Metal nanoparticle/carbon quantum dot composite as a photocatalyst for high-efficiency cyclohexane oxidation

Article abstract of DOI:10.1021/cs400913h

High-efficiency and high-selectivity catalytic oxidation of alkanes under mild conditions is a major objective of current catalysis chemistry and chemical production. Despite extensive development efforts on new catalysts for cyclohexane oxidation, curren

Full text of DOI:10.1021/cs400913h

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Article
Metal nanoparticle/carbon quantum dot composite as
photocatalyst for high efficiency cyclohexane oxidation
Ruihua Liu, Hui Huang, Haitao Li, Yang Liu, Jun Zhong, Youyong Li, Shuo Zhang, and Zhenhui Kang
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 11 Dec 2013
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Metal nanoparticle/carbon quantum dot
composite as photocatalyst for high efficiency
cyclohexane oxidation
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Ruihua Liu,1 Hui Huang,1 Haitao Li,1 Yang Liu,*1 Jun Zhong,1 Youyong Li,1 Shuo Zhang,2 and
Zhenhui Kang*1
1Institute of Functional Nano and Soft Materials (FUNSOM) & Jiangsu Key Laboratory for
Carbon–Based Functional Materials and Devices, Soochow University, Suzhou, China. Eꢀmail:
yangl@suda.edu.cn; zhkang@suda.edu.cn.
2Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese
Academy of Science, Shanghai, China
KEYWORDS: carbon quantum dots, composites, cyclohexane oxidation, photocatalysis;
selective oxidation
ABSTRACT
Highꢀefficiency and highꢀselectivity catalytic oxidation of alkanes under mild conditions is a
major aim of current catalysis chemistry and chemical production. Despite extensive
development efforts on new catalysts for cyclohexane oxidation, current commercial processes
still suffer from low conversion, poor selectivity, and excessive production of wastes. We
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demonstrate the design and synthesis of metal nanoparticles/carbon quantum dots (CQDs)
composites for highꢀefficiency and highꢀselectivity photocatalyst systems for the green oxidation
of cyclohexane. Remarkably, the present Au nanoparticles/CQDs composite photocatalyst yields
63.8% conversion efficiency and 99.9% selectivity for green oxidation of cyclohexane to
cyclohexanone using H2O2 under visible light at room temperature. Given its diversity and
versatility of structural and composition design, metal nanoparticles/CQDs composites may
provide a powerful pathway for the development of highꢀperformance catalysts and production
processes for green chemical industry.
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INTRODUCTION
One of the principal aims of catalysis chemistry in the 21st Century is highꢀefficiency and highꢀ
selectivity catalytic oxidation of alkanes,1 especially cyclohexane oxidation under mild
conditions.2 However, the high activation energy of CꢀH bond breaking in cyclohexane dictates
harsh reaction conditions, i.e. high temperature and high pressure.3 Apart from its intrinsic
importance in the chemistry of C–H activation, the selective oxidation of cyclohexane to
cyclohexanone and cyclohexanol (about 106 tons per year) is the key step of the commercial
production of nylonꢀ6 and nylonꢀ66 polymers. Despite extensive development efforts for
cyclohexane oxidization catalysts, today’s commercial processes still suffer from high
temperature (150–160 °C), high pressure (1–2 MPa), low yield (< 15%), low selectivity (< 80%),
and excessive production of wastes.4,5 Catalysts enabling cyclohexane oxidation under mild
conditions (below the boiling point of cyclohexane, 80.7 °C) would alleviate the potential hazard
of harsh reaction conditions, as well as allow for more selective and controlled reactions. As a
preferred oxidant in liquid phase, hydrogen peroxide can work at low temperature, produces only
water as a byꢀproduct and has a high oxygen uptake (47%).6ꢀ8 However, cyclohexane oxidation
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with H2O2 at low temperature using the current catalysts (such as weak acid resin,9 iron bispidine
complexes,10 Ru complex,11 Cr/Ti/Si oxides,12 NaꢀGeX,13 mesoporous TSꢀ1,14 TiꢀMCMꢀ41,15
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metal aluminophosphates,16 biomimetic systems17 and Cu–Cr2O3 ) invariably suffers from large
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amount of organic solvent (i.e. acetone and acetonitrile), poor selectivity, and low H2O2
efficiency, thus making its industrialization difficult.
