Significantly enhanced photocatalytic performance of In2O3 hollow spheres: via the coating effect of an N,S-codoped carbon layer

Article abstract of DOI:10.1039/c9ta07891f

A typical catalytic process generally consists of diffusion, adsorption and surface reaction, which should be fully taken into account for the catalyst design. In this article, we synthesized hollow spheres (N,S-C/In₂O₃ HSs) assembled from N,S-codoped carbon layer coated In₂O₃ nanoparticles, in which the porous architecture could provide diffusion channels for reactant molecules, the N,S-codoped carbon layer could enhance the separation efficiency of photogenerated carriers, and the hollow structure could improve the amount of active sites and enhance the light utilization efficiency. The obtained N,S-C/In₂O₃ HS exhibits extraordinary photocatalytic oxidative hydroxylation of arylboronic acids under blue-light irradiation. From the theoretical calculations, it was found that the carbon layer not only strengthens the adsorption of arylboronic acid reactants, but also activates the boron atom in arylboronic acids. The present work indicates that the carbon layer could be used as a medium between inorganic semiconductor photocatalysts and organic reactant molecules, thus promoting the application scope of inorganic semiconductor photocatalysts in the field of selective organic synthesis.

Full text of DOI:10.1039/c9ta07891f

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Significantly enhanced photocatalytic performance
of In2O3 hollow spheres via the coating effect of an
N,S-codoped carbon layer†
Cite this: J. Mater. Chem. A, 2019, 7,
25423
a
Liming Sun,‡a Yusheng Yuan,‡a Rong Li,‡a Wenwen Zhan,a Xiao-Jun Wang,
b
a
*
*
Yanli Zhao
and Xiguang Han
A typical catalytic process generally consists of diffusion, adsorption and surface reaction, which should be
fully taken into account for the catalyst design. In this article, we synthesized hollow spheres (N,S-C/In2O3
HSs) assembled from N,S-codoped carbon layer coated In2O3 nanoparticles, in which the porous
architecture could provide diffusion channels for reactant molecules, the N,S-codoped carbon layer
could enhance the separation efficiency of photogenerated carriers, and the hollow structure could
improve the amount of active sites and enhance the light utilization efficiency. The obtained N,S-C/
In2O3 HS exhibits extraordinary photocatalytic oxidative hydroxylation of arylboronic acids under blue-
light irradiation. From the theoretical calculations, it was found that the carbon layer not only
strengthens the adsorption of arylboronic acid reactants, but also activates the boron atom in
arylboronic acids. The present work indicates that the carbon layer could be used as a medium between
inorganic semiconductor photocatalysts and organic reactant molecules, thus promoting the application
scope of inorganic semiconductor photocatalysts in the field of selective organic synthesis.
Received 21st July 2019
Accepted 11th October 2019
DOI: 10.1039/c9ta07891f
rsc.li/materials-a
process consists of diffusion, adsorption and surface reaction,
which should be fully taken into account for the catalyst
design.21 Bulk In2O3 with low specic surface area cannot
provide enough active sites for the reactant adsorption and
surface reaction.22,23 In addition, the adsorption force of In2O3
toward organic reactant molecules is weak, which restricts the
reaction process. Thus, these disadvantages limit the applica-
tion of bulk In2O3 in photocatalytic organic reactions.
