Air atmospheric photocatalytic oxidation by ultrathin C,N-TiO2nanosheets

Article abstract of DOI:10.1039/d0gc03784b

Herein, we demonstrate the highly efficient photocatalytic sulfide oxidation reaction under mild conditions,i.e.in air, at room temperature and in the absence of a sacrificial reagent, co-catalyst or redox mediator, by using ultrathin C,N-TiO₂nanosheets as a photocatalyst.

Full text of DOI:10.1039/d0gc03784b

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Air atmospheric photocatalytic oxidation by
ultrathin C,N-TiO2 nanosheets†
Cite this: Green Chem., 2021, 23,
1165
Xiuyan Cheng,a,b Jianling Zhang,
*
a,b,c Lifei Liu,a,b Lirong Zheng,
d
Received 9th November 2020,
Accepted 12th January 2021
Fanyu Zhang,a,b Ran Duan,e Yufei Sha,a,b Zhuizhui Sua,b and Fei Xied
DOI: 10.1039/d0gc03784b
rsc.li/greenchem
Herein, we demonstrate the highly efficient photocatalytic sulfide compounds.2,7,16,17 However, the photocatalytic activities of
oxidation reaction under mild conditions, i.e. in air, at room temp- these photocatalysts for the oxidation of sulfide reaction
11,16,18
erature and in the absence of a sacrificial reagent, co-catalyst or remain low, even on using an additional oxidant (O2
or
redox mediator, by using ultrathin C,N-TiO2 nanosheets as a other oxidants4), sacrificial reagent,17 co-catalyst19 or redox
photocatalyst.
mediator.20 It is urgent to improve the efficiency of the photo-
catalytic sulfide oxidation reaction under mild conditions, e.g.,
in air, without an additional oxidant, sacrificial reagent, co-
catalyst or redox mediator, but it still remains challenging.
Here we demonstrate the high-performance photocatalytic
oxidation of thioanisole under mild conditions, i.e., air and
25 °C under sacrificial reagent-, co-catalyst- and redox
mediator-free conditions. C,N-TiO2 nanosheets (C,N-TNS) were
designed as a photocatalyst for the reaction, which exhibits
ultrathin thickness, abundant oxygen vacancies and unsatu-
rated coordination metal active sites. Moreover, the introduc-
tion of C and N into TiO2 nanosheets confers them an
enhanced absorption ability to light, and astrengthened separ-
ation capability and transfer efficiency of photoexcited
charges. Owing to these unique features, the C,N-TNS photo-
catalyst exhibits superior activity for catalyzing the selective
oxidation reactions of thioanisole under mild conditions.
The catalyst was synthesized by solvothermal treatment of
NH2-MIL-125 (Ti8O8(OH)4(BDC-NH2)6, BDC-NH2 = 2-amino-
benzene-1,4-dicarboxylate) in N,N-dimethylformamide for 24 h
at 180 °C (see synthesis details and characterization in
Fig. S1†). It presents a nanosheet morphology, as revealed by
scanning electron microscopy (SEM) and transmission elec-
tron microscopy (TEM) images (Fig. 1a and b). High-resolution
TEM (HRTEM) images show lattice fringes with a spacing of
3.52, 2.38, 1.89 and 1.48 Å (Fig. 1c–e), corresponding to the
(101), (004), (200) and (204) planes of anatase phase TiO2
(JCPDS no. 21-1272), respectively. The selected area electron
diffraction (SAED) pattern reveals that the nanosheets have a
multi-crystal structure (Fig. S2†). The four diffraction rings
from the center can be identified to be the (101), (004), (200)
and (204) planes of anatase phase TiO2 (JCPDS no. 21-1272),
respectively. Energy-dispersive X-ray spectroscopy (EDS) images
show that Ti, O, C and N elements homogeneously distribute
The selective oxidation of sulfides into sulfoxides is an impor-
tant chemical transformation, which plays a vital role in
obtaining a series of industrially and biologically essential
organic compounds.1–3 Traditionally, the reaction is carried
out through thermal oxidation processes by the use of organic
or inorganic oxidants (e.g., hydrogen peroxide,4 tert-butyl