Scheme 1. Au/CQDs composites as a photocatalyst for selective oxidation of cyclohexane in the
presence of H2O2 under visible light.
Carbon quantum dots (CQDs) with richly fluorescent and photoꢀelectrochemical properties19,20
can serve as either photoꢀinduced electron acceptors or donors.20 They can function not only as
an efficient photocatalyst (for highly selective oxidation), but also as a multiꢀfunctional
component in photocatalyst design21 to promote wider spectrum absorption and separation of
electronꢀhole, as well as to stabilize photolysis semiconductors. Metal nanoparticles (Au, Cu, etc.)
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are known to possess catalytic activities for selective oxidation reactions.22,23 Additionally,
surface plasma resonance absorption can also induce good photocatalytic abilities of metal
nanoparticles (Au, Ag, etc.).24,25 Here, we report the design of a tunable photocatalyst based on
the composite of CQDs and metal nanoparticles for the selective oxidation of cyclohexane.
Significantly, the Au nanoparticles/CQDs (Au/CQDs) composite catalyst yielded oxidation of
cyclohexane to cyclohexanone with 63.8% efficiency and >99.9% selectivity in the presence of
H2O2 under visible light at room temperature (see Scheme 1).
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Results and discussion
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Figure 1. (a) TEM image, Xꢀray diffraction (XRD) pattern (inset, upper of 1a) and energy
dispersive Xꢀray (EDX) spectrum (inset, lower of 1a) of Au/CQDs composites; (b) HRTEM
image of Au/CQDs composites; (c) The UVꢀvis absorbance spectrum of CQDs and Au/CQDs; (d)
Typical Raman spectra of CQDs and Au/CQDs composites.
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Characterization of Au/CQDs composites. The CQDs were synthesized through a method of
electrochemical ablation of graphite. As shown in Figure S1, the surface functional groups on the
CQDs are hydroxyl (ꢀOH), carboxy (C=O), and epoxide/either (CꢀOꢀC). For the present
composite catalyst, the interaction between the p/π orbits of C and d orbits of Au in AuNPs, and
the interactions between the functional groups on the CQDs (ꢀOH, C=C, C=O) and the AuNPs
are responsible for the formation and the high stability of the Au/CQDs composite catalyst.
Au/CQDs composites were synthesized by dropꢀwise addition of HAuCl4 solution to CQDs
solution with vigorously magnetic stirring in dark at room temperature (see Experimental section
for details). Figure 1a shows the transmission electron microscopy (TEM) image of the spherical
and wellꢀdispersed Au/CQDs composites with sizes of 5ꢀ15 nm. The upper inset in Figure 1a is
the energy dispersive Xꢀray (EDX) spectrum of Au/CQDs composites, where the copper peaks
are from the copper TEM grid, gold peaks from AuNPs, oxygen and carbon peaks from CQDs
and amorphous carbon film in Cu grid. Xꢀray diffraction (XRD) pattern is shown in the lower
inset in Figure 1a. All the characteristic diffraction peaks marked by “ ” can be readily indexed
to (111), (200), (220), (311) and (222) planes of metallic Au (JCPDS No. 1ꢀ1172), respectively.
The broad diffraction peak at 26o (marked by “ ”) is the characteristic peak of carbon. The highꢀ
resolution TEM image (HRTEM) of Au/CQDs composites is shown in Figure 1b. Clear lattice
spacing’s of 0.240 nm and 0.235 nm are in well agreement with the (100) crystallographic planes
of graphitic carbon and (111) planes of crystalline Au, respectively. Compared with that of
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CQDs or AuNPs, the UVꢀVisible absorption spectrum of Au/CQDs composites displays typical
absorbance in both UV and visible regions. In Figure 1c, the absorption peak at about 230 nm
represents a typical absorption of CQDs, similar to that of polycyclic aromatic hydrocarbons,26
while the absorption peak in the visible region (490 to 590 nm) is attributed to the surface
plasmon band of AuNPs.27 The Raman spectrum of Au/CQDs composites in Figure 1d shows the
Dꢀband and Gꢀband of carbon at 1343 and 1611 cmꢀ1.28 The intensity of Dꢀband and Gꢀband of
Au/CQDs composite is obviously higher than that of pure CQDs, which is due to the surface
enhancement Raman scattering effect of AuNPs.29
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Figure 2. (a) Cyclohexane adsorptionꢀdesorption isotherm curves of Au/CQDs composites;(b)
Time dependence of conversion efficiency and selectivity of cyclohexane oxidation were
investigated under dark and visible light (mole ratio of H2O2/cyclohexane is 3:1); (c) The
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conversion of cyclohexane and H2O2 under different mole ratios of H2O2/cyclohexane; (d)
Conversion and selectivity efficiency of the oxidation of cyclohexane on Au/CQDs composites
over cycling ten times (mole ratio of H2O2/cyclohexane, 3:1).
