagoda-like micro-rods synthesized by this strategy not
only have enhanced light absorption capacity, separation effi-
ciency of photo-generated electron–hole pairs and the ability to
activate reactants, but also possess directionally separated
oxidation (CuO external nanosheets) and reduction (In2O3
central axis) sites, which can effectively prevent the occurrence
of inverse reactions. The In2O3/CuO@N-C hierarchical pagoda-
like micro-rods exhibited excellent photocatalytic activity for the
CDC reaction under blue LED irradiation as expected. This work
would provide a new design idea for the comprehensive
construction of a high efficiency photocatalytic material system.
Fig. 1 (a) FE-SEM image of the MIL-68-In precursor; (b) SEM image of
the MIL-68-In@Cu-BDC precursor; (c) SEM image, (d) XRD pattern, (e)
Raman spectrum, (f) N2 adsorption/desorption isotherms and BJH
pore size distribution of the pagoda-like In2O3/CuO@N-C hetero-
epitaxial structure; (g) TEM image of a typical In2O3/CuO@N-C hier-
archical pagoda-like micro-rod; (h) HRTEM image of the central axis in
the In2O3/CuO@N-C hierarchical pagoda-like micro-rods (marked
with a red frame in Fig. 1g); (i) HRTEM image of the external sheet in
In2O3/CuO@N-C hierarchical pagoda-like micro-rods (marked with
a blue frame in Fig. 1g); (j) STEM image; and (k)–(o) EDX elemental
mapping of In, Cu, O, C and N elements for the In2O3/CuO@N-C
hierarchical pagoda-like micro-rods.
Results and discussion
The synthesis procedure of a N-doped C encapsulated CuO–
In2O3 heteroepitaxial structure (In2O3/CuO@N-C) with the
morphology of hierarchical pagoda-like micro-rods is sche-
matically depicted in Scheme 1. A well-dened MIL-68-In
precursor with a micro-rod morphology is rst synthesized by
a solvothermal method in the presence of indium(III) nitrate
tetrahydrate (In(NO3)3$4H2O) and 1,4-benzenedicarboxylic acid
in N,N-dimethylformamide (DMF) at 100 ꢀC for 4 h. Aer
dispersing the obtained MIL-68-In micro-rods in a solution of
1,4-benzenedicarboxylic acid and copper nitrate hexahydrate
(Cu(NO3)2$6H2O) at 120 ꢀC for 30 min, Cu-BDC nano-sheets
were epitaxially grown on the MIL-68-In micro-rods to form
the MIL-68-In@Cu-BDC precursor. The In2O3/CuO@N-C can be
nally obtained by carbonizing tꢀhe MIL-68-In@Cu-BDC
precursor in an Ar atmosphere at 390 C for 1 h.
peaks were consistent with a previous report,34 which conrmed
that the obtained precursor was pure phase MIL-68-In with
good crystallinity. The image from eld emission scanning
electron microscopy (FESEM, Fig. 1a) veried the formation of
solid MIL-68-In micro-rods with high uniformity and smooth
surfaces. Energy-dispersive X-ray spectroscopy (EDX) element
mapping (Fig. S2†) indicated that the In, O, C and N elements
were evenly dispersed throughout the MIL-68-In rod structure.
Aer adding MIL-68-In precursors in 1,4-benzenedicarboxylic acid
and Cu(NO3)2$6H2O solution, the XRD result (Fig. S3†) showed the
residual MIL-68-In phase and the presence of the Cu-BDC
phase,35,36 indicating the formation of the MIL-68-In@Cu-BDC
precursor. From the FESEM image (Fig. 1b), it can be found that
numerous Cu-BDC nanosheets with an oriented assembly grow
epitaxially on the surface of MIL-68-In rods to form the MIL-68-
In@Cu-BDC precursor. EDX mapping of the MIL-68-In@Cu-BDC
precursor (Fig. S4†) also showed that the In, Cu, O, C and N
elements are uniformly distributed in the precursor structure.
According to the thermogravimetric analysis (TGA) of the MIL-68-
In@Cu-BDC precursor (Fig. S5†), the pyrolysis temperature for the
carbonization of the precursor was set to 390 ꢀC.