hydroperoxide,5 trifluoroperacetic acid, urea hydroperoxide6
and iodobenzene diacetate7). This route is restricted by low
selectivity, high energy input and release of highly toxic waste,
which lead to severe environmental and ecological impacts. In
recent years, the photocatalytic oxidation of sulfides into sulf-
oxides has received increasing attention owing to its reduced
energy consumption and environmentally benign feature.8–10
Diverse kinds of heterogeneous photocatalysts have been syn-
thesized for the photocatalytic sulfide oxidation reaction,
including semiconductors (TiO2),11 metal–organic frameworks
(MOFs),3,12 organic polymers,13 covalent organic frameworks
(COFs),14 Au38S2(SAdm)20 nanoclusters,15 and hybrid
aBeijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid,
Interface and Chemical Thermodynamics, CAS Research/Education Center for
Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, P.R. China. E-mail: zhangjl@iccas.ac.cn
bSchool of Chemical Science, University of Chinese Academy of Sciences, Beijing
100049, P.R. China
cPhysical Science Laboratory, Huairou National Comprehensive Science Center,
Beijing 101400, P.R. China
dBeijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics,
Chinese Academy of Sciences, Beijing 100049, P.R. China
eCAS Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular
Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R.
China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0gc03784b
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Fig. 1 (a) SEM, (b) TEM, (c–e) HRTEM, (f) EDS elemental mapping, (g
and h) AFM image and the corresponding height profile, and (i) XRD pat-
terns of C,N-TNS and anatase phase TiO2 (JCPDS no. 21-1272). Scale
bars: 250 nm in (a), 100 nm in (b), 1 nm in (c–e) and 200 nm in (g).
in the sample (Fig. 1f). The thickness of C,N-TNS is as thin as
∼1.5 nm, as evidenced by the atomic force microscopy (AFM)
image (Fig. 1g and h). The X-ray diffraction (XRD) pattern pre-
sents four diffraction peaks at 25.4°, 38.0°, 48.2° and 62.9°,
corresponding to the (101), (004), (200) and (204) planes of
anatase phase TiO2 (JCPDS no. 21-1272), respectively (Fig. 1i),
which is in good agreement with the HRTEM result. The
broadness of the diffraction centered at 25.4° can be ascribed
to the overlap of diffraction from graphite carbon (JCPDS no.
41-1487). The C content in the catalyst was determined to be
13.84 wt% by elemental analysis, while the N content is 1.71 wt%.
The insightful chemical and structural information of C,
N-TNS was investigated by X-ray photoelectron spectroscopy
Fig. 2 (a and b) High-resolution XPS spectra of Ti 2p and O 1s of C,
N-TNS (I) and P25 (II). (c) High-resolution XPS spectra of N 1s of C,
N-TNS. (d and e) Ti K-edge XANES spectra and k3-weighted χ(k) function
of the EXAFS spectra of C,N-TNS (I), anatase type TiO2 (II), P25 (III) and
NH2-MIL-125 (IV). (f) ESR spectra of C,N-TNS (I) and P25 (II).
(XPS). The commercial Degussa P25 TiO2 was used as a refer- 397.0 eV for Ti–N is not present, indicating the absence of Ti–
ence sample (see its characterization in Fig. S3†). In the high- N bonds in C,N-TNS.28 The high-resolution C 1s XPS spectrum
resolution Ti 2p XPS spectra of C,N-TNS (Fig. 2a), the two Ti 2p further proves the partial substitution of C for the lattice tita-
peaks of Ti 2p3/2 at a binding energy of 458.5 eV and Ti 2p1/2 nium atoms to form Ti–O–C bonds in C,N-TNS (Fig. S4†).
located at 464.2 eV belong to Ti4+, which have a red shift com- Fourier transform infrared (FT-IR) spectra also give evidence
pared with those of P25 (458.8 and 464.5 eV). It implies the for the formation of oxygen vacancies, O–Ti–N and Ti–O–C
decreased electron density and lowered valence state of Ti in bonds in C,N-TNS (Fig. S5†).