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Catalytic oxidation of cyclohexane using Au/CQDs composites. The measured cyclohexane
adsorptionꢀdesorption isotherm of Au/CQDs composites (Figure 2a) exhibits the typeꢀIV curve
characteristics,30 indicating that Au/CQDs composites have good adsorption to cyclohexane. In
the catalytic experiments, H2O2 (30%) as oxidant was added dropwise to the mixture of
Au/CQDs composites (25 mg) and cyclohexane (50 mL) in a round bottom flask (250 mL) with
water condenser and continuous magnetic stirring. The oxidation products were analyzed by gas
chromatography (GC) and gas chromatographyꢀmass spectroscopy (GCꢀMS). Figure 2b shows
the time dependence of the conversion and selectivity of cyclohexane oxidation to
cyclohexanone with Au/CQDs composites as the photocatalyst driven by visible light. It reveals
that conversion efficiency increases with increasing reaction time, while the selectivity to
cyclohexanone is nearly constant at 99.9% regardless of the conversion efficiency (Table 1).
After 48 h reaction, a high conversion efficiency of 63.8% and high selectivity of over 99.9%
were achieved concurrently. The conversion of cyclohexane and H2O2 under different mole ratios
of H2O2/cyclohexane (0.0, 0.5, 1.0, 1.5, 2.0, 3.0) were also investigated and shown in Figure 2c.
With the mole ratio of H2O2/cyclohexane increasing, the conversion of cyclohexane increases
and the conversion of H2O2 slightly decreases.
The following experiments further highlight the effectiveness of the present composite
catalyst: (1) Blank reaction (i.e., without catalyst) would not produce any oxidation products
(entry 1, Table 1); (2) CQDs and AuNPs used separately as catalyst (entries 2 and 3, Table 1)
yielded cyclohexane oxidation with low conversion of only 3% and 5%, respectively; and (3) A
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mixture of CQDs and AuNPs as catalyst yielded slight improvement, but conversion and
selectivity of cyclohexane to cyclohexanone remained low at 7.0% and 76.5%, respectively
(entry 4, Table 1). In remarkable contrast, the Au/CQDs composites catalyst shows high
conversion efficiency (63.8%) and high selectivity (> 99.9%), which also remain nearly constant
over ten catalytic cycles (Figure 2d). After carefully analyzing the products, it was found that the
selectivity of C6 products is about 100% (see Table 1), without any CO2 evolved. As shown in
Figure 2d, the Au/CQDs are stable enough during the repeated experiments without exhibiting
any significant loss of catalytic activity and selectivity. After recycling ten times, Au/CQDs still
retain high catalytic activity and selectivity of cyclohexane oxidation. Moreover, the filtrate
solution test (additional 24 h reaction after removing the Au/CQDs composites from the reaction
medium) showed no catalytic activity, indicating leaching did not play an important role in the
present system. The above experiments show Au/CQDs composites indeed function as a highꢀ
performance photocatalyst in the present reaction system.
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Table 1. Different catalysts for photocatalytic oxidation of cyclohexane (mole ratio of
H2O2/cyclohexane is 3:1).
Entry
Catalyst
Oxidant
Conversion
(%)
Selectivity
Selectivity
(%)
(%)
∑selC6
ꢀ
cyclohexanone
1
2
None
None
H2O2
ꢀ
ꢀ
CQDs
3.0
99.9
100
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AuNPs
H2O2
5.0
98.4
76.5
> 99.9
54.1
49.7
50.1
64.2
100
100
100
100
100
100
100
CQDs &AuNPs
Au/CQDs
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
7.0
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63.8
30.3
22.1
23.9
14.2
Au/SiO2
Au/Carbon nanotube
Au/Graphene
Au/Graphite
Reactions were carried out in a round bottom flask (see Methods information for details).
