Engineering BiOBrxI1?x solid solutions with enhanced singlet oxygen production for photocatalytic benzylic C[sbnd]H bond activation mediated by N-hydroxyl compounds

Article abstract of DOI:10.1016/j.cclet.2021.02.006

The aerobic, selective oxidation of hydrocarbons via C[sbnd]H bond activation is still a challenge. This work shows the achievement of the room temperature visible light driven photocatalytic activation of benzylic C[sbnd]H bonds with N-hydroxysuccinimide over BiOBr_xI_1-x (0 ≤ x ≤ 1) solid solutions, whose valance bands were engineered through varying the ratio of bromide to iodide. The optimal BiOBr_0.85I_0.15 catalyst exhibited over 98% conversion ratio of ethylbenzene, which was about 3.9 and 8.9 times that of pure BiOBr and BiOI, respectively. The excellent photocatalytic activity of BiOBr_0.85I_0.15 solid solution can be ascribed to the orbital hybridization of the valence band containing both Br 4p and I 5p orbitals, which could promote photo-induced charge carrier separation and improve the generation of singlet oxygen. This work shed some light on the rational design of photocatalysts for targeted organic transformation.

Full text of DOI:10.1016/j.cclet.2021.02.006

G Model
CCLET-6135; No. of Pages 4
Chinese Chemical Letters xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Engineering BiOBrxI1ꢀx solid solutions with enhanced singlet oxygen
production for photocatalytic benzylic CꢀꢀH bond activation mediated
by N-hydroxyl compounds
*
*
Yucui Bian, Yongpan Gu, Xiaofei Zhang, Haijun Chen , Zhongjun Li
Green Catalysis Center, College of Chemistry, Zhengzhou University, Zhengzhou 450001, China
A R T I C L E I N F O
A B S T R A C T
Article history:
The aerobic, selective oxidation of hydrocarbons via CꢀꢀH bond activation is still a challenge. This work
shows the achievement of the room temperature visible light driven photocatalytic activation of benzylic
CꢀꢀH bonds with N-hydroxysuccinimide over BiOBrxI1-x (0 ꢁ x ꢁ 1) solid solutions, whose valance bands
Received 9 December 2020
Received in revised form 25 December 2020
Accepted 1 February 2021
Available online xxx
were engineered through varying the ratio of bromide to iodide. The optimal BiOBr0.85 0.15 catalyst
I
exhibited over 98% conversion ratio of ethylbenzene, which was about 3.9 and 8.9 times that of pure
BiOBr and BiOI, respectively. The excellent photocatalytic activity of BiOBr0.85 0.15 solid solution can be
I
Keywords:
ascribed to the orbital hybridization of the valence band containing both Br 4p and I 5p orbitals, which
could promote photo-induced charge carrier separation and improve the generation of singlet oxygen.
This work shed some light on the rational design of photocatalysts for targeted organic transformation.
© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
BiOBrxI1-x solid solutions
Ethylbenzene oxidation
N-hydroxysuccinimide
Photocatalysis
Singlet oxygen
Selective functionalization of benzylic sp3 CꢀꢀH bonds of
hydrocarbons with molecular oxygen (O2) plays a very important
role in pharmaceuticals and fine chemicals synthesis [1,2].
However, due to the huge resonance stabilization energy in
ground triplet state 3ƩgꢀO2, it is thermodynamically unfavorable
for O2 to directly interact with organic molecules [3]. Photo-
catalysts are usually employed to activate O2 into various reactive
oxygen species [4]. Especially, the singlet oxygen 1O2, which is an
active species with relatively mild oxidizing ability toward high
selectivity and generally associated with the synthesis of natural
products and drugs [4,5], is beneficial to selectively activating the
CꢀꢀH bonds to form oxygenated products.
via H-abstraction [12,13], which is an efficient route to achieve the
hydrocarbon oxidation under mild conditions. However, the
production of N-oxyl radicals, which is firstly induced from N-
hydroxy compounds, still requires radical initiators such as azo
compounds, metal salts, NOx, or quinone [14–16]. To tackle this
issue, heterogeneous photocatalysis is deemed as one of the most
promising way to generate the N-oxyl radicals with energy
renewable, environmentally friendly merits, which follows in
the scope of the sustainable chemistry. Whereas, up to now, only a
few specific visible light-responsive catalysts, such as C3N4, CdS,
and FeOx, could produce efficient N-oxyl radicals through oxidation
of N-hydroxyphthalimide by photogenerated holes [17–20].