C,N-TNS, which can be ascribed to the formation of the N–Ti–
To detect the local structure of C,N-TNS at the atomic level,
O and Ti–O–C bonds.21 In the O 1s XPS spectrum of C,N-TNS synchrotron extended X-ray absorption fine structure (EXAFS)
(Fig. 2b), the blue shift of Ti–O bonds (530.4 eV) compared spectra were determined. For comparison, the EXAFS spectra
with P25 (529.7 eV) is mainly due to the introduction of of three contrast samples of NH2-MIL-125, anatase type TiO2
oxygen vacancies into the TiO2 lattice. The appearance of a and P25 were also determined. The triple-pre-edge structure
binding energy peak at 533.6 eV is attributed to CvO species combined with a double white line peak is characteristic of
or adsorption of oxygen groups on the surface of C,N-TNS. The anatase type TiO2 in anatase symmetry.29 For Ti K-edge X-ray
peak at 532.1 eV corresponds to the existence of oxygen absorption near-edge structure (XANES), the absorption edge
vacancies,22 which are favorable for catalysis by providing position of C,N-TNS is at 4983.3 eV, while those for anatase
highly accessible active sites and accelerating photogenerated type TiO2 and P25 are at 4984.3 eV (Fig. 2d). It indicates the
electron–hole pair separation and transition.23–25 For P25, the existence of Tiδ+ (δ < 4) species in C,N-TNS,30 which is consist-
band at 531.4 eV is assigned to surface hydroxyl oxygen ent with XPS analyses. The radial distribution functions
(–OH).26 In the N 1s spectrum of C,N-TNS (Fig. 2c), the two (RDFs) obtained from k3-weighted normalized EXAFS data are
peaks at 401.9 and 400.0 eV correspond to oxidized nitrogen shown in Fig. 2e. The C,N-TNS catalyst presents a main peak at
and N–Ti–O linkages, respectively.27 The characteristic peak at 1.41 Å, which can be assigned to Ti–O coordinations.31 The
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peak position at 1.41 Å of Ti–O is located between those of
NH2-MIL-125 (1.31 Å) and anatase type TiO2 (1.50 Å). It implies
the unique inner-atomic distance in C,N-TNS, which can be
attributed to the abundant oxygen vacancies and the for-
mation of Ti–O–C bonds. Moreover, the signal intensity of the
Ti–O bond decreases considerably in comparison with those of
anatase TiO2 and P25, which demonstrates the loss of the Ti–O
bond and the decreased coordination number in C,N-TNS.32
By fitting for EXAFS (Fig. S6 and Table S1†), the coordination
number of Ti in C,N-TNS is 4.1, which is less than those of
anatase type TiO2 and P25 (six oxygen atoms around one Ti
atom in anatase TiO2 and P25). Moreover, the electron spin
resonance (ESR) spectrum of C,N-TNS was determined, which
is a powerful tool to detect the spin polarized charge states in
the defective TiO2 nanostructure.33 It gives a major feature
with a g-value of 2.008 (Fig. 2f), corresponding to the electron
trapped around the oxygen vacancy,34 while the signal is
absent in P25. The abundance of oxygen vacancies in C,N-TNS
is advantageous for catalysis by providing more active sites
and accelerating charge separation and transition. Based on
all the above results, a schematic model of C,N-TNS is illus-
trated in Fig. S7.†
The photocatalytic performance of C,N-TNS was investi-
gated for the selective oxidation of thioanisole reaction. 10 mg
of C,N-TNS catalyst and 0.30 mmol thioanisole were added to
5 mL of acetonitrile. The reaction was carried out at 25 °C and
in air under a 300 W Xe lamp. No additional oxidant, sacrifi-
cial reagent, co-catalyst or redox mediator was involved. The
conversion of thioanisole and the selectivity to sulfoxide were
Fig. 3 (a) Time course for the oxidation of thioanisole of C,N-TNS (I)
and P25 (II). (b) Recycle performance of C,N-TNS at 5 h using the oxi-
dation of thioanisole as a model reaction. (c) Conversions and selectiv-
ities for the oxidations of various substrates catalyzed by C,N-TNS.
