Hybridizing engineering strategy of non-lacunary (nBu4N)4W10O32 by carbon quantum dot with remarkably enhanced visible-light-catalytic oxidation performance

Article abstract of DOI:10.1016/j.apcata.2019.117261

This paper discloses an efficient strategy for the preparation of carbon quantum dot (CQD)- hybridized (nBu₄N)₄W₁₀O₃₂ (TBADT). A key step in this strategy is to hydrothermally treat CQD with TBABr solution to form the TBA⁺ micelle-encapsulated CQD, which acts as a cationized hybridizer to combine with the W₁₀O₃₂^4? anion to yield the target catalyst. XPS, UV–vis, PL and CV characterizations indicated that CQD hybridizer plays unique roles in improving the structural stability, redox capacity and visible light response of TBADT, especially in enhancing the stability of its photo-excited state. In the visible light-triggered selective oxidation of cyclohexane, toluene, ethylbenzene and benzyl alcohol with dioxygens in acetonitrile, the optimized hybrid containing 3% CQD shows much higher photo-catalytic activity than pure TBADT, affording ca. 21.7% conversion and 84.8% cyclohexanone selectivity in cyclohexane photo-oxidation, Additionally, the additive 2 M HCl further enhances the above hybridization effects, thereby significantly promoting the current photo-catalytic oxidations.

Full text of DOI:10.1016/j.apcata.2019.117261

AppliedCatalysisA,General587(2019)117261
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Hybridizing engineering strategy of non-lacunary (nBu4N)4W10O32 by
carbon quantum dot with remarkably enhanced visible-light-catalytic
oxidation performance
Anqun Sua,b, Mengke Chena, Zaihui Fua,⁎, Bo Yanga, Jialuo Shea, Feifei Wana, Chao Zhanga,
Yachun Liua
a National & Local United Engineering Laboratory for New Petrochemical Materials & Fine Utilization of Resources, Key Laboratory of Resource Fine-Processing and
advanced materials of Hunan Province and Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), College of
Chemistry and Chemical Engineering, Hunan Normal University, Changsha, 410081, China
b Hunan University of Medicine, Huaihua, 418000, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
This paper discloses an efficient strategy for the preparation of carbon quantum dot (CQD)- hybridized
Hybridizing molecular engineering
Decatungstates
Carbon quantum dot
Visible-light-catalysis
Molecular oxygen
Selective oxidation of organic compounds
(nBu4N)4W10O32 (TBADT). A key step in this strategy is to hydrothermally treat CQD with TBABr solution to
4−
form the TBA+ micelle-encapsulated CQD, which acts as a cationized hybridizer to combine with the W10O32
anion to yield the target catalyst. XPS, UV–vis, PL and CV characterizations indicated that CQD hybridizer plays
unique roles in improving the structural stability, redox capacity and visible light response of TBADT, especially
in enhancing the stability of its photo-excited state. In the visible light-triggered selective oxidation of cyclo-
hexane, toluene, ethylbenzene and benzyl alcohol with dioxygens in acetonitrile, the optimized hybrid con-
taining 3% CQD shows much higher photo-catalytic activity than pure TBADT, affording ca. 21.7% conversion
and 84.8% cyclohexanone selectivity in cyclohexane photo-oxidation, Additionally, the additive 2 M HCl further
enhances the above hybridization effects, thereby significantly promoting the current photo-catalytic oxidations.
1. Introduction
temperature and pressure [14–18]. Especially, decatungstate (DT)
anion, often used as the tetrabutylammonium salt (TBADT), is one of
The molecular oxygen (O2)-involved selective oxidation of organic
compounds under moderate conditions remains a very important and
formidable challenge research field because of its potential application
in major industrial chemical processes and its environmentally friendly
features. For example, the oxidation of cyclohexane by O2 is one of the
important industrial processes as its oxygenated products cyclohex-
anone and cyclohexanol (as KA-oil) are important intermediate pro-
ducts in the synthesis of artificial fibbers and fine chemicals [1,2]. This
oxidation process, however, always employs harsh reaction conditions
(423–573 K and 10–20 at m) to efficiently activate triplet oxygen mo-
lecules, only achieving ca. 4% cyclohexane conversion with less than
75% KA oil selectivity [3,4]. Many efficient approaches have been
exploited to enhance cyclohexane conversion and KA oil selectivity
under relatively mild reaction conditions [5–13]. Among these ap-
proaches, photo-catalytic oxidation technique has attracted increasing
interests as it can usually achieve the O2-involved selective oxidation of
various organic compounds including inert hydrocarbons under normal
the most efficient photo-catalysts reported so far and has found ex-
tensive application in the framework of photo-catalytic synthesis
[19–21], especially including the photo-catalysis oxidation of inactive
hydrocarbons by O2 [22–29], which should thank to its still-un-
paralleled reactivity [20]. Many experimental [30–33] and theoretical
[34–36] studies for DT photo-catalysis have revealed that the typical
mechanism is as follows: DT anion can be excited by light irradiation
(mainly UV light) to take place electron transition from its bridge
oxygen to tungsten atoms, thus generating its locally excited state. This
very unstable excited state rapidly decay in less than 30 ps to the actual
state (tagged wO), which is a relaxed excited state, probably of triplet
multiplicity. The generated wO species has a quantum yield of around
0.5–0.6, a lifetime of 65.5 ns and a redox potential of +2.44 V vs a
saturated calomel electrode (SCE) [36]. Due to its high reactivity, the
wO species can activate various organic substrates via hydrogen-atom
transfer (HAT) or single-electron transfer (SET) mechanism to partici-
pate selective oxidation based on O2 as an oxidant [30–33]. In the
⁎ Corresponding author.