In order to determine the composition and structure of the
obtained In2O3/CuO@N-C, the MIL-68-In precursor was rst
characterized. As shown by X-ray powder diffraction (XRD,
Fig. S1†), the precursor exhibited sharp diffraction peaks and all
Scheme 1 Schematic illustration showing the synthetic process of
monodisperse In2O3/CuO@N-C hierarchical pagoda-like micro-rods.
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The typical SEM image (Fig. 1c) of In2O3/CuO@N-C indicated
Moreover, X-ray photoelectron spectroscopy (XPS) was used
that the hierarchical rod morphology of the MIL-68-In@Cu-BDC to investigate the surface elemental chemical compositions and
precursor was well preserved aer the annealing process. the chemical state of In2O3/CuO@N-C hierarchical micro-rods.
However, compared with the MIL-68-In@Cu-BDC precursor, the The survey spectrum indicated the existence of In, Cu, O, C,
distance between the ultrathin nanosheets in In2O3/CuO@N-C and N elements in the In2O3/CuO@N-C hierarchical micro-rods
hierarchical micro-rods increased, resulting in a pagoda-like (Fig. 2a). The high resolution XPS spectrum of Cu 2p was
structure. Moreover, the ultrathin nanosheet subunit in the recorded and is shown in Fig. 2b. The two main peaks at 953.8
In2O3/CuO@N-C hierarchical pagoda-like micro-rods was con- and 933.4 eV corresponded to Cu 2p1/2 and Cu 2p3/2, and the
verted to a porous structure. The XRD pattern (Fig. 1d) indicated distance between the two peaks was 20 eV, implying a normal
that the product aer the annealing process was covered with state of Cu2+. Moreover, the shake-up satellites (denoted as Sat.)
CuO (PDF no. 00-041-0254) and In2O3 (PDF no. 00-044-1087) at 941.4, 943.9 and 962.6 eV, which are unique characteristics of
phases. The Raman spectrum of the In2O3/CuO@N-C hierar- Cu2+, also conrm the presence of CuO. Fig. 2c shows the In 3d
chical pagoda-like micro-rods (Fig. 1e) presented a D-band at XPS spectrum with double peaks, which were attributed to
1385 cmꢁ1 (associated with structural disorders and defects) typical spin–orbit split In 2d5/2 (444.7 eV) and In 2d3/2 (452.3 eV),
and a G-band at 1588 cmꢁ1 (related to graphitic carbon), which respectively. The result indicated that In(III) was the predomi-
indicated the presence of highly disordered/defective graphitic nant state in the sample. The high-resolution O 1s spectrum
carbon in In2O3/CuO@N-C hierarchical pagoda-like micro-rods. (Fig. 2d) could be deconvoluted into three major peaks centered
The porous nature of In2O3/CuO@N-C was also examined by N2 at 529.9, 530.7 and 531.9 eV corresponding to the lattice oxygen
sorption measurement (Fig. 1f), which showed a large Bru- (OV), C]O, and O–H bonds, respectively. The C 1s spectrum
nauer–Emmett–Teller (BET) surface area of about 147.1 m2 gꢁ1
.
(Fig. 2e) was composed of four deconvoluted peaks. The peaks
According to Barrett–Joyner–Halenda (BJH) analysis (inset of at 284.7 eV corresponded to the C]C/C–C bonds. The peak at
Fig. 1f), the pore size distribution is mainly centered at the 285.6 eV was assigned to the C–N bond, which conrmed the
mesoporous size range. The hierarchical structure and the nitrogen-doping in the carbon layer. The carbon atoms bonded
inside structural information of In2O3/CuO@N-C hierarchical to the carbonyl group (C–O) were represented by the peak at
pagoda-like micro-rods were further characterized by trans- 288.9 eV. The peaks of N 1s (Fig. 2f) can be ascribed to graphene-
mission electron microscopy (TEM). From Fig. 1g, it can be seen N (400.1 eV). Therefore, by combining XPS, HRTEM and EDX
that the obtained In2O3/CuO@N-C possessed a hierarchical mapping analyses of In2O3/CuO@N-C, it can be concluded that
structure with loose and porous characteristics. High resolution
transmission electron microscopy (HRTEM) was used to inves-
tigate the composition of each part in the In2O3/CuO@N-C
hierarchical pagoda-like micro-rods. The HRTEM image of the
central axis in In2O3/CuO@N-C hierarchical pagoda-like micro-
rods (Fig. 1h) exhibited that the lattice distance was 0.292 nm,
which corresponded to the (222) plane of In2O3. On the other
hand, a lattice distance of 0.252 nm belonging to the CuO (1ꢀ11)
plane appeared in the HRTEM image of the external sheet in
In2O3/CuO@N-C hierarchical pagoda-like micro-rods (Fig. 1i).
