Oxygen-mediated formation of MoSx-doped hollow carbon dots for visible light-driven photocatalysis

Article abstract of DOI:10.1039/c6ta04278c

It is of great interest to modulate the photocatalytic activity of nanomaterials by varying their composition at the atomic scale and their nanostructure. Herein, we demonstrate a bottom-up approach for the synthesis of MoS_x-doped hollow carbon

Full text of DOI:10.1039/c6ta04278c

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Journal of Materials Chemistry A
PAPER
Oxygen-mediated Formation of MoSx-doped Hollow Carbon Dots
for Visible Light-driven Photocatalysis†
Received 00th January 20xx,
Accepted 00th January 20xx
Jung Hyun Park, Faizan Raza, Su-Ji Jeon, DaBin Yim, Hye-In Kim, Tae-Woog Kang, and Jong-Ho
Kim*
DOI: 10.1039/x0xx00000x
It is of great interest to modulate the photocatalytic activity of nanomaterials by varying their composition at atomic scale
and their nanostructure. Herein, we demonstrated a bottom-up approach for synthesis of MoSx-doped hollow carbon dots
www.rsc.org/
(MoSHCDs) as a photocatalyst for the visible light-driven aerobic oxidative coupling of amines. The molecular oxygen-
assisted solvothermal reaction of a MoS2 nanosheets/N-methyl-2-pyrrolidone dispersion provided MoSHCDs with a unique
hollow interior of 6-7 nm as well as with doping of a large portion of pyridinic N atoms and MoSx. As compared to typical
carbon dots not bearing hollow structure, as-prepared MoSHCDs exhibited more excellent photocatalytic activity in the
oxidative coupling reactions of various amines under visible light irradiation at 25 oC. Mechanistic investigations suggested
that doping with pyridinic N atoms and MoSx was responsible for the significantly improved photocatalytic activity of
MoSHCDs in the photocatalysis. The reaction mechanism of the oxidative coupling reactions of amines that were effectively
promoted by MoSHCDs under visible light irradiation was also fully examined.
Aerobic oxidation of amino compounds is an effective method
for producing various imines, which has been conducted in the
presence of metal catalysts under generally harsh conditions.14
Introduction
Carbon nanodots, such as carbon dots (CDs) and graphene
quantum dots (GQDs), have received tremendous attention in
diverse research fields due to their superior properties
Recently, several environmentally-benign approaches utilizing
visible light as a driving force were reported for the aerobic
oxidation of various amines into corresponding imines.15-19
These methods showed promise in the photocatalyzed
production of imines with high yields. However, to effectively
promote the aerobic oxidation of amines under visible light
irradiation, precisely designed photocatalysts are needed.
Well-defined CDs are potentially effective candidates for
photocatalyzing the aerobic oxidative coupling of amines
under visible light irradiation since they were found to strongly
absorb visible light and emit intense fluorescence with high
quantum yields in the visible range, but, there have been no
reports to date.20-22 In particular, modulating the
photocatalytic activity of CDs via doping with MoSx in this
reaction is important since MoSx can promote O2 activation, an
essential step in the aerobic oxidative coupling reaction of
amines.23
Herein, we report a novel approach for the bottom-up
synthesis of MoSx-doped hollow CDs (MoSHCDs) that can
promote the visible light-driven oxidative coupling reaction of
various amines at room temperature (rt). MoSHCDs were readily
synthesized via the molecular oxygen-assisted solvothermal
reaction of a MoS2 nanosheets/N-methyl-2-pyrrolidone (NMP)
dispersion. After full characterization of the MoSHCDs
properties, their photocatalytic activity in the aerobic oxidative
coupling reaction of amines under visible light irradiation was
investigated, and then was compared to that of typical carbon
dots without hollow structure. Additionally, in this study we
compared to conventional semiconductor quantum dots.1,
2
The physicochemical properties of CDs and GQDs can be tuned
effectively by heteroatom-doping3-5 or surface passivation with
organic and inorganic components.6-9 In this regard, numerous
efforts have been devoted to improving their quantum yield
(QY) via doping and surface passivation, and then mostly to
demonstrating their potential as an alternative to organic
fluorescent dyes in bioimaging.1 Conversely, only a few studies
have demonstrated how heteroatom doping and surface
passivation can alter the catalytic activity of carbon
nanodots.10-12 In particular, investigations regarding how
doping and functionalization can modulate the photocatalytic
activity of carbon nanodots in useful organic transformations
driven by visible light are lacking, despite the fact that their
activity could be significantly altered. Therefore, the precise
design of carbon nanodots with high photocatalytic activity for
the visible light-driven transformation of organic compounds is
still needed.