1
determined by H nuclear magnetic resonance (1H NMR) ana-
lysis, using 1,3,5-trioxane as an internal standard. For com-
parison, the photocatalytic performances of P25 for the selec-
After the reaction, the catalyst was recovered and its re-
tive oxidation of thioanisole were also studied under the same usability for the reaction was tested. There is no obvious deterio-
experimental conditions. As shown in Fig. 3a, P25 shows a ration of the conversion from thioanisole to methyl phenyl sulf-
poor catalytic performance for the selective oxidation of thioa- oxide after reusing five times, indicating the stability of C,
nisole over a prolonged reaction time. At 6 h, the conversion of N-TNS during photocatalysis (Fig. 3b). The XRD pattern and
thioanisole is only 29.3% as catalyzed by P25, with a selectivity TEM image of the reused C,N-TNS show that the catalyst can
to methyl phenyl sulfoxide of 92.2% (Fig. S8†). Remarkably, maintain its morphology and structural integrity (Fig. S10†).
there is a significant increase in thioanisole conversion as
The photocatalytic activities of C,N-TNS for the oxidations
catalyzed by C,N-TNS and the conversion of thioanisole of other sulfide derivatives were explored. The results reveal
reaches 100% at 6 h, with a selectivity to methyl phenyl sulfox- that C,N-TNS can catalyze various sulfide derivatives to their
ide of 95.0% (Fig. S9†). It is worth noting that the photo- corresponding sulfoxides with high conversions and selectiv-
catalytic performance of C,N-TNS is much better than those of ities under mild conditions (Fig. 3c and Fig. S9†). The substi-
the reported photocatalysts under an O2 atmosphere. For tution of a larger functional group (–phenyl) results in a
example, the conversion of 0.30 mmol thioanisole at 10 h is decreased conversion, which can be attributed to the steric
84% as catalyzed by 40 mg of TiO2,18 the conversion of effect of the larger group that hinders the contact of larger
0.30 mmol thioanisole at 12 h is 76.4% as catalyzed by 40 mg reactant molecules and the catalyst surface.36 The photo-
Co–N–TiO2,35 the conversion of 0.25 mmol thioanisole at 18 h catalytic performance of C,N-TNS for catalyzing the thioanisole
is 79% as catalyzed by 14.8 mg Ir–Zr MOF,12 the conversion of derivatives is superior to those of the reported catalysts,
18
35
0.2 mmol thioanisole at 6 h is 99.9% as catalyzed by 30 mg including h-LZU1 in air,14 TiO2 and Co–N–TiO2 under an
C60/g-C3N4,7 etc. Moreover, the activity of C,N-TNS is much O2 atmosphere. Moreover, the catalytic performances of C,
higher than that of the recently reported COF photocatalyst at N-TNS for the photocatalytic oxidations of benzylamine to
air, which needs 22 h for the complete conversion of 0.3 mmol N-benzylbenzaldimine and ethylbenzene to acetophenone
thioanisole as catalyzed by 10 mg h-LZU1.14 A detailed com- were detected (see experimental details in the ESI†).