Introduction
Visible light photoredox catalysis is a powerful tool in organic
synthesis on account of its unique mechanism, high functional
group compatibility, and mild reaction nature.1–6 Among
numerous existing photocatalysts, metal oxides have drawn
much attention owing to their low processing cost, suitable
energy bandgap, and chemical stability against photo-corrosion
even in harsh environments.7–12 In2O3 with a bandgap of 2.8 eV
has been proven as a visible-light active photocatalyst.13–17 In2O3
possesses a suitable band position to drive the necessary redox
reactions, d10 electronic conguration of In3+ ions, excellent
conductivity, and high stability in aqueous medium.18–20
However, In2O3-based photocatalysts for organic synthesis,
especially for those with high value-added products, have still
not reached their full efficiency. In general, a typical catalytic
Because phenol is a very important intermediate for many
pharmaceutical molecules, natural products and polymers24–28
and the main product of photocatalytic oxidative hydroxylation
of arylboronic acids (OHAA) is high value-added phenol, OHAA
was selected as the target reaction in this study. The carbon
coating layer on the In2O3 surface could strengthen the
adsorption of arylboronic acid reactants by the p–p stacking
interaction between the graphite ring of the carbon layer and
the aromatic ring of arylboronic acids. Moreover, the carbon
layer with high electrical conductivity could improve the
transfer of photogenerated electrons from In2O3 to the carbon
layer and enhance the separation efficiency of photogenerated
electrons and holes,29,30 leading to enhanced photocatalytic
activity.31–35
Herein, we synthesized hollow spheres (N,S-C/In2O3 HSs)
assembled from N,S-codoped carbon coated In2O3 nano-
particles by thermal treatment of the 2,1,3-benzothiadiazole
modied In(OH)(2,5-PDC) (2,5-pyridinedicarboxylic acid ¼ 2,5-
PDC) coordination compound at an optimized temperature.
The as-synthesized N,S-C/In2O3 HS possesses several favorable
aJiangsu 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
bDivision of Chemistry and Biological Chemistry, School of Physical and Mathematical
Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371.
E-mail: zhaoyanli@ntu.edu.sg
† Electronic supplementary information (ESI) available: Experimental details,
powder XRD, FTIR, TGA, SEM images, EDX, photoluminescence spectra, time
courses of oxidation hydroxylation reactions, UV-vis absorption spectra, 1H
NMR spectra, and control experiments for photocatalytic OHAA. See DOI:
10.1039/c9ta07891f
‡ These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. A, 2019, 7, 25423–25432 | 25423

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features for photocatalytic reactions. (1) A uniform and intimate and distance, it could be concluded that the adsorption of
carbon layer on the surface of In2O3 nanoparticles was achieved arylboronic acid on the graphite (001) surface is energetically
by thermal decomposition of the coordination compound, more favorable than that on In2O3 (100), (110) and (111)
because the metal ions could be uniformly separated by the surfaces, attributing to the p–p stacking interaction between
organic linker at a molecular level.36–44 (2) The hollow structure the graphite ring and the aromatic unit of arylboronic acid.
of the N,S-C/In2O3 HS is benecial to not only increase the Thus, the theoretical studies suggest that the carbon layer could
amount of active sites for photocatalytic reactions, but also gain serve as a medium to strengthen the adsorption of the aryl-
much more incident light through absorption and multiple boronic acid reactant on the N,S-C/In2O3 HS.
scattering and reection effects to enhance the light utilization
On the basis of the theoretical calculation results, the carbon
efficiency.45 (3) The porous shell of the N,S-C/In2O3 HS could layer was uniformly coated on In2O3 nanoparticles by thermal
provide channels to promote the diffusion of reactant mole- treatment of the 2,1,3-benzothiadiazole modied In(OH)(2,5-
cules into the hollow interior for photocatalysis.46,47 (4) N and S PDC) coordination compound. Thus, the obtained N,S-C/In2O3
codoping could remarkably enhance the electron mobility of HS was assembled by N,S-codoped carbon coated In2O3 nano-
the carbon layer and further improve the photogenerated particles (Scheme 1). In particular, the monodisperse
carriers.48–50 As a result, the N,S-C/In2O3 HS exhibited extraor- In(OH)(2,5-PDC) hollow structure as the template was synthe-
dinary photocatalytic OHAA under blue-light irradiation. The sized by using indium(III) nitrate hydrate (In(NO3)3$4H2O) as the
present work indicates that the carbon layer could be used as metal salt, 2,5-PDC as the coordination ligand, and 2,1,3-ben-
a medium between inorganic semiconductor photocatalysts zothiadiazole as the modulator in a mixture of N,N-dime-
and organic reactant molecules for expanding the application thylformamide (DMF) and H2O via a hydrothermal method at
ꢁ
scope of inorganic semiconductor photocatalysts in the eld of 120 ꢁC for 6 h. Aer calcination in an Ar atmosphere at 350 C
organic synthesis.
for 60 min, the N,S-C/In2O3 HS was readily obtained.