Imines are important organic compounds for the synthesis of
pharmaceuticals, agrochemicals, dyes and fine chemicals.13
Department of Chemical Engineering, Hanyang University, Ansan 426-791,
Republic of Korea
* E-mail: kjh75@hanyang.ac.kr
† Electronic Supplementary Information (ESI) available: Materials, experimental
procedures, and additional supporting data. See DOI: 10.1039/x0xx00000x
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demonstrated the mechanism responsible for the outstanding benzylamine and a 0.3 mmol portion of heptyVlaiemw AinrtieclewOnelirnee
DOI: 10.1039/C6TA04278C
photocatalytic activity of MoSHCDs in the aerobic oxidative added to the reaction mixture.
coupling of amines as well as the mechanism for this
photocatalysis.
The reaction yield was calculated using pre-synthesized imine
products from amine and aldehyde.24, 25 To synthesize imine
product, 5 mmol of amine, 5 mmol of aldehyde and 0.5 mg of
MgSO4 were added to 5 ml of CH2Cl2. The reaction mixture was
stirred for 5 h at room temperature and then MgSO4 was
removed by filtering. The solvent was vaporized using rotary
evaporator, and unreacted amine and aldehyde were
vaporized under vacuum at 60 oC. The calibration curve was
obtained using toluene as an internal standard and each imine
product to calculate the yield of the oxidative coupling
reaction of amines. Except N-(4-(tert-butyl)benzyl)-1-(4-(tert-
butyl)phenyl) methanimine, all other products exhibited a
same slope of calibration curves.
Experimental
Materials
Molybdenum
methylbenzylamine, 4-methoxybenzylamine, piperonylamine,
4-chlorobenzylamine, 2-picolyamine, 2-
disulfide
(MoS2),
benzylamine,
4-
thiophenemethylamine, 1-phenylethylamine, dibenzylamine,
benzoquinone and coumarin 153 were purchased from Sigma-
Aldrich
(USA).
4-tert-Butylbenzylamine,
2,4-
dichlorobenzylamine, and heptylamine were purchased from
Tokyo Chemical Industry (Japan). 2-Methylbenzylamine, 4-
fluorobenzylamine, tetra-n-butylammonium perchlorate, and
Ag wire were purchased from Alfa Aesar (USA) and used
without purification. Acetonitrile, N-methyl-2-pyrrolidone,
ethanol and toluene were purchased from Dae-Jung Chemicals
(Korea). Dialysis membrane (spectra/Por®, MWCO: 1,000) and
alumina membrane filter (Anodisc 47, 20nm) were provided by
Spectrum® Laboratories (USA) and WhatmanTM (USA),
respectively.
To study the effect of light power intensity and wavelength
on the catalytic activity of MoSHCDs, a xenon lamp (300 W,
Newport, USA) with a 400 nm long wave pass cut-on filter and
a liquid filter was utilized. For the study of power intensity
effect on the catalytic activity, the light intensity was varied
from 14, 70, to 210 mW/cm2 by adjusting the distance
between the reactor and the lamp. For study of light
wavelength effect on the catalytic activity, a 590 nm long wave
pass cut-on filter was equipped into the xenon lamp with a 210
mW/cm2 of power.