Moreover, from Fig. 1i, it can be found that the CuO was
encapsulated by a few layered carbon shell. These results of
HRTEM indicated that the central axis of In2O3/CuO@N-C
hierarchical pagoda-like micro-rods consisted of In2O3, the
external nanosheets were composed of CuO, and the outside of
the micro-rods was coated with carbon layers. This conclusion
was further veried using the STEM image (Fig. 1j) and energy-
dispersive X-ray (EDX) mapping. As shown in Fig. 1k, the In
element was concentrated on the central axis of In2O3/CuO@N-
C hierarchical pagoda-like micro-rods. On the other hand, the
Cu element was distributed on the external nanosheets of
In2O3/CuO@N-C hierarchical pagoda-like micro-rods (Fig. 1l).
From Fig. 1m–o, it could be found that the O, C and N elements
were homogeneously distributed in the hierarchical structure.
The EDX mapping results manifested that the In2O3/CuO@N-C
hierarchical pagoda-like micro-rods were composed of the
In2O3 central axis and CuO external sheets, and coated by N-
doped C layers, and they were consistent with the results of
HRTEM.
Fig. 2 XPS spectrum of In2O3/CuO@N-C hierarchical pagoda-like
micro-rods, (a) survey spectrum and high-resolution XPS spectrum: (b)
Cu 2p, (c) In 3d, (d) O 1s, (e) C 1s, and (f) N 1s.
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N-doped C layer-coated hierarchical pagoda-like micro-rods, the In2O3/CuO@N-C sample possessed improved separation
which were constituted by the In2O3 central axis and CuO efficiency of photo-generated electron–hole pairs due to In2O3/
external nanosheets, have been fabricated.
CuO heterojunctions and the external N-doped C layer. Photo-
The formation of In2O3/CuO heterojunctions and an external luminescence (PL) emission spectra of the photocatalysts also
N-doped C layer offered directional transfer channels for photo- provided information on charge-separation efficiency. As shown
generated electrons, which could effectively increase the sepa- in Fig. 3d, the PL intensity presented a clear trend of In2O3/
ration efficiency of electrons and holes. In order to characterize CuO@N-C < In2O3@N-C < CuO@N-C < In2O3/CuO, which
the migration and separation of photo-generated electrons and indicated that In2O3/CuO@N-C possessed the highest separa-
holes, linear sweep voltammetry (LSV) curves, photocurrent tion rate of photo-induced charges among these samples. A
density potential (PDP), electrochemical impedance spectra uniform coating of the N-doped carbon layer and hetero-
(EIS) and photoluminescence spectra of four samples (In2O3/ junctions were the determining factors responsible for this
CuO@N-C, In2O3@N-C (corresponding preparation and char- order of charge separation efficiency. Thus, the obtained In2O3/
acterization are shown in ESI Fig. S6†), CuO@N-C (Fig. S7†) and CuO@N-C hierarchical pagoda-like micro-rods possessed good
In2O3/CuO (Fig. S8†)) have been characterized, respectively. The separation efficiency of photo-generated electron–hole pairs.
