Synthesis of carbon quantum dots/SiO2 porous nanocomposites and their catalytic ability for photo-enhanced hydrocarbon selective oxidation

Article abstract of DOI:10.1039/c3dt51165k

We report a facile hydrolytic process for the preparation of CQDs/SiO ₂ porous nanocomposites, which show high catalytic activity and stability for the selective oxidation of cis-cyclooctene under visible light irradiation, with TBHP as a radical initiator and oxygen (in the air) as an oxidant at 80 °C.

Full text of DOI:10.1039/c3dt51165k

Dalton
Transactions
View Article Online
View Journal | View Issue
COMMUNICATION
Synthesis of carbon quantum dots/SiO2 porous
nanocomposites and their catalytic ability for photo-
Cite this: Dalton Trans., 2013, 42, 10380
Received 4th May 2013,
Accepted 28th May 2013
enhanced hydrocarbon selective oxidation†
DOI: 10.1039/c3dt51165k
Xiao Han,‡ Yuzhi Han,‡ Hui Huang, Hengchao Zhang, Xing Zhang, Ruihua Liu,
Yang Liu* and Zhenhui Kang*
www.rsc.org/dalton
We report a facile hydrolytic process for the preparation of CQDs/ previous work, complex photocatalysts (TiO2/CQDs, SiO2/
SiO2 porous nanocomposites, which show high catalytic activity CQDs, Fe2O3/CQDs, ZnO/CQDs, Cu2O/CQDs, etc.) have been
and stability for the selective oxidation of cis-cyclooctene under successfully constructed and applied in the organic contami-
visible light irradiation, with TBHP as a radical initiator and nants photodegradation with visible or near infrared light
oxygen (in the air) as an oxidant at 80 °C.
excitation.18–22 However, the study on the CQDs’ catalytic
ability for the hydrocarbon selective oxidation is really scarce.
Two serious issues for nanoparticles during the real cata-
lytic reaction conditions are the catalyst aggregation and catalyst
loss in recycling, and these two points can be effectively solved
by selecting appropriate catalyst support materials.23,24 Porous
SiO2 beads are regarded as promising supports to enhance the
catalytic performance by suppressing catalyst aggregation,
improving reactive substance and charge transfer, and promot-
ing catalyst stability.23,25–27
In this study, we developed a facile hydrolytic process for
the preparation of CQDs/SiO2 nanocomposites with a pore size
of about 5.8 nm. These CQDs/SiO2 porous nanocomposites
show high catalytic activity (31.73% conversion based on cis-
cyclooctene and 89.13% selectivity for 2-hydroxycyclooctanone)
for the selective oxidation of cis-cyclooctene under visible light
irradiation, with TBHP as a radical initiator and oxygen (in air)
as an oxidant at 80 °C. Recycling catalytic experiments con-
firmed that the CQDs/SiO2 nanocomposites are stable enough
with no significant change in the catalytic activity and selecti-
vity after 5 times recycling. An illustration of the present
synthesis strategy for the construction of CQDs/SiO2 porous
nanocomposites is shown in Scheme 1.
Nanocatalysis is one of the core issues of nanoscience
and nanochemistry, and plays a significant role in today’s
chemical industry, environmental and energy-source pro-
blems. Although the attractive catalytic performance of noble
metal nanoparticles (e.g. Au, Ag, Pt and Pd) has been demon-
strated in a wide range of reactions including selective oxi-
dation, hydrocarbon conversion reactions, hydrogenation,
and reduction,1–6 the high cost is still the major obstacle for
the large-scale production and further application of noble-
metal nanocatalysts. Selective oxidation is a simple and
important pathway for functionalizing organic hydrocarbon
molecules, and new developments in this field cover the major
areas of the modern chemical industry.7–9 Therefore, developing
non-metal nanocatalysts used in selective catalytic oxidation is
still highly desirable.