Preparation of MoSx-doped Hollow Carbon Dots (MoSHCDs)
Results and Discussion
Bottom-up synthesis of MoSHCDs
To synthesize MoSHCDs, bulk MoS2 (500mg) was added into N-
methyl-2-pyrrolidone (NMP, 200ml), and the resulting solution
was sonicated using a probe sonicator (140 W) for 3 h in an ice
bath. The mixture was then centrifuged at 500 g-force for 60
min, and a 75 % portion of supernatant was collected to obtain
an exfoliated MoS2 nanosheets/NMP dispersion. Then, a 20 ml
portion of MoS2 nanosheets/NMP dispersion in a two-necked
round bottom flask was heated up with stirring for 3 h at 140
oC under O2 or Ar atmosphere. After cooling down to room
temperature, the reaction mixture was filtered with alumina
membrane filter (20 nm pore), and the filtrate was
To synthesize MoSHCDs, a MoS2 nanosheets/NMP dispersion
was obtained via a simple liquid exfoliation of bulk MoS2 in
concentrated under vacuum at 100oC for
6 h. The
concentrated solution was diluted with 20 mL of water, which
was then purified via dialysis with a membrane tube (MWCO
1,000) for 1 day. The product, MoSHCDs, were lyophilized at - 40
oC overnight.
Aerobic oxidative coupling reaction of amines under visible light
irradiation
A photocatalyst (2 mg) was dispersed in 10 ml of an
acetonitrile and NMP mixture (7 : 3) in a glass vial with an open
top septa cap via light sonication. A 0.1 mmol portion of amine
and 5 l of toluene as an internal standard were added into
the catalyst solution. Then, the mixture was stirred in the
presence of molecular O2 under visible light irradiation using
two 60 W cool white LED lamps (LED SIGN ART, Korean) at
room temperature. The reaction was monitored by gas
chromatography as a function of time. After a certain period of
time, the reaction yields were calculated. For the
heterocoupling reaction of amines, a 0.1 mmol portion of
Figure 1. Real-time monitoring of MoSHCDs formation. (a) Optical and (b) fluorescence
images of a MoS2 nanosheets/NMP dispersion as a function of reaction time at 140 oC
under O2. (c) Optical and (d) fluorescence images of
a MoS2 nanosheets/NMP
dispersion under Ar gas. (e) Absorption spectra of MoSHCDs, and (f) Absorbance of
various solutions at 372 nm as a function of reaction time at 140 oC. (w/o: without)
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Figure 2. Characterization of MoSHCDs. (a) TEM image of MoSHCDs (Inset: size distribution). (b) AFM image of MoSHCDs (Inset: thickness distribution), and (c) height
profile for a red line in (b). (d) HR-TEM and (e) magnified images of MoSHCDs. (f) Lattice structure of MoSHCDs (Inset: FFT pattern).
NMP, as previously reported.26 Then, the MoS2/NMP solution
o
was heated at 140 C under O2 atmosphere. As shown in Fig.
Characterization of MoSHCDs
1a, the solution color turned very quickly into dark yellow,
which exhibited very strong fluorescence under a 365 nm
Next, we analyzed the structure of MoSHCDs using
ultraviolet (UV) lamp (Fig. 1b). However, under an Ar
atmosphere, no change of the solution color was observed,
even 3 h after heating (Fig. 1c) and no fluorescence emission
appeared (Fig. 1d). This result indicates that molecular oxygen
plays
a crucial role in the formation of MoSHCDs. For
comparison, pure NMP not containing MoS2 nanosheets was
o
also heated at 140 C for 3 h under O2. As shown in Fig. S1a,
the solution color rarely changed until 2 h, but turned light
yellow after 3 h. This solution also showed weak fluorescence
(Fig. S1b), but the obtained fluorescent nanomaterials
(denoted NMP-CDs) were typical CDs without hollow
structure, as discussed later. Pure NMP did not change color in
3 h after heating under an Ar atmosphere (Figs. S1c and S1d).