LSV curves (Fig. 3a) showed that the In2O3/CuO@N-C has
The cross-dehydrogenative coupling (CDC) reaction was
a higher cathodic current density for the reduction of water to used as the target reaction to investigate the visible-light pho-
H2 than In2O3@N-C, CuO@N-C and In2O3/CuO, resulting from tocatalytic activity of In2O3/CuO@N-C hierarchical pagoda-like
the accelerated charge transfer ability caused by the synergistic micro-rods. In order to determine the conditions for photo-
effects of the In2O3/CuO heterojunctions and N-doped C layer catalysis, comparative experiments have been performed in the
coating. The separation efficiency of photo-generated electrons absence of light and a catalyst (Table S1†). The result indicated
and holes was directly characterized by measuring the photo- that this reaction did not occur without any one of them (light
current density of four different samples. As shown in Fig. 3b, and catalyst). Therefore, the photocatalytic reaction was carried
In2O3/CuO@N-C exhibited higher photo-current density than out under blue light emitting diode (LED, l ¼ 450 nm, 3 W)
In2O3/N-C and CuO/N-C samples, respectively, indicating that irradiation in a Schlenk tube (Fig. 4a). In order to systematically
the presence of In2O3/CuO heterojunctions can increase the study the inuence of composition and structure on the pho-
separation efficiency. Furthermore, In2O3/CuO@N-C showed tocatalytic activity of In2O3/CuO@N-C hierarchical pagoda-like
higher photo-current density than In2O3/CuO, which proved micro-rods, In2O3@N-C, CuO@N-C, In2O3/CuO and DNCPH
that the N-doped C layer can further accelerate the electron (dodecahedral N-doped C-coated In2O3/CuO p–n hetero-
transfer, promoting the separation of photo-generated electrons junction, the corresponding preparation and characterization
and holes in space. The EIS results (Fig. 3c) indicated that the are shown in ESI Fig. S9†) were used as reference samples. The
In2O3/CuO@N-C sample has the smallest semicircle in the composition of reference sample DNCPH was the same as that
Nyquist plot among these four samples. The smallest semicircle of the In2O3/CuO@N-C hierarchical pagoda-like micro-rods,
indicated the lowest charge-transfer resistance, which could that is, it was also composed of N-doped C-coated In2O3/CuO
warrant efficient transportation and separation of photo- nanoparticles. However, different from the directional separa-
generated carriers. Based on the above results and discussion, tion arrangement of In2O3 and CuO in In2O3/CuO@N-C hier-
archical pagoda-like micro-rods, In2O3 and CuO nanoparticles
were irregularly mixed in DNCPH. As shown in Fig. 4b, the
photocatalytic activities of In2O3/CuO@N-C hierarchical
pagoda-like micro-rods and DNCPH were higher than those of
In2O3@N-C, CuO@N-C and In2O3/CuO. The 1H NMR spectra of
the reactants and product are shown in Fig. S10.† The results
indicated that the In2O3/CuO heterostructure and N-doped C
layer played a positive effect on the photocatalytic CDC reaction.
Compared with DNCPH, the better photocatalytic activity of
In2O3/CuO@N-C hierarchical pagoda-like micro-rods declared
that the heterojunction structure of the In2O3 and CuO direc-
tional separation arrangement was more conducive to the CDC
reaction. The reason for this phenomenon was that in In2O3/
CuO@N-C hierarchical pagoda-like micro-rods, In2O3 and CuO
separated in space were used as reduction and oxidation sites to
enrich photo-generated electrons and photo-generated holes,
respectively, which not only realized the effective separation of
photo-generated electron–hole pairs, but also effectively
inhibited the inverse reaction of the photocatalytic CDC reac-
tion. The cycling stability of the In2O3/CuO@N-C catalysts was
Fig. 3 (a) Linear sweep voltammetry (LSV) curves, (b) photo-current
density curves, (c) electrochemical impedance spectroscopy Nyquist
checked by repeating the same reaction process for four cycles.
As shown in Fig. 4c, In2O3/CuO@N-C catalysts exhibited high
plots, (d) photoluminescence spectra of In2O3/CuO@N-C, In2O3@N-
C, CuO@N-C and In2O3/CuO samples.
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Fig. 5 Electron density difference maps of 2-phenyl-1,2,3,4-tetrahy-
droisoquinoline (a) before and (b) after the adsorption on the graphite
(001) surface; electron density difference maps of indole (c) before and
(d) after the adsorption on the graphite (001) surface.
0.31e to 0.27e (Table S2†). The bond length of Ca–H1 stretched
˚
˚
Fig. 4 (a) Photocatalytic equation and conditions of the CDC reaction,
(b) reactant yield with In2O3/CuO@N-C, In2O3@N-C, CuO@N-C,
DNCPH and In2O3/CuO as photocatalysts, (c) cycling stability using
In2O3/CuO@N-C as the photocatalyst, and (d) photocatalytic perfor-
mance of In2O3/CuO@N-C in various indole substrates with different
N-aryl-tetrahydroisoquinoline derivatives.