E-mail address: fzhhnnu@126.com (Z. Fu).
https://doi.org/10.1016/j.apcata.2019.117261
Received 29 May 2019; Received in revised form 20 August 2019; Accepted 13 September 2019
Availableonline16September2019
0926-860X/©2019ElsevierB.V.Allrightsreserved.

A. Su, et al.
AppliedCatalysisA,General587(2019)117261
improvement of DT photo-catalysis system, many strategies are focused
on the development of heterogeneous DT photo-catalytic processes to
recover catalyst [37] and only a few reports are revolved the use of co-
catalysts [23,38] or additives [39] to improve photo-catalytic perfor-
mance of DT anion. However, the current DT photocatalysis system is
still limited to its inborn inadequacies related to the unstable structure,
unsatisfactory redox recycling, poor response to visible light and photo-
excited state lifetime. Consequently, it is highly desirable to develop a
novel hybridizing engineering (HE) strategy to comprehensively ad-
dress these shortcomings of DT as a visible-light-responsive catalyst.
Carbon quantum dot (CQD) with size below 10 nm is a typically
quasi-spherical nanoparticle, predominantly consisting of a sp2 hy-
bridized graphitic core functionalized with polar carboxyl or hydroxyl
groups on the surface [40–43]. The unique attributes of CQD such as
low cost and toxicity, broad optical absorptions, tunable fluorescence
emissions, excellent physicochemical and photochemical stability and
so on [44,45], render it very attractive for a wide range of applications,
including chemical sensing, biosensing, bioimaging, drug delivery,
photodynamic therapy, photocatalysis and electrocatalysis [46,47]. In
particular its excellent electron-accepting and -transport behaviors and
processes are rather important to the separation of photo-generated
charge carries, which have been successfully applied to significantly
enhance the photo-catalytic efficiency for the CQD-modified TiO2
[48–51], Bi2WO6 [52], Fe3O4 [53], C3N4 [54], Cu2O [55], Ag3PO4 [56]
and BiOX [57] composites. However, to the best of our knowledge, the
CQD-hybridized DT catalyst and its photochemistry are not reported so
far.
and heated at 200 °C for 5 h. After cooling to room temperature, the
carbonized solution was dialyzed for 24 h to get obtain a purified so-
lution thereof. Finally, the purified solution was dealt with freeze-
drying to obtain CQD solid. An UV–vis spectrum in Fig. S1 (Supporting
Information (SI)) indicated that CQD exhibits its typical absorption
curve and its aqueous solution can emit a strong blue fluorescence
under light illumination of 365 nm. Additionally, A TEM image in Fig.
S2a illuminated that pure CQD nanoparticles are quasi-spherical and
mainly located between 2–5 nm, in line with the previous literature
results [59,60].
2.3. Fabrication of CQD-hybridized TBADT catalysts
CQD-hybridized TBADT catalysts (tagged CQD/TBADT) were syn-
thesized via hydrothermal reaction. Typically, 0.01 mol TBABr and a
certain amount of CQD were dissolved into 6 mL deionized water under
ultrasonic wave, then, the resulting solution was transferred to a 10 mL
Teflon-lined autoclave and treated at 100 °C for 24 h under autogenous
pressure to obtain the TBA+ micelle-encapsulated CQD solution, which
was marked as solution A. Next, 6.4 g (19.4 mmol) Na2WO4·2H2O was
dissolved with 40 mL water, followed by the addition of 13.4 mL of 3 M
HCl aqueous solution, the acidified solution was heated to boiling for
5–10 min to obtain a decatungstate acid solution (marked as solution
B). The above solution A was added to the solution B at 100 °C under
continuous stirring and the brown solid was gradually precipitated from
the reaction solution. After cooling, the obtained precipitate was
filtered, repeatedly washed with water and ethanol and then dried
under vacuum at 60 °C for 24 h to yield the goal product. The hy-
bridizing amount of CQD in the hybrid catalysts was scheduled for
1–4% by simply controlling the weight ratio of CQD to TBADT and its
practical hybridizing amount estimated by TGA method was ca. 0.82,
1.96, 3.04 and 4.22 wt%, respectively (Fig. S3 and Table S1), basically
being consistent with the scheduled one. Additionally, FT-IR spectra of
CQD/TBADT samples are in Fig. S4 and a TEM image of 3% CQD/
TBADT is in Fig. S2b further supported that CQD has been successfully
introduced into TBADT.
Inspired by the above-mentioned research progresses of CQD-hy-
bridized semiconductor oxide photocatalysts and the successful devel-
opment of metal-, photosensitizer- or ligand-hybridized lacunary
polyoxometalates (POMs) [58], we guess that CQD may be used to
enhance the solar light harvesting efficiency of DT anion and the se-
paration efficiency of its photo-generated charge carries if the non-la-
cunary DT can be successfully hybridized by this larger size neutral
sensitizer. Herein, we would report a novel and practical HE strategy to
resolve the difficulties encountered in the CQD-hybridized TBADT. A
key step in this HE strategy is to hydrothermally treat the CQD with
TBABr solution to form the TBA+ cationic micelle-encapsulated CQD,
2.4. Visible-light-catalytic oxidation experiments
4−
which acts as a cationized sensitizer to combine with W10O32
anion
via electrostatic interaction to produce the target catalyst. To the best of
our knowledge, this is the first successful example of using the in-
expensive and readily available CQD to hybridize the non-lacunary
TBADT, which can show the remarkably enhanced photo-catalytic
performance for the visible light-driven selective oxidation of organic
compounds with O2. This work not only provides an effective HE
strategy for the preparation of the CQD-hybridized DT photo-catalysts,
but also opens up new voyages for the extensive application of DTs in
the field of visible light catalysis.