Therefore, it would be significant and highly desirable to explore
other efficient photocatalytic system to regulate N-oxyl radicals
generation for activating CꢀꢀH bonds.
The direct selective activation of C(sp3)ꢀꢀH bonds has attracted
enormous attentions [6], to which great efforts have been devoted
including the use of transition metal complexes as homogeneous
catalysts and the heterogeneous catalysis with rather stringent
reaction conditions, such as high temperature and pressure [7,8],
while it is still a hot topic to selectively oxidize these inert bonds
under mild conditions due to the high bond dissociation energy of
C(sp3)ꢀꢀH bonds (70–130 kcal/mol) [9–11]. Alternatively, the CꢀꢀH
bonds can readily react withN-oxyl radicals to form carbon radicals
Solid solution materials have been intensively investigated in
photocatalysis field due to their flexible and effective modulation
of the energy band structures [21,22]. Among them, bismuth
oxyhalide solid solution, as a new type of two dimensional material
with open layered structure possessing attractive physicochemical
properties, for example, internal electric field [23], low toxicity,
adjustable optical absorptions, and enhanced charge separation,
was extensively used for photocatalytic degradation of organic
pollutants [24], reduction of CO2 [25], and elimination of NO [26].
Moreover, the regulation of band structure of solid solution could
also adjust the activation capacity of reactive oxygen species.
* Corresponding authors.
E-mail addresses: chenhaijun@zzu.edu.cn (H. Chen), lizhongjun@zzu.edu.cn
(Z. Li).
https://doi.org/10.1016/j.cclet.2021.02.006
1001-8417/© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Y. Bian, Y. Gu, X. Zhang et al., Engineering BiOBrxI1ꢀx solid solutions with enhanced singlet oxygen production for
photocatalytic benzylic CꢀꢀH bond activation mediated by N-hydroxyl compounds, Chin. Chem. Lett., https://doi.org/10.1016/j.
cclet.2021.02.006

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Chinese Chemical Letters xxx (xxxx) xxx–xxx
Inspired by the above investigations, we intended to engineer
solid solution to achieve selective benzylic CꢀꢀH bond activation
through regulating specific reactive oxygen species production
involved by succinimide-N-oxyl radicals (SINO). Herein, the
BiOBrxI1-x solid solutions with adjustable band structure and
redox potentials were used to study their photocatalytic perfor-
mance for aerobic oxidation of sp3CꢀꢀH bonds to corresponding
oxygenated products at room temperature with the assistance of
N-hydroxysuccinimide (NHSI). The possible mechanism of syner-
gistic effect of enhanced carrier separation and singlet oxygen
production by solid solution toward improved photocatalytic
performance was revealed.
The crystal phase purity of the as-prepared BiOBrxI1-x solid
solutions (denoted as BBI-x, 0 ꢁ x ꢁ 1, hereafter) was identified by
X-ray diffraction (XRD) measurements in Fig. 1a. The diffraction
peaks of the pure BiOBr (BB) and BiOI (BI), which are in keeping
with the standard XRD patterns of BiOBr (JCPDS No. 09-0393) and
BiOI (JCPDS No. 10-0445) in the tetragonal phase, respectively,
demonstrating the tetragonal structure of the BBI-x. When the
value of x decreased, the peaks of BBI-x presented a slight shift
Fig. 2. SEM, TEM, and HRTEM images of (a), (d) and (g) BB; (b), (e) and (h) BBI-0.85;
(c), (f) and (i) BI catalysts, respectively.
toward lower 2u angles, which was arising from the replacement of
bromide ions by iodide ions with a larger radius (Br: 1.95 Å vs. I:
2.16 Å). In addition, the changes in the lattice parameters of the
solid solution further confirm that iodide ions were successfully
incorporated into the BB lattice, and the interplanar lattice spacing
of BBI-x was calculated by the Bragg's law in Table S1 (Supporting
information) [27,28]. The lattice parameter c exhibited a nearly
linear relationship with the Br mole fraction (Fig. 1b). Notably, the
calculated value of c deviated from Vegard’s law, which may be
mainly due to the weakness of anion-anion interactions across the
interface between two adjacent halide anion layers [28]. These
regular movements in XRD peaks position and the variation of the
c indicate that the obtained catalysts are solid solutions rather than
a mixture of BB and BI [24].