Carbon quantum dots (CQDs) have gradually become
a rising star in the nanocarbon family due to their benign,
abundant, nontoxic and inexpensive nature.10,11 The strong
and tunable fluorescence of CQDs promotes their wide appli-
cations in catalysis, bio-imaging, optoelectronics, sensors
and surface-enhanced Raman scattering.12–16 A recent study
shows that the photoexcited CQDs are both excellent electron
donors and electron acceptors.17 Such remarkable photo-
induced electron transfer properties offer CQDs new opportu-
nities to be used as a class of photoactive nanocatalysts. In our
In our experiments, CQDs were synthesized by using the
electrochemical method developed by our research group,18
Institute of Functional Nano & Soft Materials (FUNSOM) and Jiangsu Key Laboratory
for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou,
China. E-mail: yangl@suda.edu.cn, zhkang@suda.edu.cn
†Electronic supplementary information (ESI) available: Experimental pro-
cedures; SEM and TEM images, nitrogen adsorption–desorption isotherm, pore
size distribution curve and FTIR spectrum of the obtained catalysts; detailed
catalytic performance of nanomaterials. See DOI: 10.1039/c3dt51165k
‡These authors contributed equally to this work.
Scheme 1 Schematic procedure for the construction of CQDs/SiO2 porous
nanocomposites.
10380 | Dalton Trans., 2013, 42, 10380–10383
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Communication
Fig. 2 (a) Raman spectrum of CQDs/SiO2 nanocomposites. (b) DRS absorption
spectra of CQDs/SiO2 nanocomposites, SiO2 nanospheres and SiQDs/SiO2
nanocomposites.
the curve exhibits a type-IV isotherm, and the hysteresis loop
in the adsorption–desorption isotherm is characteristic of
mesopores,2,28 with the pore size of about 5.8 nm. What’s
more, the Brunauer–Emmett–Teller (BET) specific surface area
of the CQDs/SiO2 nanocomposites was measured to be
272.08 m2 g−1, suggesting that the obtained CQDs/SiO2 nano-
composites have a large surface area.
Raman and Fourier transform infrared (FTIR) spectra were
also recorded to further confirm the existence of CQDs in the
CQDs/SiO2 nanocomposites. As shown in Fig. 2a, two charac-
teristic peaks located at 1335 and 1607 cm−1 are observed in
the Raman spectrum of CQDs/SiO2 nanocomposites, corres-
ponding to the D-band and the G-band of carbon, respecti-
vely.18 For the FTIR spectrum of CQDs/SiO2 nanocomposites
(as shown in Fig. S4†), the peak located at 1405 cm−1 is related
Fig. 1 (a) SEM and (b) TEM images of CQDs/SiO2 nanocomposites. Insert in (a)
is SEM image of typical single particle of CQDs/SiO2 nanocomposites (scale bar:
20 nm). Inserts in (b) are TEM image of typical single particle (scale bar: 20 nm)
and HRTEM image of CQDs on CQDs/SiO2 nanocomposites (scale bar: 2 nm). (c)
and (d) are dark field STEM images of CQDs/SiO2 nanocomposites. Corres-
ponding EDX elemental mapping images of (e) C, (f) Si and (g) O (scale bar:
20 nm).
and the transmission electron microscopy (TEM) image is to O–C–O symmetric vibration,21 providing a further support
for the formation of CQDs/SiO2 nanocomposites. Other bands
obtained by a facile hydrolytic process (experimental details are attributed to SiO2.