Power X-ray diffraction (XRD) was used to characterize the
crystal structure of the In(OH)(2,5-PDC) precursor. The diffrac-
tion peaks of In(OH)(2,5-PDC) basically proved its good crys-
tallinity (Fig. S2†). Fourier-transform infrared spectroscopy
(FTIR) shows the n(C–N) vibrational mode (Fig. S3†), further
indicating the coordination between In and 2,5-PDC. The
morphology of the synthesized In(OH)(2,5-PDC) precursor was
studied by scanning electron microscopy (SEM). Fig. S4a† shows
that In(OH)(2,5-PDC) possesses highly monodisperse and
regular spheres with a rough surface. The SEM image also
indicates that the spheres have a diameter of about 400–600 nm
(Fig. S5a†). The energy-dispersive X-ray (EDX) mapping of
Results and discussion
The adsorption of arylboronic acid on graphite (001), In2O3
(100), In2O3 (110), and In2O3 (111) surfaces was investigated by
the density functional theory (DFT) calculations. The corre-
sponding top view and side view of stable adsorption congu-
rations are shown in Fig. 1 and S1.† The calculated adsorption
energies for arylboronic acid adsorbing on these surfaces were
ꢀ7.40 eV, ꢀ3.08 eV, ꢀ3.22 eV, and ꢀ0.85 eV, and the distance
between arylboronic acid and these surfaces was 1.86, 2.35,
˚
2.27, and 2.59 A, respectively. Based on the adsorption energies
Fig. 1 Top view of optimized stable configuration for the adsorption of arylboronic acid on (a) graphite (001), (b) In2O3 (100), (c) In2O3 (110), and
(d) In2O3 (111) surfaces.
25424 | J. Mater. Chem. A, 2019, 7, 25423–25432
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Scheme 1 Schematic illustration showing the synthetic process of the N,S-C/In2O3 HS.
In(OH)(2,5-PDC) exhibits that In, O, C, N, and S elements are C, N, and S elements and the organic ligand 2,5-PDC having C
uniformly distributed (Fig. S4b–g†), indicating the integration and N elements could be easily pyrolyzed in situ to form the N,S-
of these elements onto the spheres. The thermal behavior of codoped carbon layer. The obtained N,S-codoped carbon layer
In(OH)(2,5-PDC) was investigated by thermogravimetric anal- could enhance the conductivity of In2O3 nanoparticles, facili-
ysis (TGA). The TGA curve (Fig. S6†) shows that the initial 10% tating effective separation of photogenerated electrons and
weight loss below 200 ꢁC corresponds to the volatilization of holes.
surface-adsorbed solvent molecules. The weight loss between
High-resolution TEM (HRTEM) and selected-area electron
300 and 400 ꢁC might be attributed to the carbonization of the diffraction (SAED) patterns were then used to obtain more
In(OH)(2,5-PDC) precursor. Based on the TGA results, the detailed structural information of the N,S-C/In2O3 HS. As shown
annealing condition for the pyrolysis of the In(OH)(2,5-PDC) in Fig. 3a, the sharp contrast between hollow interior and outer
precursor was set to 350 ꢁC in an Ar atmosphere for 1 h to shells clearly reveals the inner cavity. The wall thickness of the
completely transform it into the cubic In2O3 phase.