These control experiments indicated that MoS2 nanosheets
affect the structure of MoSHCDs as well as accelerate their
formation. The formation kinetics of MoSHCDs was
quantitatively monitored by measuring their absorption
characteristics, which exhibited strong absorption in the visible
range as well as in the UV region (Fig. 1e). The absorbance of
MoSHCDs at 372 nm was plotted as a function of the heating
time (Fig. 1f), showing the formation of MoSHCDs was
accelerated only in the presence of both O2 and MoS2
nanosheets. However, the absorbance of pure NMP heated at
140 oC for 3 h, in which a few NMP-CDs formed, did not
increase as much as MoSHCDs, indicating that the formation of
NMP-CDs did not effectively occur (w/o MoS2_O2 in Fig. 1f and
Fig. S2).
Figure 3. XPS analysis of MoSHCDs. XPS spectra for (a) C1s, (b) O1s, (c) N1s, (d) Mo 3d,
(e) S2p and (f) wide scan.
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Figure 4. Optical property of MoSHCDs. (a) Excitation-dependent emission of MoSHCDs,
(b) photoluminescence excitation (PLE) spectrum at an emission of 500 nm and
photoluminescence (PL) spectrum of MoSHCDs at an excitation of 400 nm.
transmission electron microscopy (TEM) and atomic force
microscopy (AFM), as shown in Figure 2. The structures were
quasi-spherical in shape with an average size of 22.13 nm,
which is larger than most of the CDs previously reported (Fig.
2a). In addition, MoSHCDs were 3.43 nm in topographical height
(Figs. 2b and 2c). They had a bright center as shown in Fig. 2d,
indicating a hollow interior. Figure 2e more clearly shows the
6.5-nm hollow interior of MoSHCDs and a 7.5-nm carbon shell.
In addition, the carbon shell had a well-defined crystal
structure with a lattice spacing of 0.297 nm (Fig. 2f), consistent
with the basal spacing of graphite. The X-ray diffraction (XRD)
spectrum of MoSHCDs reveals that the interlayer spacing of
MoSHCDs was 0.37 nm (Fig. S4), which is very close to that of
graphite.27 We also analyzed NMP-CDs obtained by heating
pure NMP under an O2 atmosphere. Interestingly, NMP-CDs
had no hollow interior (Figs. S3a and S3b) and were much
smaller in size (ca. 5 nm) with a lattice spacing (0.21 nm)
smaller than MoSHCDs. These results indicate the presence of
MoS2 nanosheets during a formation reaction led to hollow-
structured CDs.
The chemical composition of MoSHCDs was analyzed using X-
ray photoelectron spectroscopy (XPS) and was compared with
NMP-CDs. As shown in Figure 3, MoSHCDs were mainly
composed of C, N, O, S and Mo, while NMP-CDs consisted of C,
N and O atoms (Figs. S3c-S3f). As shown in the C1s spectrum of
MoSHCDs (Fig. 3a), the peaks for C=C and C-C bonds appeared
at 284.8 and 284.2 eV, respectively. In addition, the peaks
observed at 285.8, 286.4 and 287.63 eV can be assigned to C-
S/C-N/C-O, C=O and O-C=O bonds. The Mo-O chemical bond
Figure 5. Visible light-driven aerobic oxidative coupling of BA catalyzed by MoSHCDs. (a)
Reaction scheme, (b) Product yields at various reaction conditions and (c) Potential
energy diagram for MoSHCDs, O2 and BA. (d) Light power and wavelength effect on the
photocatalytic activity of MoSHCDs in the oxidative coupling reaction of BA.