shown in Fig. S1.† The CQDs/SiO2 nanocomposites were
are shown in the ESI†). Fig. 1a and the inset exhibit the typical
scanning electron microscopy (SEM) images of the obtained
Besides CQDs/SiO2 nanocomposites, SiO2 nanospheres
(SEM image shown in Fig. S5†) and SiQDs/SiO2 nanocompo-
CQDs/SiO2 nanocomposites, which reveal their spherical sites (TEM and HRTEM images shown in Fig. S6†) were also
shape with an average size of about 100 nm. A TEM image of synthesized for comparison. BET test results show that, com-
pared with porous CQDs/SiO2 nanocomposites, SiO2 nano-
shows the TEM images with high magnification, from which spheres and SiQDs/SiO2 nanocomposites have much smaller
we can clearly observe that CQDs with the size of about 5 nm surface areas (12 m2 g−1 and 14 m2
−1, respectively), and
almost no pores can be found in them, as shown in Fig. S7.†
CQDs/SiO2 nanocomposites is shown in Fig. 1b. Fig. S2†
g
(indexed by yellow circles) have been uniformly dispersed on
the surface of SiO2 nanospheres. The well-resolved lattice Fig. 2b displays the diffuse reflectance spectroscopy (DRS) of
fringe spacing of 0.212 nm in the high resolution TEM the as-prepared three nanocatalysts. It can be clearly seen
that CQDs/SiO2 nanocomposites can absorb significantly more
(HRTEM) image (lower inset of Fig. 1b) agrees well with (100)
plane spacing of graphitic carbon. The dark field scanning light in the full recorded spectral range compared with
transmission electron microscopy (STEM) images of CQDs/ others, indicating that CQDs/SiO2 nanocomposites have the
better light restriction ability, which may lead to their high
corresponding EDX elemental mapping images of C, Si and O photocatalytic activity for target reactions.
SiO2 nanocomposites are shown in Fig. 1c and 1d. And the
(as shown in Fig. 1e–g) indicate that CQDs are well-dispersed
in the SiO2 nanospheres. The weak signals of the additional C
The photocatalytic behavior of CQDs/SiO2 nanocomposites
was explored for the selective oxidation of cis-cyclooctene
element from the background are attributed to the carbon under visible light (150 W Xenon lamp) irradiation, with TBHP
film coated on the copper grid (Fig. 1e). All the above results as a radical initiator and oxygen (in the air) as an oxidant at
80 °C. The conversion and selectivity of cis-cyclooctene were
prove that CQDs/SiO2 nanocomposites have been successfully
synthesized.
recorded by GC-MS (carrier gas (N2) at 140 K; temperature
In order to check whether the obtained CQDs/SiO2 program 60 °C, 1 min, 15 °C min−1, 180 °C, 15 min; split ratio,
10 : 1; injector, 300 °C; detector, 300 °C), and the details
nanocomposites are porous materials, we measured the nitrogen
adsorption–desorption isotherm and the pore size distribution are shown in Fig. 3a and Table S1.† We can see that a
curve of the CQDs/SiO2 nanocomposites. As shown in Fig. S3,† high conversion efficiency of 31.73% and a high selectivity of
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 10380–10383 | 10381

View Article Online
Communication
Dalton Transactions
Next, the photo-responsive catalytic property of CQDs/SiO2
nanocomposites for the selective oxidation of cis-cyclooctene
was also studied. Fig. 3d and Table S4† give the relationship
between the conversion of cis-cyclooctene/selectivity of
2-hydroxycyclooctanone and reaction time with CQDs/SiO2
nanocomposites as catalysts with or without (in dark) visible
light irradiation. It can be clearly observed that, when CQDs/
SiO2 nanocomposites were used as catalysts, the conversion of
cis-cyclooctene under visible light irradiation was almost 3
times higher than that in the dark environment, and the trend
of selectivity for 2-hydroxycyclooctanone is consistent with
the conversion efficiency. It can be found that light irradiation
is necessary for the high conversion efficiency and high selecti-
vity for the CQDs/SiO2 nanocomposites catalyzed selective
oxidation of cis-cyclooctene.
In general, good catalysts should be stable enough for their
practical application. Therefore, the repeated selective oxi-
dation of cis-cyclooctene was carried out 5 times using CQDs/
SiO2 nanocomposites as catalysts. As shown in Fig. S10,† the
catalytic activity and selectivity exhibit no evident change, indi-
cating the catalytic stability of CQDs/SiO2 nanocomposites.