Visible-light-catalytic oxidation experiments were conducted on a
self-designed photo-reactor equipped with a water-cooled condenser
and an oxygen storage vessel (1 atm). A 35 W tungsten–bromine lamp
(with an UV light filter, light intensity, 535 mW/cm2) was used as a
light source. The whole lighting reaction was operated in the closed
reactor under normal temperature and pressure and its specific oper-
ating conditions and analytical method for the oxygenated products
could be found in our recent publication [39].
2.5. Characterization
2. Experimental
X-ray photoelectron spectroscopy (XPS) of the samples was mea-
sured on a VG Multi Lab 2000 system with a monochromatic Mg-Kα
source operated at 20 kV. Transmission electron microscopy (TEM)
images of the samples were obtained from a JEOL JEM-2100 trans-
mission electron microscope at an accelerating voltage of 200 kV.
Liquid UV–vis spectra of the samples in MeCN were recorded from 200
to 800 nm on UV-2450 spectrophotometer (Shimadzu, Japan) and
UV–vis diffuse reflectance spectra (DRS) of the solid samples were re-
corded from 200 to 800 nm using an UV–vis spectrophotometer (UV-
3310) with BaSO4 as a reference. Transmission FT-IR spectra of the
samples were recorded from 400 to 4000 cm−1 on a Nicolet Nexus
510 P FT-IR spectroscopy using a KBr disk. Photo-luminescence (PL)
measurements of the samples in MeCN were carried out on fluorescence
spectrophotometer (HITACHI F-7000) at room temperature. Cyclic
voltammetric (CV) experiments of the samples in MeCN were measured
with an electrochemical analyzer (CHI 650e Chenhua Instrument
2.1. Material and sample preparation
All chemicals were analytical grade and used without further pur-
ification. Sodium tungstate (Na2WO4·2H2O), tetrabutylammonium
bromide (TBABr), ethylenediamine, citric acid, cyclohexane, toluene,
ethylbenzene, benzyl alcohol, acetonitrile (MeCN) were purchased from
Sinopharm and used without further treatment. Distilled water was
used throughout this experiment.
2.2. Fabrication of CQD
CQD solid was fabricated according to Zhu et al.’s previous work
followed by freeze-drying [59]. In general, 1 g citric acid and 335 μL
ethylenediamine were dissolved in 10 mL deionized water, then the
resulting solution was transferred into 25 mL Teflon-lined autoclave
2

A. Su, et al.
AppliedCatalysisA,General587(2019)117261
Table 1
Visible light-driven oxygenation of cyclohexane with O2 catalyzed by CQD-hybridized TBADT samples in various media.a
b
Entry
CQD hybridizing amount (wt%)
Additive (mL)
MeCN (mL)
Conv. (mol%)
Selectivity of productsc (%)
Cyclohexanol
Cyclohexanone
1
2
3
4
–
–
–
–
–
–
–
–
5.5
5.5
5.5
5.5
5.5
5.5
5.5
5.0
5.4
4.9
4.9
4.9
4.9
4.9
8.50
26.67
16.93
16.45
15.25
10.96
26.34
23.65
14.71
22.65
11.68
25.30
27.30
31.94
22.29
73.33
83.07
83.55
84.75
89.04
73.66
76.35
85.29
77.35
88.32
74.70
72.70
68.06
77.71
1.0
2.0
3.0
4.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
17.40
20.47
21.73
20.01
8.35
10.82
24.51
24.96
30.30
29.31
25.44
24.49
25.97
5
6d
7e
8
H2O (0.5 mL)
12 M HCl (0.1 mL)
2 M HCl (0.6 mL)
2 M H3PO4 (0.6 mL)
2 M H2SO4 (0.6 mL)
2 M HAc (0.6 mL)
2 M PhSO3H (0.6 mL)
9
10
11
12
13
14
a
Cyclohexane (1.0 mmol), catalyst (0.012 mmol), O2 (1 atm), temperature (25 °C), time (12 h), using 35 W of tungsten–bromine lamp as visible light source.
Cyclohexane conversion = (∑content (mmol) of each product measured via GC analysis/adding cyclohexane amount (mmol)) × 100%.
Product selectivity = the content of this product/∑content (mmol) of each product × 100%.
The mixture of 3% CQD and TBADT without any treatment.
3% CQD/TBADT samples obtained under hydrothermal treatment for 12 h.
b
c
d
e
Company). Thermogravimetric analysis (TGA) of the samples was
performed on a NETZSCH-STA 409PC at 20 °C/min heating rate under a
flowing air atmosphere (10 mL/min).
a slight improvement in the conversion and selectivity (Entry 7) owing
to its low practical hybridizing amount of CQD (See Fig. S3 and Table
S1). These findings support that only the CQD inside TBADT can play a
unique role in improving photo-catalytic performance of TBADT. En-
tries 8–14 list the data for the 3% CQD/TBADT-catalyzed this photo-
oxidation in the presence of water or various acidic additives. The ad-
ditive water or 12 M HCl slightly promotes the current photo-catalytic
oxidation, achieving about 24.5–25.0% conversion (Entries 8–9). No-
tably, a mixed solution of the above two additives (tagged 2 M HCl)
may further promote this photo-catalytic oxidation, providing ca. 30%
conversion and as high as 88% cyclohexanone selectivity (Entry 10).