The morphology and microstructure of the as-synthesized
BBI-x catalysts were examined by scanning electron microscope
(SEM), N2 adsorption, and transmission electron microscope
(TEM). As depicted by Fig. 2a, when none of iodide ion was
doped, the obtained BB catalyst exhibited a bent sheet-like two-
dimensional morphology, which still presented in BBI-0.85
(BiOBr0.85I0.15) catalyst when 15% molar ratio of bromine was
TEM (HRTEM) images revealed the high degree of crystallinity
and the clear lattice fringes of catalysts in Figs. 2g–i. For BBI-0.85
solid solution, the d-spacing of 0.280 nm were coincided with the
(110) plane of the tetragonal structure (Fig. 2h). Moreover, the
interplanar lattice spacing value of BBI-0.85 falls in between that of
0.278 nm of pure BB and 0.285 nm of BI (Figs. 2g, i), which further
confirms the formation of solid solution containing mixed halides.
And the energy-dispersive X-ray spectrometer mapping images
showed that the Bi, O, Br, and I elements were evenly distributed in
BBI-0.85 nanosheet in Fig. S3 (Supporting information).
The UV–vis diffuse reflectance spectra analysis was adopted to
characterize the optical properties of BBI-x solid solutions (Fig. S4a
in Supporting information). With the increasing of the content of
iodine, the optical absorption edges of the BBI-x catalysts
underwent a red shift from 440 nm of BB to 664 nm of BI. The
energy band gap (Eg) of BBI-x (x = 0, 0.2, 0.4, 0.6, 0.85, and 1.0) were
calculated to be 1.68, 1.75, 1.80, 1.96, 2.17, and 2.59 eV (Fig. S4b in
Supporting information), respectively. Besides, the conduction
band (CB) of BBI-x solid solutions can be determined by Mott–
Schottky measurements (Fig. S4c in Supporting information) [29],
and the valence band (VB) edges of catalysts could be obtained by
the formula of EVB = Eg – ECB, which were found to be 1.40,1.46,1.57,
1.75, 1.93, and 2.38 V vs. NHE for BI, BBI-0.2, BBI-0.4, BBI-0.6, BBI-
0.85, and BB, respectively (Fig. S4d in Supporting information). It
has been revealed by density functional theory calculations that
the CB of BBI-x was mainly composed of Bi 6p orbitals while the VB
was consist of Bi 6s, O 2p, Br 4p, and I 5p orbitals and the doping of
iodine into BB gave rise to upshift of the VB potential due to the
hybridization of Br and I np orbitals, which resulted in the electrons
being photo-excited more easily [24]. Accordingly, the light
absorption was broadened and the band structures regulated
through the solid solution engineering.
replaced by iodine, while BBI-0.6 (BiOBr0.6 0.4
I
), BBI-0.4 (BiO-
Br0.4I0.6), BBI-0.2 (BiOBr0.2 0.8), and BI catalysts showed spherical
I
structure assembled by the nanosheet, as illustrated in Fig. S1
(Supporting information) and Figs. 2b and c. These results
demonstrate that the content of iodine has a remarkable effect
on the assembly mode of the solid solution. Table S2 and Fig. S2
(Supporting information) showed that the surface areas of BB,
BBI-0.85, and BI were calculated to be 6.04 m2/g, 22.40 m2/g, and
29.21 m2/g, respectively. Further, the TEM images in Figs. 2d–f
clearly showed the similar nanosheet structure of BB, BBI-0.85, and
BI, which were in line with the SEM images. The high-resolution
The aerobic visible light photo-oxidation of ethylbenzene was
selected as
a model reaction in Scheme 1 to assess the
photocatalytic performance of the solid solutions. As shown in
Fig. 3a, NHSI or BBI-0.85 solid solution alone could not oxidize
ethylbenzene under photoirradiation in the presence of O2. A
Fig. 1. (a) X-ray diffraction patterns of BBI-x catalysts. (b) Variation of the lattice
parameter c of the BBI-x solid solutions with Br mole fraction.
Scheme 1. Aerobic oxidation of ethylbenzene.