was also observed at 530.2 eV (Fig. 3b). Notably, MoSHCDs had
a larger portion of pyridinic N (398.4 eV) rather than pyrrolic N
(399.9 eV), as shown in Figure 3c. However, NMP-CDs had
mostly pyrrolic N (Fig. S3e). This variation in nitrogen species
alters their photocatalytic activity in the aerobic oxidative
coupling of amines. The specific peaks of Mo4+ (228.7, 231.8
eV), Mo5+ (230.3, 233.41 eV) and Mo6+ (232.0 and 235.3 eV)
were clearly observed (Fig. 3d), indicating that the MoSHCDs
were successfully doped with Mo atoms. According to the
result of inductively coupled plasma atomic emission
spectroscopy (ICP-AES), 0.15 wt % of Mo was included in the
MoSHCDs. In the S2p spectrum of MoSHCDs (Fig. 3e), bridging
2-
S2 and apical S2- appeared at 164.9 eV.28 The additional peak
at 165.9 eV was assigned to SO2.29 The XPS analysis reveals
that the atomic ratio between molybdenum and sulfur atoms
was 1:1.17, indicating that molybdenum sulfide species were
embedded into the CDs. The several oxidation states of
molybdenum sulfide could be generated through the oxidation
of dissolved MoS2 by NMP radicals and oxygen during
solvothermal reactions.
Scheme 1. Reaction mechanism for the aerobic oxidative coupling of amines photocatalyzed by MoSHCDs.
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Table 1. Oxidative coupling reactions of various amines photocatalyzed by MoSHCDsa
Yield
(%)b
Entry
Substrate
Product
Time
1
12
95
95
2
3
12
12
94
98
4
5
6
12
12
(24)
64
(97) c
12
12
88
89
7
8
12
(24)
60
(87) c
12
(24)
50
(56) c
9
10
12
86
12
(24)
23
11
12
(11) c
12
12
94
13
67d
aAmine (0.1 mmol), MoSHCDs (2 mg), acetonitrile:NMP (7:3), cool white LED lamp (60W). bYield calculated by gas chromatography using toluene as an internal standard. cYield
for 24 h. dBenzylamine (0.1 mmol) and heptylamine (0.3 mmol) were used for the reaction.
To investigate the mechanism responsible for the formation bearing five- and six-membered heterocycles are carbonized.
of MoSHCDs, we analyzed the MoS2 nanosheets/NMP reaction We speculated that the H2O molecules could produce nano-
mixture at the middle of the reaction using nuclear magnetic bubbles at elevated temperatures higher than their boiling
resonance (NMR) spectroscopy. As shown in Figure S6, the point during polymerization/carbonization, which might create
characteristic peaks for the formation of several oxidized a hollow interior in MoSHCDs.
products of NMP such as N-methylsuccinimide, 4-
We investigated the fluorescent property of MoSHCDs, which
(methylamino)-4-oxobutanoic acid and 2-pyrrolidone were showed excitation wavelength-dependent fluorescence
observed. Reportedly, NMP can be oxidized by molecular O2 at emission (Fig. 4a). As previously reported,2, 9 this fluorescence
high temperatures to produce these oxidized products.30, 31 shift can be attributed to the presence of different surface
During the course of NMP oxidation, H2O is also produced as a states and inhomogeneity in the MoSHCD size. The MoSHCDs
byproduct. This oxidation process is significantly accelerated exhibited excitation and emission maxima at 372 and 487 nm,
by MoS2 nanosheets, resulting in the rapid production of respectively (Fig. 4b) and a quantum yield of 13%. NMP-CDs
polymers bearing five- and six-membered heterocycles as well showed similar fluorescence emission, but exhibited much
as abundant H2O in the reaction mixture. Then, these polymers weaker absorption in visible region (Fig. S5).