Among the tested other acidic solutions, 2 M phosphoric acid has a si-
milar promoted effect to 2 M HCl for this photo-catalysis oxidation, but
it causes the selectivity of cyclohexanone to drop by 10% (Entry 11).
Other three 2 M acidic solutions are far behind 2 M HCl in improving
cyclohexane conversion and cyclohexanone selectivity (Entries 12–14).
The promotion effect of the additives water and especially 2 M HCl
solution on the DT photo-catalysis oxidation system has been studied in
detail by us using UV–vis and PL spectra and CV measurements, which
is likely due to the following contributions of these two additives [39]:
i) Water plays important roles in stabilizing the structure of DT anion
under light illumination and in improving the redox cycling of DT anion
in MeCN; ii) 12 M HCl solution can enhance the oxidative capacity and
redox cycling of DT anion as well as stabilize the photo-excited states of
DT anion. iii) The co-existence of such two additives plays a good sy-
nergistic role in promoting the current photo-catalytic oxidation. Based
on the current reaction results, the promotion effect of these additives
on the photo-catalytic oxidation performance of CQD/TBADT should
also originate from the aforementioned contributions.
In the following experiments, the influence of various reaction
parameters on the above photo-catalysis oxidation was checked using
the best 3% CQD/TBADT as a catalyst and the results are shown in
Fig. 1. Fig. 1a illuminates that cyclohexane conversion, as expected,
continuously increases from 13.0 to 21.7% when the dosage of catalyst
in pure MeCN is enhanced from 0.3 to 1.5% (Fig. 1a), along with a
gradual increase in cyclohexanone selectivity, from 79.6 to 86.6%.
Thereafter, a further increase in catalyst dosage results in a slightly
reduction in the conversion, which should be due to the fact that the
solubility of the catalyst in MeCN becomes worse at its high dosage,
thus leading to reduced light permeability. Fig. 1b shows that when
illumination time is between 6 and 12 h, the conversion is continuously
and remarkably enhanced from 20.3 to 30.3% with an increase of the
3. Results and discussion
3.1. Photocatalytic oxidation
The photo-catalytic performances of TBADT and its hybrid catalysts
with different hybridizing amount of CQD were assessed using the
oxidation of cyclohexane with molecular oxygen (O2) in MeCN under
visible light illumination and the results are listed in Table 1. As we
previously reported [39], TBADT shows low activity for this photo-
oxidation, only affording 8.5% cyclohexane conversion with about 73%
cyclohexanone selectivity under sustained exposure to visible light for
12 h (Entry 1). Entries 2–5 show that cyclohexane conversion and se-
lectivity are significantly enhanced upon these CQD-hybridized TBADT
catalysts and such enhanced effect is gradually strengthened with in-
creasing the hybridizing amount of CQD. For example, the highest
conversion of 21.7% is achieved over 3% CQD/TBADT and the highest
selectivity for cyclohexanone (89%) is obtained over 4% CQD/TBADT.
Notably, the carbon balance analysis and the HPLC analysis of the
photocatalytic oxidation of cyclohexane, cyclohexanol and cyclohex-
anone (see SI) indicate that the further oxidation of the generated KA
oils in the current photo-catalytic system is almost negligible, but can
not when KA oils are substrates alone. These findings support that the
current photo-catalytic oxidation system has an excellent selectivity for
KA oils, in consistence with the results reported by Molinari and co-
workers [61]. The possible reason why cyclohexane is oxidized with a
much higher probability than its oxygenated products KA oils in its
practical photo-reaction system is that it has a particularly high con-
centration and can provide twelve identically oxidizable C–H bonds.
The control experiments using a variety of monochromatic visible light
as an external source of illumination described in SI indicated that this
photo-catalytic oxidation is only driven by a violet light at 400 nm,
which is consistent with the absorption curves of these two catalysts in
the visible region (See Fig. 4C). TBADT and 3% CQD/TBADT achieve an
apparent quantum efficiency (AQE) of 4.4% and 8.3%, respectively,
under violet light. Entry 6 shows that the direct addition of 3% CQD
into TBADT photo-catalytic system is invalid to improve this oxidation.
And a control sample CQD/TBADT prepared using the mixed solution of
3% CQD and TBABr under 12 h of hydrothermal treatment only exhibits
3

A. Su, et al.
AppliedCatalysisA,General587(2019)117261
Fig. 1. Influence of various parameters on the 3% CQD/TBADT-photocatalyzed oxidation of cyclohexane (1.0 mmol) by O2 in MeCN. (a) Influence of catalyst
concentration (other conditions: MeCN, 5.5 mL; illumination time, 12 h); (b) Influence of illumination time (other conditions: catalyst, 0.012 mmol; MeCN, 4.9 mL;
2 M HCl, 0.6 mL); (c) Influence of 12 M HCl amount (other conditions: catalyst, 0.012 mmol; MeCN, 4.9 mL; water, 0.5 mL; illumination time, 12 h); (d) Influence of
water amount (other conditions: catalyst, 0.012 mmol; MeCN, 5.3-4.2 mL; 12 M HCl, 0.1 mL; illumination time, 12 h).