from 1.11 A to 1.12 A. The charge redistribution and the elon-
gated bond length indicated that the interaction between Ca
and H1 was weakened and the Ca site was activated. As a result,
the activated Ca site was easily attacked by the active group and
then dehyrogenated. Similar activation also occurred when
indole was adsorbed on the graphite (001) surface. Aer
adsorption (Fig. 5d), the charge exchange between the Cb atom
and its bonding H atom decreased compared with that before
adsorption (Fig. 5c). The Mulliken charge on Cb and its bonding
H increased from ꢁ0.40e to ꢁ0.39e and decreased from 0.33e to
0.32e, respectively. The bond length of Ca–H was elongated from
performance cycling stability without any obvious loss of photo-
catalytic efficiency. The catalysts well maintained their crystal
structure and morphology aer four cycles of photocatalytic
reaction (Fig. S11†). Various indole substrates were reacted with
different N-aryl-tetrahydroisoquinoline derivatives, and the
yields of coupling target products were about 88–99% in each
case (Fig. 4d). The above results showed that the In2O3/CuO@N-
C catalysts exhibited excellent catalytic efficiency and high
stability for the CDC photocatalytic reaction.
˚
˚
1.08 A to 1.09 A. In the adsorbed indole molecule, the Cb site
was activated and can be attacked leading to dehydrogenation.
The results of the above theoretical calculation indicated that
the coated carbon layer in the In2O3/CuO@N-C hierarchical
pagoda-like micro-rods has a good adsorption and activation
effect
on
the
two
reactants
(2-phenyl-1,2,3,4-
The activation effect of the carbon layer on reactants was
investigated using the electron density difference maps of 2-
phenyl-1,2,3,4-tetrahydroisoquinoline and indole before and
aer adsorption on the graphite (001) surface. The calculated
adsorption energies for 2-phenyl-1,2,3,4-tetrahydroisoquinoline
and indole on the graphite (001) surface were ꢁ0.67 and
ꢁ0.45 eV, respectively. The corresponding geometric congu-
rations of 2-phenyl-1,2,3,4-tetrahydroisoquinoline and indole
adsorbed on the graphite (001) surface are shown in Fig. S12.†
By comparing the electron density difference maps of 2-phenyl-
1,2,3,4-tetrahydroisoquinoline before (Fig. 5a) and aer
(Fig. 5b) adsorption on the graphite (001) surface, it can be
found that the charge redistribution occurred between Ca and H
atoms. Compared with that before adsorption, the charge
exchange between Ca and H1 atoms of 2-phenyl-1,2,3,4-
tetrahydroisoquinoline aer adsorption became weaker. The
calculated Mulliken charge on Ca increased from ꢁ0.46e to
ꢁ0.42e, meanwhile, the Mulliken charge on H1 decreased from
tetrahydroisoquinoline and indole) of the CDC reaction.
Based on the above experimental and theoretical analyses,
the possible mechanism of the CDC photocatalytic reaction
catalysed by In2O3/CuO@N-C hierarchical pagoda-like micro-
rods is proposed and shown in Scheme 2. The loose lamellar
structure of In2O3/CuO@N-C hierarchical pagoda-like micro-
rods provided a structural basis for the multi-level reection
of incident light between CuO nanosheets, which was condu-
cive to improving the utilization of incident light and generated
more photo-generated electron–hole pairs. The UV-vis absorp-
tion spectra of In2O3/CuO@N-C, In2O3@N-C, CuO@N-C and
In2O3/CuO were investigated (Fig. S13†) and they veried this
conclusion. Under blue LED light irradiation, electrons were
excited from the valence band (VB) of CuO to its conduction
band (CB), leaving holes in the VB. The photo-generated elec-
trons then transferred to the CB of In2O3, due to the formation
of CuO–In2O3 p–n heterojunctions.16 In order to further inves-
tigate the migration path of photo-generated electrons in the
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order to verify the active group of this CDC photocatalytic
reaction catalyzed by In2O3/CuO@N-C composites, 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO),
a
well-known O2cꢁ
scavenger, was added to the reaction mixture under standard
conditions. Compared with the 99% yield of the product in the
absence of TEMPO, only 7.4% yield of the product was obtained
in its presence. Combined with the experimental results that
the In2O3/CuO@N-C composites generated a large number of
O2cꢁ radicals, it could be inferred that the O2cꢁ radicals were the
main active groups in this CDC photocatalytic reaction.