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MoSHCDs was lower than for the oxidation of BA,VaielwloAwrtiicnlegOntlhinee
DOI: 10.1039/C6TA04278C
electron transfer from BA to MoSHCDs. This electron transfer
Visible-light driven oxidative coupling reaction of amines
was indirectly confirmed by measuring the fluorescence
quenching of MoSHCDs by BA. As shown in Figure S8, the
fluorescence of MoSHCDs was gradually diminished with
increased concentration of BA, indicating the electron transfer
from BA to MoSHCDs. This reductive fluorescence quenching
was also observed in other fluorophores.35-37
Based on the experimental results, we proposed the reaction
mechanism for the aerobic oxidative coupling of amines
photocatalyzed by MoSHCDs (Scheme 1). After MoSHCDs absorb
visible light, the excited electron is transferred to O2 to
produce superoxide and the electron of BA is transferred to
MoSHCDs to yield the oxidized BA. These two intermediates
then react to produce benzylimine along with H2O2 as a
byproduct, which was confirmed by NMR (Fig. S9). Finally,
benzylimine reacts with another BA to produce the product
NBBA.
We investigated the effect of light power density and
wavelength on the photocatalytic activity of MoSHCDs during
the reaction of BA (Fig. 5d). As the light power density
increased from 14, 70, and to 210 mW/cm2 (> 400 nm), the
reaction yields gradually increased from 33 to 98%. As
expected, the higher density of photons can create the more
number of excited electrons in the conduction band of
MoSHCDs for triggering the reaction, leading to an increase in
the product yield. When a wavelength longer than 590 nm, in
which the absorption of MoSHCDs was significantly reduced
(Fig. 1e), was incident into the reaction solution, however, the
production yield for NBBA decreased to 34% due to insufficient
photon energy. This dependency of the product yields on light
power density and wavelength is characteristic for typical
photocatalysis. Hence, we clearly suggest that MoSHCDs
photocatalyze the aerobic oxidative coupling of BA to produce
the corresponding product. We additionally calculated the
apparent quantum efficiency of MoSHCDs for the
photocatalyzed transformation of benzylamine to the
corresponding product at 410 nm, which was 0.95%
(experimental details in SI).
The aerobic oxidative coupling reactions of various amines
photocatalyzed by MoSHCDs were investigated (Table 1). BAs
bearing electron donating groups were converted into the
corresponding products at high yields (entries 1-4). However,
the product yields of BAs with electron withdrawing groups
slightly decreased (entries 5-8). Heterocyclic amino
compounds such as pyridine-2-ylmethanamine and thiophene-
methylamine were also examined (entry 9-10); they were
converted into the corresponding products with slightly lower
yields than for BA derivatives. The 1-phenylethylamine (entry
11) was not an active substrate in MoSHCDs-photocatalyzed
oxidative coupling reaction due to steric hindrance. Secondary
dibenzylamine also highly yielded the corresponding imine
(entry 12). In addition, an oxidative cross-coupling reaction
between BA and heptylamine produced a moderate yield of
product (entry 13). These results show that MoSHCDs are
promising photocatalysts to effectively promote the oxidative
coupling reactions of various amines.