time. After that, it slowly increases to 35.8% with further prolonging
time to 24 h. On the whole, the selectivity of cyclohexanone is im-
proved slightly by extending illumination time. Fig. 1c shows the effect
of HCl amount on photo-oxidation, when the amount of HCl is raised
from 0.12 to 1.2 mmol, cyclohexane conversion ascends from 23.0 to
30.3%. After that, the conversion is slightly reduced with the further
increase of HCl amount, which is different from our previous report that
the conversion over pure TBADT rapidly decreases when HCl amount is
higher than 1.2 mmol [39]. This indicates that the CQD-hybridized
TBADT is more tolerant to the additive HCl compared to pure TBADT,
which may originate from the interactions of CQD with HCl, as sup-
ported by UV–vis spectra in Fig. S7. Cyclohexanone selectivity slowly
ascends from 78.9 to 88.3% with the amount of HCl, which should be
due to the enhanced redox capacity of the catalyst under protonation of
HCl [39]. Fig. 1d displays that when the addition amount of water is
enhanced from 0 to 0.5 mL, the conversion continuously rises from 21.4
to 30.3%. Thereafter, an attempt to further enhance water amount to
1.2 mL contrarily results in a significant reduction in the conversion,
along with a decrease in cyclohexanone selectivity. This may be be-
cause an excess amount of water leads to a remarkable decrease in
catalyst solubility, on the other hand, it may cause the leaching of the
water-soluble CQD from the hybrid catalyst.
Fig. 2 is the irradiation time-dependence of cyclohexane conversion
for two TBADT and 3% CQD/TBADT photo-catalytic oxygenation sys-
tems. Within the irradiation time range of the investigation (6–36 h),
the conversion in the TBADT-MeCN system continuously increases with
the time until about 30 h. After that, the conversion barely varies with
further extending the time. Notably, the conversion in the 3% CQD/
TBADT- MeCN system not only increases quickly with the time in the
Fig. 2. A comparison of irradiation time-dependence of cyclohexane conversion
for two photo-catalytic oxygenation systems (catalyst amount, 0.012 mmol;
cyclohexane, 1 mmol). 1: TBADT in pure MeCN (5.5 mL); 2: TBADT in MeCN
(4.9 mL) with 2 M HCl (0.6 mL); 3: 3% CQD/TBADT in pure MeCN (5.5 mL); 4:
3% CQD/TBADT in MeCN (4.9 mL) with 2 M HCl (0.6 mL).
early stage of photoreaction (6–12 h) but also always maintains an in-
creasing trend within the examined illumination time, implying that the
hybridizing of CQD probably improves the photo-catalytic activity and
stability of TBADT under photo-reaction conditions. When 2 M HCl as
an additive is introduced into these two photo-catalytic systems, the
4

A. Su, et al.
AppliedCatalysisA,General587(2019)117261
Table 2
clearly in the hybrid sample (Fig. 3c–d) and their BEs have different
degree of shifts compared to those of pure CQD (Fig. S5). In addition,
the carbon content of this hybrid sample by XPS analysis is about 1%
higher than pure TBADT (see Table S2). These findings support the
successful doping of CQD and its strong interaction with TBADT
[57,60]. Notably, another hybrid sample with hybridizing 3% CQD
hardly presents the XPS signals of W4f in its survey XPS spectrum (Fig.
S5). We can draw the following conclusions from the present XPS re-
sults: i) The change in the binding energies of the W and O atoms of DT
anion and the C and N atoms of CQD in the hybrid sample indicates that
CQD interacts with DT anion and the existence of this interaction
provides an evidence that larger size of CQD (2–5 nm) can be in-
troduced into TBADT, in consistence with the TEM, TGA and FT-IR
results shown in Figs. S2–S4. ii) It is likely that the CQD inside TBADT
are tightly attached around DT anions mainly through the actions of
their polar groups, so that the detection for the W atoms of DT anions
through XPS surface analysis technique becomes very difficult at high
hybridizing amount of CQD. And iii) the hybridizing of CQD can render
the W and O atoms of TBADT an enhanced electropositivity and elec-
tronegativity, respectively, such a enhanced ionic bonding capacity
between W and O atoms should help to improve the structural stability
of CQD/TBADT [67], as supported by the accompanying UV–vis
spectra.
Visible light-driven oxygenation of other substrates with O2 catalyzed by 3%
CQD/TBADT in MeCN.a
Entry Substrate
2 M HCl
(mL)
Conv.(mol
%)
Main products (Selectivity/
%)
1
2
Toluene
Toluene
Ethylbenzene
Ethylbenzene
Benzyl alcohol
–
0.6
–
0.6
–
6.35
PhCHO(94.54), BA (5.46)
PhCHO (88.51), BA(11.49)
α-PEA(9.23), ACP(90.77)
α-PEA(21.29), ACP(78.71)
PhCHO(69.96), PhCOOH
(30.04)
11.63
11.92
20.42
56.90
3b
4b
5
6
a
Benzyl alcohol 0.6
63.63
PhCHO(59.29), PhCOOH
(40.71)
Substrate (1.0 mmol), catalyst (0.012 mmol), acetonitrile, O2 (1 atm),
temperature (25 °C), time (12 h), using 35 W of tungsten–bromine lamp as
visible light source.
b
The calculated substrate conversion based on the main oxygenation pro-
ducts; α-PEA and ACP presented phenethylalcohol and acetophenone, respec-
tively.
conversion rapidly ascends with the time in the early stage of photo-
reaction (6–15 h), but its increasing becomes remarkably slow in the
late stage of photo-reaction and this slowing trend is especially out-
standing upon TBADT. This indicates that 2 M HCl solution as an acidic
additive can further improve the photo-catalytic activity of the two
catalysts but has no significant role in enhancing their stability under
reaction conditions, as supported by the following UV–vis spectra.