2

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Fig. 3. (a) Photocatalytic oxidation of ethylbenzene over different reaction
conditions. (b) Dependences of oxidation products yield on the NHSI concentration
in the ethylbenzene oxidation over BBI-0.85 catalyst. (c) Time-dependence of
oxidation products yield over BBI-0.85. (d) Photocatalytic oxidation of ethylbenzene
over diverse catalysts. Conditions: 10 mg of catalyst, 0.1 mmol of ethylbenzene,
3 mg of NHSI, 3.0 mL of CH3CN, O2 at 1 atm, room temperature, blue LED light
(455 nm, 8 W), 5 h.
Fig. 4. (a) Photocurrent-time curves. (b) FT-IR spectra of NHSI and adsorbed NHSI
on BBI-0.85 catalyst. (c) Ethylbenzene conversions over BBI-0.85 catalyst with
different scavengers. (d) EPR spectra of 1O2 over different catalysts in the presence
of TEMP.
(Table S3 in Supporting information). The longer charge carrier
lifetime of solid solutions than that of the monocomponent ones
indicates that the hybridization of the halogen orbitals in the
valence band contributes to prolonging the lifetime of the photo-
generated charge and the BBI-0.85 catalyst has the best photo-
induced charge separation efficiency. Due to the hybridization of
different halogen np orbitals of solid solutions, the mobility of
photon-induced holes is decreased, whereas electron mobility is
not affected [24,30]. Consequently, the BBI-x solid solution
displayed considerably reduced recombination efficiency of
photogenerated electron holes and enhanced photocatalytic
performances.
The oxidation potential of the semiconductor is a key factor that
drives the chemical transformation, the more positive of the VB
petential, the higher oxidation ability of the photocatalyst. A
volcano relationship was obtained between VB potential and
photocatalytic performance of BBI-x solid solutions in Fig. S7
(Supporting information). Notably the redox potential of NHSI/
SINO was determined to be 1.1 V vs. NHE by the cyclic voltammetry
in Fig. S8 (Supporting information). Therefore, the VB potential in
the range of 1.40–2.38 V of the BBI-x solid solutions could all be
capable of oxidizing NHSI to produce SINO radical, which was the
initially reactive species for CꢀꢀH bond activation [17,18]. In
addition, the strong interaction between NHSI and BBI-0.85 surface
was clearly demonstrated by Fourier transform infrared spectros-
combination of BBI-0.85 and NHSI can catalyze ethylbenzene with
98% conversion in atmospheric O2 under 455 nm irradiation.
Nevertheless, almost no conversion occurred in the mixture of
NHSI and BBI-0.85 photocatalyst in the dark or in the absence of O2.
These experiments clearly demonstrate that BBI-0.85, NHSI, and
O2were essential to the process of photocatalytic CꢀꢀH bond
activation. Fig. 3b exhibited that the ethylbenzene conversion
initially increased with the amounts of NHSI arising, when the
quantity of NHSI reached to 3 mg, the conversion reached a
plateau. As shown in Fig. 3c, the total amounts of oxidized products
increased sharply in three hours, and the intermediate product
phenylethanol was further oxidized to acetophenone with
increasing irradiation time.
To investigate the relationship between the catalytic perform-
ances and the variation of iodide concentration, the photocatalytic
activity of a range of BBI-x catalysts (x = 0, 0.2, 0.4, 0.6, 0.8, 0.85, 0.9,
0.95,1) were investigated. As shown in Fig. 3d, the catalytic activity
initially increased and then decreased with the increasing of the
content of iodide, and BBI-0.85 photocatalyst exhibited the highest
conversion rate with 98%, which was about 3.9 and 8.9 times that
of BB and BI catalysts, respectively. The apparent quantum
efficiency (AQE) can reach 0.72% at 445 nm over BBI-0.85 catalyst.
These results indicate that the construction of solid solution can
significantly improve the photocatalytic activity.
To explore the origin of the excellent photoactivity of solid
solutions, we evaluated the photogenerated electron holes
separation efficiency of these catalysts by (photo)electrochemical
measurements. Photocurrent-time curves of BBI-0.85 exhibited a
higher photocurrent density in contrast to the other four catalysts
(BI, BBI-0.4, BBI-0.6, and BB) in Fig. 4a. And Fig. S5 (Supporting
information) showed that the diameter of the arc radius of BBI-0.85
was clearly much smaller than the other four photocatalysts in
electrochemical impedance spectroscopy measurements. The
time-resolved photoluminescence spectra was further applied to
uncover the detailed information with respect to the carrier
kinetics of the solid solutions (Fig. S6 in Supporting information).