The reaction pathway of the CQDs/SiO2 nanocomposites
catalyst system possibly is similar to that of the Au/C and
metal-modified SiNW catalyst systems, in that TBHP acts as a
free radical initiator with oxygen in air as the oxidant, while
Fig. 3 Reaction profile of the selective oxidation of cis-cyclooctene: (a) the
relationship between the conversion of cis-cyclooctene/selectivity to different
products and reaction time with 100 nm CQDs/SiO2 nanocomposites as cata-
lysts. The relationship between the conversion of cis-cyclooctene/selectivity of
2-hydroxycyclooctanone and reaction time with (b) CQDs/SiO2 nanocomposites
with different sizes (100 and 250 nm) as catalysts, (c) CQDs/SiO2 nanocompo-
sites, SiQDs/SiO2 nanocomposites and SiO2 nanospheres as catalysts, respecti-
vely, (d) CQDs/SiO2 nanocomposites as catalysts under visible light irradiation or
in the dark.
2-hydroxycyclooctanone up to 89.13% were achieved simul- the CvC bond in cis-cyclooctene is directly oxidized by a per-
taneously after 24 h reaction. The selectivities to epoxycyclo- oxide species generated from TBHP and oxygen.7,8 As reported,
octane and 1,2-cyclooctanediol fell to 5.95% and 4.05% from the photoexcited CQDs are both excellent electron donors
15.89% and 26.67% with increasing reaction time. So we can and electron acceptors,17 which can undoubtedly enhance
conclude that epoxycyclooctane and 1,2-cyclooctanediol are the oxidation of cis-cyclooctene in the presence of TBHP and
formed initially as primary products, and 2-hydroxycyclo- oxygen. Furthermore, the CQDs on CQDs/SiO2 nanocompo-
octanone is formed as a consequence of sequential oxidation.
sites possess graphite fragment structure, which is similar to
In a further set of experiments, we investigate the size effect that of polycyclic aromatic hydrocarbons. So for selective oxi-
of CQDs/SiO2 nanocomposites (SEM images are shown in dation of cis-cyclooctene, the π–π interaction between the conju-
Fig. 1a and Fig. S8†) catalysts on the selective oxidation of cis- gated structure of CQDs and cis-cyclooctene is beneficial for
cyclooctene. The detailed conversion of cis-cyclooctene and the enrichment of cis-cyclooctene on the surface of CQDs/SiO2
selectivity of 2-hydroxycyclooctanone are shown in Fig. 3b and nanocomposites. In addition, as shown in Fig. S3,† the CQDs/
Table S2,† which reveal that the smaller sized CQDs/SiO2 nano- SiO2 nanocomposite is a kind of porous structure with a pore
composites exhibit a higher conversion efficiency and higher size of about 5.8 nm, and CQDs were loaded both on the
selectivity of 2-hydroxycyclooctanone for the cis-cyclooctene surface and inside of the nanocomposites. During the catalytic
oxidation. The reason can be ascribed to the higher surface reaction process, the reagents can be effectively absorbed by
area (272.08 cm2 g−1) of the small sized CQDs/SiO2 nano- the porous CQDs/SiO2 nanocomposites, then diffuse to the
composites (as shown in Fig. S3a and S9†).
inside of catalysts through the pore. This process facilitates
In the following experiments, the catalytic performance of the sufficient contact between reagents and CQDs, leading to
SiO2 nanospheres and SiQDs/SiO2 nanocomposites in the the enhanced catalytic efficiency of the selective oxidation.
selective oxidation of cis-cyclooctene was further studied under Finally, compared with pure SiO2, the combination of CQDs
the same conditions to confirm the important role of CQDs, with SiO2 enhances the light response and utilization (Fig. 2b).
and the detailed results are shown in Fig. 3c and Table S3.† The above aspects together account for the high photocatalytic
In the three nanomaterials, CQDs/SiO2 nanocomposites as cata- activity of CQDs/SiO2 nanocomposites.