Finally, the visible-light-catalytic oxidation of other organic sub-
strates such as toluene (PhMe), ethylbenzene (EB) and benzyl alcohol
(BA) by O2 in MeCN was examined using 3% CQD/ TBADT as a re-
presentative catalyst. As shown in Table 2. The current photo-catalysis
system is still effective to the oxidation of such three aromatic com-
pounds whether 2 M HCl solution is present or not, respectively pro-
viding BA and benzaldehyde (PhCHO) for PhMe, acetophenone (ACP)
and α-phenethylalcohol (α-PEA) for EB, PhCHO and benzoic acid
(PhCOOH) for BA as the main oxygenated products. The reactivity of
three aromatic substrates over this hybrid catalyst presents an in-
creasing sequence of BA > EB > PhMe, which just coincides with
their gradually decreasing side chain C–H bond energies [62]. Also, the
photo-catalysis oxidation of these substrates is promoted dramatically
in the presence of 2 M HCl solution, resulting in an almost doubled
conversion for PhMe or EB. These results obtained under the visible-
light-catalysis of such hybrid catalyst are much better than those pre-
viously reported in pure TBADT photo-catalysis [39].
3.2.2. UV-vis spectra
Liquid UV–vis absorption spectra of TBADT and 3% CQD/TBADT in
MeCN were carefully studied and the results are shown in Fig. 4. Fig. 4a
displays that TBADT with low concentration exhibits two characteristic
bands in 250–350 nm in its UV–vis spectrum, in agreement with the
previous works [68,69]. The band at 320 nm is ascribed to oxygen to
tungsten charge transition (LMCT) of four linear WeOeW bridge bonds
in the W10O324− structure [70] and another weak band at 265 nm likely
corresponds to LMCT process of the unstable structural subunit
2−
6−
[W5O16
]
[71] or the keggin structural [H2W12O40
]
[72]. An
UV–vis spectrum of the hybrid sample (see curve 2 in Fig. 4a) only
exhibits the two LMCT bands of TBADT, but two characteristic bands of
pure CQD at 240 and 344 nm shown in Fig. S1, which are respectively
assigned to π-π* transition of an polycyclic aromatic C]C systems and
n-π* transition of the C]O band or other related groups [73–75], are
hardly found in this hybrid sample as they overlap with those belonged
to TBADT. Notably, the hybridizing of CQD does not affect the centered
wavelength of these two LMCT bands, but it improves the strength of
the structural band at 320 nm. A ratio (R = I320
/ I265 nm) for the
nm
integral areas of such two LMCT bands may reflect the quality of
TBADT [67] and the calculated R values for pure TBADT and its hy-
brids-containing 1–4% CQD (Fig. S6) are ca. 0.72, 0.74, 0.75, 0.77 and
0.72, respectively (Table S4), indicating that the quality of TBADT is
slightly and gradually improved with the hybridizing amount of CQD
and is optimal at a 3% hybridizing amount, consistent with the above
reaction and XPS results. According to the literature proposed method
[76,77], the energy bandgap (Eg/eV) of these two liquid samples in
MeCN was obtained by treating liquid UV–vis spectral curves of Fig. 4a
(see Fig. 4b) and the obtained Eg values of TBADT and 3%CQD/TBADT
in pure MeCN are 3.06 and 2.99 eV, respectively, which are basically
consistent with the Eg values measured by the UV–vis DRS spectra of
these two solid samples (see Fig. S9 and Table S3), indicating that CQD
hybridization only slightly decreases the Eg of TBADT. Fig. 4c illus-
trates that TBADT itself in MeCN has a very weak response to the visible
light of about 400 nm at its high concentration (curve 1). A high con-
centration of 3% CQD/TBADT exhibits an obviously enhanced visible
absorption in 400–800 nm and its absorption curve clearly displays a
tail extending out into the visible to near-IR regions arisen from CQD
[78] (curve 5), suggesting that CQD may play a role in utilizing full
spectrum of sunlight for photo-catalytic system [57,79]. The influence
of the additives water and 12 M HCl on the visible spectra
(400–800 nm) of these two samples were also studied. As shown in
3.2. Study on the mechanism of CQD hybridizing
3.2.1. XPS
The chemical states for the surface atoms of pure TBADT, CQD and
two typical hybrid samples respectively hybridizing 1 and 3% CQD
were investigated by X-ray photo-electron spectroscopy (XPS) and the
high resolution XPS spectra for the surface W, O, C and N atoms of
TBADT and 1% CQD/TBADT are shown in Fig. 3 (the survey XPS
spectra of four samples, the high resolution XPS spectra for CQD and
3% CQD/TBADT and the composition of the surface elements for 1%
CQD/TBADT and TBADT could be found in Fig. S5 and Table S2). As
shown in Fig. 3a, the two energy peaks for W4f5/2 and W4f7/2 in pure
TBADT are located at 37.06 and 34.94 eV, respectively, and these two
peaks in the hybrid sample presents a shift of about 0.6 eV toward a
higher energy region. The O1s peak in TBADT appears at 531.17 eV and
it in the hybrid sample is located at a low energy region of 530.27 eV
(Fig. 3b). This indicates that CQD hybridization can result in an in-
crease in the binding energy (BE) of W atoms but a decrease in the BE of
O
atoms. Additionally, two C1s signals assigned to the C]O
(286.21 eV) and C–N (O) (285.41 eV) of CQD [63,64] and one N1s
signal arisen from the C(O)-NH (401.84 eV) of CQD [65,66] are found
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AppliedCatalysisA,General587(2019)117261
Fig. 3. High resolution XPS spectra of pure TBADT and 1% CQD/TBADT.