It can be seen that the calculated average lifetime of the carriers
copy (FT-IR) tests (Fig. 4b). The clear red-shift of C O vibration to
¼
1670 cmꢀ1 of BBI-0.85 mixing with NHSI, compared with free NHSI
at 1708 cmꢀ1, signifies the adsorption of NHSI on BBI-0.85 surface,
which would facilitate the charge transfer [17]. Although, the
oxidation of NHSI to SINO radical could be smoothly proceeded
over all the solid solutions, the highest oxidation activity for
ethylbenzene over BBI-0.85 indicates the electron holes separation
was the rate determining step taking (photo)electrochemical
results into account.
Further, to get insights into the main reactive species during the
photocatalytic process, we carried out a series of radical trapping
experiments as shown in Fig. 4c. Adding 2,6-ditert-butyl-4-
methylphenol (BHT) can almost inhibit the reaction completely,
indicating a carbon-centered radical was involved, which was
firstly generated by the trigger of SINO radicals [17]. When
(
tavg = (A1t12 + A2t22)/(A1t1 + A2t2)) were 7.07
which was longer than that of BI (6.12 s), BBI-0.2 (6.58
BBI-0.4 (6.37 s), BBI-0.6 (6.29 s), and BB (6.18 s), respectively
m
s for the BBI-0.85,
s),
m
m
ꢂ
ꢂꢀ
m
m
m
O2 capture reagent of 1, 4-benzoquinone (BQ) and OH capture
3

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Chinese Chemical Letters xxx (xxxx) xxx–xxx
reagent of isopropanol (IPA) were added separately [31], we
obtained a slight decrease in ethylbenzene conversion, indicating
solution showed an enhanced photocatalytic activity with
promoted charge separation efficiency and enhanced genera-
tion of singlet oxygen. This study exposes that a rational
regulation of solid solution photocatalysts can modulate the
activation of oxygen in more challenging organic transforma-
tion reactions.
ꢂ
ꢂꢀ
that O2 and OH were not key intermediates in the reaction. As
electron capture reagents of K2S2O8 (KPS) as well as hole capture
reagents of (NH4)2C2O4 (AO) and NaHCO3 (SB) were added [32], the
ethylbenzene conversion were reduced from 98% to 71%, 62%, and
50%, respectively, verifying the more important role of hole.
Notably, the hole would directly oxidize the NHSI to produce SINO
radical and then promote the cleavage of CꢀꢀH bonds [17,19]. The
moderate inhibition role of hole scavenger suggests the SINO
radical may also be generated by the other way.
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgments
It is noteworthy that the L-tryptophan (L-Trp) can dramatically
suppress the conversion of ethylbenzene, proving that singlet
oxygen plays a vital role in the oxidation reaction (Fig. 4c).
Additionally, electron paramagnetic resonance (EPR) measure-
ments were performed with 2,2,6,6-tetramethylpiperidine (TEMP)
as a trapping agent. As shown in Fig. 4d, a 1:1:1 triplet signal with
g-value of 2.006 appeared in the presence of BBI-0.85 under
photoirradiation, in accordance with TEMP-1O2 [33,34]. The higher
singlet oxygen signal intensity over BBI-0.85 nanosheets than
those of BB and BI catalysts indicated the prominent production of
singlet oxygen. Considering that the reaction can hardly occur
without NHSI and the oxidation of NHSI to SINO radical was the
initial step in ethylbenzene oxidation, which was traditionally
considered to be triggered by holes generated in photocatalyst.
While the SINO radical still existed to promote the photo-oxidation
of ethylbenzene as evidenced by the hole capture experiments.
Therefore, we conclude that besides the oxidation of holes, the 1O2
would also oxidize NHSI to produce SINO radical with H2O2 as the
by-product, which was supported by the ultraviolet visible
spectroscopy detective experiments in Fig. S9 (Supporting
information).
This work was supported by the National Natural Science
Foundation of China (No. 21671176) and Post-doctoral Foundation
of Henan Province.
Appendix A. Supplementary data
Supplementary material related to this article can be
found, in the online version, at doi:https://doi.org/10.1016/j.
cclet.2021.02.006.
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In conclusion, BBI-x solid solution photocatalysts has been
successfully constructed for the selective photocatalytic oxidation
of sp3CꢀꢀH bonds under mild conditions. The BiOBr0.85 0.15
I
solid
4