Conclusions
In summary, the novel In2O3/CuO@N-C hierarchical pagoda-
Scheme 2 Schematic showing the proposed mechanism of the CDC
photocatalytic reaction catalysed by In2O3/CuO@N-C hierarchical like micro-rods with effective separation of redox sites have
pagoda-like micro-rods.
been successfully synthesized by utilizing a MOF-on-MOF het-
erostructure with epitaxial relationship as the precursor. The
obtained In2O3/CuO@N-C possessed good separation efficiency
In2O3/CuO@N-C composites, the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) of the In2O3 (100)/N-doped graphite (001) interface
were calculated at the G-point (Fig. S14†). It could be found that
the electronic densities of the HOMO are almost localized
around the O atoms of In2O3, while the electronic densities of
the LUMO are attributed to the N and C atoms of graphite. This
indicated that the photo-generated electrons accumulated on
In2O3 would further migrate to the coated N-doped carbon
layer. So far, the photo-generated electrons and holes were
separated in space. The photo-generated holes on CuO nano-
sheets reacted with 2-phenyl-1,2,3,4-tetrahydroisoquinoline (Ra)
to form the radical cation Rac+ as a result of the existence of
lone-pair electrons on the N atom in Ra. However, the photo-
generated electrons on the In2O3 central axis reduced oxygen
of photo-generated electron–hole pairs, attributed to the
formation of the CuO–In2O3 p–n heterojunction. The loose
lamellar structure of In2O3/CuO@N-C hierarchical pagoda-like
micro-rods enhanced the light-harvesting capability by the
multi-level reection of incident light between CuO nanosheets.
For the CDC photocatalytic reaction, the coated N-doped carbon
layer not only ensured the effective separation and smooth
migration of photo-generated electrons and holes, but also had
a signicant activation effect on both reactants (2-phenyl-
1,2,3,4-tetrahydroisoquinoline and indole). The directional
separation of CuO (external nanosheets) and In2O3 (central axis)
as oxidation and reduction sites was important to react with 2-
phenyl-1,2,3,4-tetrahydroisoquinoline and indole, respectively,
which effectively avoids the inverse reaction of CDC. Therefore,
the obtained In2O3/CuO@N-C hierarchical pagoda-like micro-
rods exhibited excellent photocatalytic activity for the CDC
reaction.
molecules to superoxide radicals (O2cꢁ). The generation of cO2
ꢁ
in the In2O3/CuO@N-C photocatalyst was further proved by
using the N,N,N0,N0-tetramethyl-phenylenediamine (TMPD)
reagent. The principle of detection was based on the fact that
photogenerated electrons can transfer from the TMPD molecule
Conflicts of interest
ꢁ
to O2, which can produce cO2 and a blue-colored product
There are no conicts to declare.
(absorption peaks at 612 nm and 563 nm); therefore, the
amount of free radicals could be determined by detecting the
intensity of the two absorption peaks. Five samples (In2O3/
CuO@N-C, DNCPH, In2O3@N-C, CuO@N-C and In2O3/CuO)
produced characteristic absorption peaks aer reacting with
TMPD (Fig. S15†). The characteristic absorption peak intensity
of these ve samples followed the order of In2O3/CuO@N-C >
DNCPH > In2O3@N-C > CuO@N-C > In2O3/CuO, which corre-
sponded well with the trend of the separation and migration
ability of photogenerated carriers. The generated O2cꢁ attacked
the Ca site of Ra activated by the coated carbon layer and
captured the H atom on this site to form the intermediate Rac.
Furthermore, the intermediate Rac was oxidized to the imine
cation Ra4. The activated Cb site of indole due to adsorption on
the coated carbon layer accepted the photo-generated electrons
on In2O3 and dehydrogenated to produce the nucleophile
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21671085, 21701063 and 22005126), the
Natural Science Foundation of Jiangsu Province (BK20191466)
and Xuzhou City (KC20061), the Project Funded by the Priority
Academic Program Development of Jiangsu Higher Education
Institutions, the Postgraduate Research and Practical Innova-
tion Program of Jiangsu Province (KYCX20_2251), and the
Jiangsu Overseas Visiting Scholar Program for University
Prominent Yong & Middle-Aged Teachers and Presidents.
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