curves 2–4 and 6–8 of Fig. 4c, the absorption curves of the two samples
in 400–800 nm rarely vary in the presence of water (curves 2 and 6) but
can obviously rise under the action of 12 M HCl (curves 3 and 7), and
this elevating effect is more pronounced upon the CQD-hybridized
sample. Notably, curves 4 and 8 show that this elevating effect of 12 M
HCl is obviously weakened when it is diluted with water to 2 M con-
centration. Additionally, Fig. S7 illuminates that the effect of these
additives on the visible spectrum (400–800 nm) of pure CQD in MeCN
presents the similar rules as described above. These findings suggest
that water as an additive only exerts weak hydrogen bond interactions
on TBADT or CQD, and HCl shows strong interactions with TBADT or
CQD by its protonation [35], which can significantly enhance the
TBADT and especially CQD/TBADT photo-catalytic systems to use
visible light. Table S3 shows that the Eg values of TBADT and 3% CQD/
TBADT in MeCN are slightly reduced by about 0.02 eV in the presence
of 2 M HCl or water and decreased by about 0.04 eV in the presence of
12 M HCl, indicating that the influence of these additives on the energy
bandgap of TBADT is almost negligible.
structural band of 3% CQD/TBADT is similar to our previously reported
TBADT system [63], but the hybridizing of CQD is obviously slow down
the light illumination-driven decay of TBADT’s structural band at
320 nm in pure MeCN, as supported by the conversion curve 3 in Fig. 2.
Additionally, the hybridizing of CQD can significantly prevent 12 M HCl
from damaging the structural band. The quality of 3% CQD/TBADT and
TBADT under the condition of simulating real photo-reaction was es-
timated using the relative intensity of the structural band (R = I320 nm
/
I265 nm) from the UV–vis spectral curves in Figs. 4d and S8 and the
obtained R values are listed in Table S5. It is seen from in Table S5 that
these R values can well reflect the above influence rules of the reaction
media on the stability of both DT catalysts under light illumination.
These findings further support that the hybridization of CQD can make
TBADT have a more stable structure and greater tolerance to the ad-
ditive HCl, thus showing a significant improvement in photo-catalytic
activity in MeCN, especially in the presence of 12 M HCl and water
additives.
We previously reported that TBADT in pure MeCN is structurally
unstable and prone to degradation to other unstable subunits under the
condition of simulating real photo-reaction [39,67]. And the presence
of water can significantly inhibit this degradation owing an unique
stabilization on TBADT [80], but 12 M HCl can promote this degrada-
tion due to its protonation. In order to study whether the hybridizing of
CQD can improve the structural stability of TBADT under photo- re-
action condition, the UV–vis spectral change of 3% CQD/TBADT in
MeCN was monitored after irradiation with uninterrupted visible light
for 12 h and the results are shown in Fig. 4d (the UV–vis spectral change
of pure TBADT recorded under the same conditions could be found in
Fig. S8). Fig. 4d shows that the effect of visible light illumination on the
3.2.3. Photoluminescence (PL) spectra
PL spectral technique was used to study the photo-catalysis systems
of TBADT and its hybrids. As shown in Fig. 5a, TBADT displays a strong
and broad PL signal at 350–550 nm in MeCN under 320 nm light ex-
citation. The hybridizing of CQD does not change the maximum emis-
sion wavelength of this PL peak, supporting that it hardly affects the
energy bandgap of TBADT, consistent with the above UV–vis spectral
results. But the hybridizing of CQD results in a remarkable decay of this
PL signal and such decay tendency is becoming apparent with the in-
crease of CQD hybridization, indicating that it can significantly reduce
the recombination probability of the photo-generated electron-hole
pairs of TBADT owing to its excellent electron-accepting and -transport
6

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AppliedCatalysisA,General587(2019)117261
Fig. 4. (a) UV–vis spectra of TBADT and 3% CQD/TBADT (1.2 × 10−4 M) in MeCN. (b) The λabs of TBADT and 3% CQD/TBADT. (c) The visible amplified absorption
edges of TBADT (1–4) and 3% CQD/TBADT (5–8) with high concentration (2.2 × 10−3 M) in various media. (d) 3% CQD/TBADT in various media containing
1 mmol cyclohexane after visible light irradiation for 12 h.
behaviors [60]. The effect of the additives on the PL intensity of DT
anion was further investigated using 3% CQD/TBADT as re-
signal of this hybrid than that of TBADT (Figs. 5b vs S10). Additionally,
Fig. S11 shows that pure CQD, under 320 nm light excitation, also ex-
hibits a strong fluorescence emission with a central wavelength of
450 nm in MeCN-water and especially pure water, in consistence with
the previous reports [59]. Under the action of 12 M HCl, the PL signal of
CQD in MeCN-water is obviously enhanced and takes place blue-shift of
25 nm, and this enhanced effect of 12 M HCl may be weakened when it
a
presentative sample, as shown in Fig. 5b, the PL intensity of this hybrid
sample is slightly enhanced in the presence of water but reduced in the
presence of 2 M and especially 12 M HCl, which is consistent with our
previously reported results in pure TBADT photo-catalysis system [67].
Notably, HCl solution can exert a stronger quenching effect on the PL
Fig. 5. (a) PL spectra of TBADT and CQD/ TBADT hybrids (3.0 × 10−4M) in pure MeCN. 1: Pure TBADT; 2–5: Corresponding to 1, 2, 3 and 4% CQD/TBADT hybrids.
(b) PL spectra of 3% CQD/TBADT (3.0 × 10−4M) in MeCN containing additives. 1: pure MeCN; 2: MeCN (5.0 mL) and H2O (0.5 mL); 3: MeCN (5.4 mL) and 12 M HCl
(0.1 mL); 4: MeCN (4.9 mL) and 2 M HCl (0.6 mL).