parison of the photocatalytic performance of C,N-TNS with Benzylamine can be converted to N-benzylbenzaldimine com-
those of the reported photocatalysts is listed in Table S2.†
pletely at 3 h, while the conversion of ethylbenzene reaches
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100% at 24 h, with a selectivity to acetophenone of 75.0% photoluminescence (PL) spectroscopy, time-resolved fluo-
(Fig. 3c and Fig. S11 and S12†). rescence decay spectroscopy (TRFDS), photocurrent measure-
The optical properties and electronic band structures of the ments and electrochemical impedance spectroscopy (EIS). As
catalyst were determined. From ultraviolet-visible diffused shown in Fig. 4d, the PL spectrum of C,N-TNS under photo-
reflectance spectra (UV-Vis DRS), it is evident that C,N-TNS excitation at 300 nm shows a much lower emission intensity
shows a significantly enhanced absorbance than P25, with a than P25, suggesting a reduced recombination rate of electrons
long-tail light absorption in the visible light region (Fig. 4a). It and holes for C,N-TNS.41 TRFDS measurements were con-
can be ascribed to the modification of the electronic states ducted to reveal the interfacial transition of the photogene-
from the dopants C and N.37 The optical band gaps were esti- rated charge carriers (Fig. S13†). The average PL lifetime for C,
mated to be 3.31 and 3.39 eV for C,N-TNS and P25, respect- N-TNS is 2498 ps, which is much longer than that of P25
ively, from the Tauc plots converted from UV-Vis DRS (inset of (1387 ps). It indicates that C,N-TNS can offer more opportu-
Fig. 4a). Mott–Schottky measurements conducted at frequen- nities for free charges to participate in the surface photoreac-
cies of 500 and 1000 Hz reveal that the conduction band (CB) tion, thereby gaining enhanced photocatalytic activity.42 For
edges of C,N-TNS and P25 are −1.00 and −0.88 V versus Ag/ EIS experiment (Fig. 4e), the C,N-TNS catalyst exhibits lower re-
AgCl (vs. Ag/AgCl) (Fig. 4b), respectively. Such a more negative sistance and smaller radius compared to that of P25, facilitat-
flat-band potential of C,N-TNS makes it more desirable for the ing interfacial charge transfer.43 Remarkably, the photocurrent
photocatalytic oxidation of thioanisole.38–40 The valence band density of C,N-TNS is ∼10.4 times higher than that of P25
(VB) edges of C,N-TNS and P25 are calculated to be 2.31 and (Fig. 4f), further proving the greater separation efficiency of
2.51 V vs. Ag/AgCl, respectively (Fig. 4c). The narrowed band photogenerated electron–hole pairs and the less rate of charge
gap of C,N-TNS can facilitate the electron excitation from the recombination of C,N-TNS.44
valence band to the conduction band in C,N-TNS under light
irradiation and thus enhance its photocatalytic activity.
To analyze the reactive oxygen species (ROS) and their con-
tributions during the thioanisole oxidation reaction, a series
The separation capability and transfer efficiency of photo- of quenching experiments using different scavengers were per-
excited charges in the photocatalysts were investigated by formed. p-Benzoquinone (BQ) and beta-carotene were added
as scavengers to capture superoxide radicals (O2•−) and singlet
oxygen (1O2) during the photocatalytic reaction process,
respectively (Table S3†).45 The results indicate that 1O2 and
•−
O2 are the ROS for the photocatalytic oxidation of thioani-
sole. Furthermore, ESR measurements were conducted to gain
more insights into the involved ROS. 5,5-Dimethyl-1-pyrroline
N-oxide (DMPO) and 2,2,6,6-tetramethylpiperidine (TEMP)
•−
serve as trapping agents to probe O2 and 1O2, respectively
1
(Fig. S14†).45 It is revealed that O2 is the predominant ROS in
the photocatalytic oxidation process. Based on all the above
results, a possible reaction mechanism for the oxidation of
thioanisole is proposed. Electron–hole pairs are excited under
visible light irradiation. Photo-generated electrons migrate
from the VB to the CB and transfer to the surface of C,N-TNS,
and then react with the O2 dissolved in the solution to
produce 1O2 via triplet energy transfer. Thioanisole is activated
by the photo-generated holes on C,N-TNS and further oxidized
to methyl phenyl sulfoxide by the generated 1O2.