Then, we evaluated the photocatalytic activity of MoSHCDs in
the oxidative coupling reaction of amines under visible light
irradiation (Fig. 5a). As a model substance, benzylamine (BA,
10.7 mg) was used to yield a corresponding product, N-
benzylidenebenzylamine (NBBA), catalyzed by MoSHCDs (2 mg)
under a cool white LED lamp (60 W, > 400 nm). The reaction
was conducted at rt under O2 atmosphere. Figure 5b shows
that BA was converted into NBBA with high yield (97%),
indicating that MoSHCDs are very active photocatalysts in this
reaction. However, NMP-CDs (2 mg) showed much weaker
photocatalytic activity in the oxidative coupling of BA (21%
yield). This comparison suggests that doping and chemical
composition can significantly alter the photocatalytic activity
of CDs in the reaction. The much higher photoactivity of
MoSHCDs in the aerobic oxidation of amines is due to the
greater portion of pyridinic nitrogen in MoSHCDs than NMP-CDs
that have mostly pyrrolic nitrogen. Since pyridinic nitrogen is
more basic than pyrrolic nitrogen, the aerobic oxidation of
amines can be enhanced by MoSHCDs. According to the
simulation results, pyridinic nitrogen was found a more active
species for oxygen reduction reactions.32 Another important
reason for the outstanding photocatalytic activity of MoSHCDs is
MoSx doping, which facilitates O2 activation, a crucial step in
this reaction.23
To understand the reaction mechanism responsible for the
aerobic oxidative coupling of amines photocatalyzed by
MoSHCDs, further control experiments were performed. As
shown in Figure 5b, BA was rarely converted into NBBA in the
absence of MoSHCDs. In addition, the oxidative coupling of BA
did not occur without light irradiation, even in the presence of
MoSHCDs. After removing O2 by purging with Ar gas, the
product yield dramatically decreased to 5.6% in the presence
of MoSHCDs under light irradiation. This small amount of
product might be produced by coupling of benzylimine, which
could be generated by dehydrogenation of benzylamine, with
other benzylamine as reported in other literatures.15, 33 These
results clearly indicate that the oxidative coupling of BA into
NBBA was photocatalyzed by MoSHCDs under an O2
atmosphere. According to a previous report,34 O2 is converted
into superoxide by obtaining an excited electron from a
photocatalyst, and then this superoxide reacts with an oxidized
amine to produce an imine intermediate. To confirm this in the
MoSHCDs-catalyzed oxidative coupling of amine, benzoquinone
(BQ), a superoxide scavenger, was added into the reaction
mixture. As shown in Figure 5b, the production yield of NBBA
significantly decreased to 2.9%. This result indicates that the
O2 activation through an electron transfer from MoSHCDs is a
crucial step in the aerobic oxidative coupling reactions. The
plausibility for electron transfer from MoSHCDs to O2 was
confirmed by measuring the edge positions of MoSHCDs using
cyclic voltammetry (Fig. S7). As shown in the energy diagram
(Fig. 5c), the potential of the conduction band of MoSHCDs was
higher than that required for the conversion of O2 into
superoxide. Additionally, the potential of the valence band of
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12. M. Favaro, L. Ferrighi, G. Fazio, L. Colazzo, C. Di Vaentin, C.
Conclusion
Durante, F. Sedona, A. Gennaro, S. Agnoli and G.VGierwaAnroticzlzeiO, nAliCneS
DOI: 10.1039/C6TA04278C
In conclusion, we developed a unique hollow interior of
photocatalyst MoSHCDs doped with a large portion of pyridinic
N atoms and MoSx, which showed outstanding photocatalytic
activity in the aerobic oxidative couplings of various amines
under visible light irradiation at room temperature.
Mechanistic investigations revealed that modulating the type
of doping at atomic level and nanostructure was crucial for the
significantly enhanced photocatalytic activity of MoSHCDs in
visible light-driven catalysis. We expect that this chemical
approach for a control over doping and nanostructures will be
extended to designing effective photocatalysts for diverse
photocatalytic reactions.
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Acknowledgements
This work was supported by the Basic Science Research
Program (NRF-2014R1A2A1A11051877 and 2008-0061891) of
the National Research Foundation of Korea (NRF), funded by
the Ministry of Science, ICT and Future Planning. It was also
supported by the Human Resources Development program
(No.20154030200680) of the Korea Institute of Energy
Technology Evaluation and Planning (KETEP), through a grant
funded by the Korea government Ministry of Trade, Industry
and Energy.
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Conflict of Interest
The authors declare no competing financial interest.
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Journal of Materials Chemistry A
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DOI: 10.1039/C6TA04278C
Graphical Abstract
MoSx-doped hollow carbon dots exhibit outstanding
photocatalytic activity in the aerobic oxidative amine coupling
reaction at room temperature.