lysts yielded the highest catalytic activity (31.73% conversion
based on cis-cyclooctene and 89.13% selectivity for 2-hydroxy-
cyclooctanone), and the catalytic ability sequence was: CQDs/
SiO2 nanocomposites>SiQDs/SiO2 nanocomposites>SiO2 nano-
Conclusions
spheres. All the above results prove that CQDs indeed make a In summary, we demonstrate a facile hydrolytic process for the
major contribution to the high catalytic activity of CQDs/SiO2 preparation of CQDs/SiO2 porous nanocomposites with a large
nanocomposites for the selective oxidation of cis-cyclooctene.
BET specific surface area and high visible light sensitivity. For
10382 | Dalton Trans., 2013, 42, 10380–10383
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Communication
the selective oxidation of cis-cyclooctene under visible light
irradiation, with TBHP as a radical initiator and oxygen (in the
air) as an oxidant at 80 °C, the CQDs/SiO2 nanocomposites
exhibit high catalytic activity (31.73% conversion based on
cis-cyclooctene and 89.13% selectivity for 2-hydroxycyclo-
8 C. H. A. Tsang, Y. Liu, Z. H. Kang, D. D. D. Ma, N. B. Wong
and S. T. Lee, Chem. Commun., 2009, 5829–5831.
9 Y. X. Zhang, X. Han, R. H. Liu, Y. Liu, H. Huang,
J. M. Zhang, H. Yu and Z. H. Kang, J. Phys. Chem. C, 2012,
116, 20363–20367.
octanone). Compared to SiQDs/SiO2 nanocomposites and 10 S. N. Baker and G. A. Baker, Angew. Chem., Int. Ed., 2010,
SiO2 nanospheres, CQDs/SiO2 nanocomposites exhibit the 49, 6726–6744.
enhanced catalytic ability. In addition, a set of experiments 11 H. T. Li, Z. H. Kang, Y. Liu and S. T. Lee, J. Mater. Chem.,
prove the size of the CQDs/SiO2 nanocomposites and the light 2012, 22, 24230–24253.
irradiation play important roles in the catalytic process. 12 B. Y. Yu and S. Y. Kwak, J. Mater. Chem., 2012, 22, 8345–
The present work provides an exciting prospect for the high 8353.
performance novel catalyst design and fabrication towards 13 L. Cao, X. Wang, M. J. Meziani, F. S. Lu, H. F. Wang,
new energy sources and environmental issues. Future work
will focus on the functionalization of other nanomaterials
(MnO2, Fe3O4, SnO2, and CuO, etc.) with CQDs.
P. J. G. Luo, Y. Lin, B. A. Harruff, L. M. Veca, D. Murray,
S. Y. Xie and Y. P. Sun, J. Am. Chem. Soc., 2007, 129, 11318–
11319.
14 X. Zhang, F. Wang, H. Huang, H. T. Li, X. Han, Y. Liu and
Z. H. Kang, Nanoscale, 2013, 5, 2274–2278.
15 S. J. Zhu, Q. N. Meng, L. Wang, J. H. Zhang, Y. B. Song,
H. Jin, K. Zhang, H. C. Sun, H. Y. Wang and B. Yang,
Angew. Chem., Int. Ed., 2013, 52, 3953–3957.
16 P. H. Luo, C. Li and G. Q. Shi, Phys. Chem. Chem. Phys.,
2012, 14, 7360–7366.
17 X. Wang, L. Cao, F. S. Lu, M. J. Meziani, H. T. Li, G. Qi,
B. Zhou, B. A. Harruff, F. Kermarrec and Y. P. Sun, Chem.
Commun., 2009, 3774–3776.
18 H. Ming, Z. Ma, Y. Liu, K. M. Pan, H. Yu, F. Wang and
Z. H. Kang, Dalton Trans., 2012, 41, 9526–9531.
19 H. T. Li, X. D. He, Z. H. Kang, H. Huang, Y. Liu, J. L. Liu,
S. Y. Lian, C. H. A. Tsang, X. B. Yang and S. T. Lee, Angew.
Chem., Int. Ed., 2010, 49, 4430–4434.