The formation of the above C,N-TNS from the solvothermal
treatment of NH-MIL-125 is interesting and the evolution
process was investigated by varying the reaction time. At a short
time of 2 h, the NH2-MIL-125@TiO2 composite is produced, pre-
senting as polyhedra in the average size of 450 nm (see XRD pat-
terns and SEM images in Fig. S15 and S16†). Pure TiO2 is
formed at 6 h and the polyhedra gradually transform into
nanosheets when the reaction time is as long as 18 h. FT-IR and
EXAFS spectra indicate that the NH2-MIL-125 solid particles
undergo a linker loss during the post-solvothermal process and
Fig. 4 (a) UV-Vis DRS spectra and band gap energies (the inset) of C,
N-TNS (I) and P25 (II); (b) Mott–Schottky plots; (c) band alignments (the
potentials were recorded vs. Ag/AgCl electrode); (d) PL spectra with the
the loss of the Ti–O bond in the samples obtained at 6, 12, 18
and 24 h gradually increases (Fig. S17 and S18†). The catalytic
activities of the above samples for thioanisole oxidation were
excitation wavelength of 300 nm; (e and f) EIS spectra and transient
photocurrent measurements under a 300 W Xe lamp (350 < λ < 780 nm). investigated under the same experimental conditions as those
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for Fig. 3, while the reaction time was fixed at 4 h. For the
samples synthesized at 2, 6, 12, 18 and 24 h, the thioanisole
conversions are 3.2%, 13.3%, 30.7%, 43.9% and 59.7%,
respectively. Obviously, the C,N-TNS obtained from the solvo-
thermal treatment of NH2-MIL-125 for 24 h has the maximum
activity to thioanisole. It can be attributed to its ultrathin
nanosheet morphology, highly accessible active sites on the
surface and abundant oxygen vacancies, which facilitate the
substrate−photocatalyst contact and promote mass transport.
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Conclusions
In summary, we designed a C,N-TiO2 nanosheet catalyst for
the photocatalytic sulfide oxidation reaction, which possesses
many particular characteristics including an ultrathin thick-
ness (∼1.5 nm), abundant oxygen vacancies, plentiful unsatu-
rated coordination metal sites on the nanosheet surface, out-
standing light absorbance, a strong separation capability of
photogenerated electron–hole pairs and weak charge recombi-
nation. Owing to these unique features, the C,N-TiO2
nanosheet photocatalyst exhibits high performance for the
photo-oxidation of thioanisole to methyl phenyl sulfoxide in
air, at room temperature and without the use of an additional
oxidant, sacrificial reagent, co-catalyst or redox mediator. The
activity and selectivity of the C,N-TiO2 nanosheet photocatalyst
for the photo-oxidation of thioanisole to methyl phenyl sulfox-
ide are much higher than those of the reported photocatalysts
under harsh reaction conditions (e.g., using an additional
oxidant, sacrificial reagent, co-catalyst or redox mediator). This
study provides new possibilities for realizing the photocatalytic
oxidation reactions under mild conditions by nanostructure
engineering of a single catalyst, which avoids the complex syn-
thesis process and phase separation that usually occur for
composite catalysts. We anticipate that the C,N-TNS photo-
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Conflicts of interest
19 H. Zhang, Q. Huang, W. J. Zhang, C. Y. Pan, J. Wang,
C. X. Ai, J. T. Tang and G. P. Yu, ChemPhotoChem, 2019, 3,
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There are no conflicts to declare.
20 H. M. Hao, Z. Wang, J.-L. Shi, X. Li and X. J. Lang,
ChemCatChem, 2018, 10, 4545–4554.
Acknowledgements
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The authors acknowledge the financial support from the
Ministry
of
Science
and
Technology
of
China
(2017YFA0403003), the National Natural Science Foundation of
China (21525316, 21673254), the Chinese Academy of Sciences
(QYZDY-SSW-SLH013) and the Beijing Municipal Science &
Technology Commission (Z191100007219009).
24 X. Y. Fan, Y. Zhang and K. D. Zhong, Nanoscale, 2018, 10,
20443–20452.
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