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AppliedCatalysisA,General587(2019)117261
3.3. Proposed mechanism of CQD hybridization
3.3.1. Preparation mechanism
In order to explore the preparation mechanism of CQD-hybridized
TBADT, the mixed solution of CQD and TBABr before and after hy-
drothermal treatment was studied by TEM and PL characterization
techniques and the recorded TEM images and PL spectra are shown in
Figs. S2c–d and S12, respectively. According to the above character-
ization results, we propose that TBA cations, under hydrothermal
condition, can be introduced on the surface of CQD via ion exchange
and absorption, then these introduced TBA cations may induce free TBA
cations in solution to aggregate around CQD, leading to the formation
of TBA cationic micelle-encapsulated CQD particles. Finally, the CQD
cationized by TBA micelle can be combined with DT anion via elec-
trostatic interaction to produce the hybridized TBADT (Scheme 1).
Notably, the additional interaction of the polar groups of CQD with DT
anions, as previously observed in the silica-loaded TBADT [61,82], also
helps to improve the stability of the hybrid catalyst.
Fig. 6. Cyclic voltammograms (CVs) of TBADT and its hybrids (2.4 × 10−3 M)
in MeCN (9.8 mL) containing 2 M HCl (1.2 mL). 1: Pure TBADT; 2–5:
Corresponding to 1, 2, 3 and 4% CQD/TBADT hybrids.
3.3.2. Mediated mechanism
According to the present results, as well as the previous studies
[48–57], the mediated mechanism of CQD hybridization to significantly
enhance the visible light catalytic activity of TBADT was proposed as
follows: (Scheme 2): It is generally accepted that TBADT may be excited
by mainly UV light to generate its locally excited state via electron
transition from its bridge oxygen to tungsten atoms, followed by that
such very unstable excited state will rapidly decay to a very reactive
species with an electron deficient oxygen center (marked as wO). Such
wO species with a super strong oxidation ability can directly oxidize
various organic compounds including inert hydrocarbons (RH) to yield
its one electron reduced state and a carbon centered radical species (R·)
via a hydrogen atom abstraction or electron transfer pathway. The
following re-oxidation of the reduced DT to its starting state (catalysis
cycling) and the formation of the oxygenated products can be achieved
under the participation of O2 [30–33]. Enhanced effective separation of
photo-generated charge carriers is generally considered to be one main
approach to improve the photo-catalysis performance of TBADT and the
hybridizer CQD, as supported by our present studies, plays a key role in
this aspect. In addition, this hybridizer also helps to improve the quality
of TBADT and its structural stability under photoreaction condition, as
well as to enhance the visible light harvesting efficiency and redox
capacity of TBADT. Notably, the above favorable effects of the hy-
bridizer CQD on TBADT may be further strengthened by a relay func-
tion of HCl aqueous solution.
is diluted with water to 2 M concentration, suggesting that a surface
passivation of CQD that can enhance its PL intensity [81] may occur
under strong protonation of HCl. These findings support that HCl per-
haps plays a relay function in accelerating a transfer of the photo-ex-
cited electrons from TBADT to CQD through it simultaneously exerts
strong protonation effects on TBADT and CQD.
3.2.4. Cyclic voltammograms (CVs)
We previously investigated the cyclic voltammograms (CVs) of GCE
for TBADT in various reaction media and found that the oxidative ca-
pacity of TBADT in MeCN can be enhanced in the presence of HCl so-
lution, but reduced in the existence of water. Furthermore, the redox
recycling of TBADT in MeCN may be improved in the presence of HCl
solution [39,67]. Herein, the CVs of GCE for TBADT and 3% CQD/
TBADT were measured in the optimal reaction media (MeCN-2 M HCl)
and the obtained results are shown in Fig. 6. A CV curve of TBADT
exhibits
a pair of quasi-reversible redox waves at (−0.061 +
(−0.107))/2 = −0.084 V attributable to the redox process between
W6+ and W5+ ions, in agreement with our previous report [39]. No-
tably, the hybridizing of CQD can cause such a pair of redox waves a
slight shift to a more positive potential and this positive shift becomes
more noticeable with the increase of CQD hybridization (Table S6),
supporting that CQD hybridization may strengthen the redox capacity
of TBADT in the presence of 2 M HCl. Notably, the differences between
its oxidative and reductive peaks listed in Table S6 support that the
increase of CQD hybridization may slightly improve the redox cycling
of TBADT.
4. Conclusion
In summary, for the first time we have developed a novel HE
strategy for the synthesis of CQD-hybridized TBADT catalysts with the
following merits: i) This HE strategy is very simple and efficient; ii) This
HE strategy can enhance the structural stability and redox capacity of
TBADT, especially improve the separation of the photo-generated
Scheme 1. Proposed a schematic diagram for the CQD-hybridized TBADT.
8

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AppliedCatalysisA,General587(2019)117261
Scheme 2. Proposed a mechanism for the CQD hybridization-significantly enhanced visible light-driven photo-catalysis activity of TBADT.
electron-hole pairs of TBADT and its visible light harvesting efficiency;
iii) This HE strategy may remarkably improve the photo-catalytic per-
formance of TBADT in visible light-triggered selective oxidation of inert
hydrocarbons with O2 in MeCN. With such kind of robust visible-light-
responsive hybridized catalysts in hand, we are interested in further
exploring their potential applications to other important visible-light-
catalysis reactions such as the selective oxidation of biomass-derived
compounds by O2.
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We acknowledge the financial support for this work by the National
Natural Science Fund of China (21676079, 21546010), the Natural
Science Fund of Hunan Province (2018JJ3335, 14JJ2148, 11JJ6008,
10JJ2007), Hunan 2011 Collaborative Innovation Center of Chemical
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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.apcata.2019.117261.
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