spheres is about 30–40 nm. The corresponding SAED pattern
Powder XRD measurement (Fig. S7†) of the N,S-C/In2O3 HS (inset of Fig. 3a) shows a set of concentric rings, matching well
product aer the calcination indicates that it belongs to the with cubic In2O3. A representative HRTEM image is presented
cubic In2O3 phase (PDF no. 00-006-0416). Field emission scan- in Fig. 3b. The surface of small In2O3 particles was coated by
ning electron microscopy (FESEM) was employed to charac- a continuous carbon layer with an average thickness of about
terize the morphology (Fig. 2a) of the N,S-C/In2O3 HS. The 2 nm. Furthermore, the HRTEM image exhibits a very clear
overall hollow sphere morphology is well preserved with a 350– interplanar distance with d-spacings of 0.25 nm and 0.29 nm,
500 nm diameter (Fig. S5b†) and the sphere surface is rough. corresponding to the (400) and (222) planes of cubic In2O3,
The magnied image (Fig. S8†) reveals that the hollow sphere is respectively. These results further conrm that the phase of
assembled by small nanoparticles. In order to further under- small particles belongs to cubic In2O3. The N2 adsorption/
stand the structural information, transmission electron desorption isotherms were used to investigate the porosity of
microscopy (TEM) was used to characterize the N,S-C/In2O3 HS. the N,S-C/In2O3 HS. The Brunauer–Emmett–Teller measure-
The bright and shade contrast in the low-magnication TEM ments indicate that the N,S-C/In2O3 HS has a surface area of
image (Fig. 2b) indicates the internal hollow nature of the N,S- about 122.5 m2 gꢀ1, and Barrett–Joyner–Halenda pore size
C/In2O3 HS. The TEM image also supports that the hollow distribution is mainly centered at about 4 nm and 6 nm
sphere is assembled by small nanoparticles. The structure and (Fig. 3c).
composition of the N,S-C/In2O3 HS were then studied by scan-
The surface chemical state and composition of the N,S-C/
ning transmission electron microscopy (STEM) and elemental In2O3 HS were further characterized by X-ray photoelectron
mapping. The STEM image (Fig. 2c) conrms the hollow spectroscopy (XPS). As shown in Fig. 3d, there are a series of
structure with a porous shell. The elemental mapping (Fig. 2d– peaks corresponding to In, O, C, N, and S elements in the survey
h) shows that In, O, C, N, and S elements are uniformly spectrum of the N,S-C/In2O3 HS. The two prominent peaks at
distributed in the entire spherical architecture. The elemental 451.6 and 444.1 eV from the high-resolution XPS of In 3d
proles (Fig. S9†) further conrm the existence of these (Fig. 3e) are assigned to In 2d3/2 and In 2d5/2 respectively,
elements. Therefore, the indium ion in the In(OH)(2,5-PDC) proving that indium oxide is only in the form of In2O3 in
sphere is converted to In2O3 nanoparticles during the pyrol- the N,S-C/In2O3 HS. Fig. 3f displays the XPS spectrum of O 1s,
ysis, while the introduced 2,1,3-benzothiadiazole containing which can be coherently tted into three peaks of 529.4, 530.9
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Fig. 2 (a) FESEM and (b) TEM images of the N,S-C/In2O3 HS and (c) STEM image and (d–h) corresponding EDX mapping of In, O, C, N, and S
elements from the N,S-C/In2O3 HS.
and 531.9 eV. The binding energy at 529.4 eV is attributed to the C]C bond (284.8 eV), C–N bond (285.7 eV), C–O bond (287.0
lattice oxygen in the In2O3 phase (OL). The medium intensity eV) and C]O bond (288.2 eV), respectively, conrming the
peak at 530.9 eV is assigned to the O2ꢀ ion in oxygen-decient codoping of N and S elements into the carbon layer. Fig. 3h
regions (OV). The intensity peak at 531.9 eV could be due to shows the XPS spectrum of N 1s with three peaks located at
chemisorbed and dissociated oxygen species (OHꢀ, Oꢀ and O2
)
398.7, 399.4 and 400.1 eV, which can be attributed to pyridinic N
ꢀ
on the surface. The high-resolution C 1s spectrum (Fig. 3g) can (398.7 eV), pyrrolic N (399.4 eV) and N-graphene (400.1 eV),
be tted into ve peaks of 284.1, 284.8, 285.7, 287.0 and respectively. N doping in the carbon layer, particularly the
288.2 eV. These peaks correspond to the C–S bond (284.1 eV), pyridinic N accounted for 43.6% of total N species in the N,S-C/
25426 | J. Mater. Chem. A, 2019, 7, 25423–25432
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