Acknowledgements
This work is supported by the National Basic Research
Program of China (973 Program) (2012CB825800,
2013CB932702), the National Natural Science Foundation of
China (51132006, 21073127, 21071104), the Specialized
Research Fund for the Doctoral Program of Higher Education
(20123201110018), a Suzhou Planning Project of Science and
Technology (ZXG2012028), a Foundation for the Author of
National Excellent Doctoral Dissertation of China (200929),
and a project funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions.
20 H. C. Zhang, H. Ming, S. Y. Lian, H. Huang, H. T. Li,
L. L. Zhang, Y. Liu, Z. H. Kang and S. T. Lee, Dalton Trans.,
2011, 40, 10822–10825.
Notes and references
21 H. Yu, H. C. Zhang, H. Huang, Y. Liu, H. T. Li, H. Ming
and Z. H. Kang, New J. Chem., 2012, 36, 1031–1035.
22 H. T. Li, R. H. Liu, Y. Liu, H. Huang, H. Yu, H. Ming,
S. Y. Lian, S. T. Lee and Z. H. Kang, J. Mater. Chem., 2012,
22, 17470–17475.
1 A. S. K. Hashmi and G. J. Hutchings, Angew. Chem., Int. Ed.,
2006, 45, 7896–7936.
2 H. C. Zhang, H. Huang, Y. Liu, X. Han, Z. Ma, L. L. Zhang,
H. T. Li and Z. H. Kang, J. Mater. Chem., 2012, 22, 20182–
20185.
23 Y. J. Wang, D. P. Wilkinson and J. J. Zhang, Chem. Rev.,
2011, 111, 7625–7651.
3 Z. J. Chen, Z. H. Guan, M. R. Li, Q. H. Yang and C. Li,
Angew. Chem., Int. Ed., 2011, 50, 4913–4917.
4 E. Schmidt, C. Bucher, G. Santarossa, T. Mallat, R. Gilmour
and A. Baiker, J. Catal., 2012, 289, 238–248.
5 H. Huang, H. C. Zhang, Z. Ma, Y. Liu, H. Ming, H. T. Li and
Z. H. Kang, Nanoscale, 2012, 4, 4964–4967.
6 H. Chen, Y. Shao, Z. Y. Xu, H. Q. Wan, Y. Q. Wan,
S. R. Zheng and D. Q. Zhu, Appl. Catal., B, 2011, 105, 255–
262.
24 Z. S. Wu, S. B. Yang, Y. Sun, K. Parvez, X. L. Feng and
K. Müllen, J. Am. Chem. Soc., 2012, 134, 9082–9085.
25 L. Wang, D. M. Xing, Y. H. Liu, Y. H. Cai, Z. G. Shao,
Y. F. Zhai, H. X. Zhong, B. L. Yi and H. M. Zhang, J. Power
Sources, 2006, 161, 61–67.
26 K. Qian, W. X. Huang, J. Fang, S. S. Lv, B. He, Z. Q. Jiang
and S. Q. Wei, J. Catal., 2008, 255, 269–278.
27 X. H. Mo, J. Gao, N. Umnajkaseam and J. G. Goodwin Jr.,
J. Catal., 2009, 267, 167–176.
28 Z. Chen, Z. M. Cui, F. Niu, L. Jiang and W. G. Song, Chem.
Commun., 2010, 46, 6524–6526.
7 M. D. Hughes, Y. J. Xu, P. Jenkins, P. McMorn, P. Landon,
D. I. Enache, A. F. Carley, G. A. Attard, G. J. Hutchings,
F. King, E. H. Stitt, P. Johnston, K. Griffin and C. J. Kiely,
Nature, 2005, 437, 1132–1135.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 10380–10383 | 10383