Conversion and selectivity were determined by GC analysis; the variance of values is estimated
to be less than ±2%.
Cyclohexane oxidation was further carried out with the Au/CQDs composites catalyst driven
by light of different wavelengths (Figure 3a). Figure 3b shows that the best conversion efficiency
was obtained in green light, whose wavelength matches the surface plasma resonance zone of
AuNPs (490ꢀ590 nm, see Figure S2).27 However, under the same conditions (visible light, room
temperature), the use of AuNPs supported on SiO2 spheres (Au/SiO2 nanospheres; TEM and
HRTEM images, see Supporting information Figure S3) as catalyst only yielded a low
conversion efficiency of 30.3% and low selectivity of 54.1% for cyclohexanone (entry 6, Table
1). While, other carbon based catalysts (such as Au/Carbon nanotubes, Au/graphite, and
Au/Graphene) also could not get good photocatalytic abilities on the cyclohexane oxidation
(entries 7, 8 and 9 Table 1). The totality of the experiments shows that the high conversion and
selectivity in the present photocatalyst system should be attributed to the collective contribution
and interactions of CQDs and AuNPs.
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Figure 3. Time dependence of the conversion and selectivity of the oxidation of cyclohexane
with Au/CQDs composites as photocatalyst (mole ratio of H2O2/cyclohexane is 3:1) driven by
different lighting conditions: (a) under dark, UV light (high pressure mercury lamp, main
wavelength 365 nm), Visible light (Xe lamp with visibleꢀCUT filter to cut off light of
wavelength < 420 nm), and IR light (Xe lamp with visibleꢀCUT filter to cut off light of
wavelength < 700 nm); (b) blue (400 nm), green (525 nm), red (620 nm), and IR (850 nm) light,
using high power (>1 W) LED as light source.
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Interaction between AuNPs and CQDs. Xꢀray absorption spectroscopy (XAS, elementꢀ
specific spectroscopic technique for structural information of different species) experiments were
performed to investigate the electronic structure of Au/CQDs composites.31 Au L3ꢀedge (Xꢀray
absorption near edge structure, XANES) and (Extended xꢀray absorption fine structure, EXAFS)
data for Au/CQDs composites with and without visible light irradiation were collected. The
XANES data are shown in Figure 4a and the Fourier transform of the EXAFS data in R space
shown in Figure 5a. The XANES data for Au/CQDs composites with and without visible light
irradiation also show very similar spectral shape. In contrast, obvious differences between lightꢀ
on and ꢀoff can be found in the Fourier transform of the EXAFS data in the R space. The main
peak around 2.6 Å can be attributed to AuꢀAu bonds. Au/CQDs composites before light
irradiation show a main peak at 2.6 Å indicating the existence of AuNPs, and the peak intensity
is lower than that for Au foil revealing the nature of AuNPs.32 Strikingly, when the same
Au/CQDs composites were exposed to visible light, a new peak at 1.5 Å can be observed. This
peak could be attributed to the interaction between CQDs and AuNPs, such as AuꢀC bonds.32
Also, a further decrease of the main peak intensity can be observed, suggesting decrease of Auꢀ
Au bonds with formation of AuꢀC bonds. In followed the experiments, Au L3ꢀedge XANES and
EXAFS data for Au/CQDs composites were collected during the in situ photocatalytic reaction.
Under the in situ reaction condition, the XANES data for Au/CQDs composites (Figure 4b) is
also very similar to that for Au NPs and Au foil. While, as shown in Figure 5b, under the in situ
reaction condition, there is no obvious signal on the formation of AuꢀC bonds between CQDs
and AuNPs, even if a further decrease of the main peak (2.6 Å) intensity can be observed under
visible light. The disappearing of the AuꢀC bonds signal further confirms that the photocatalytic
reaction is really occurred on the interface of AuNPs and the CQDs.
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Figure 4. (a) Au L3ꢀedge XANES spectra of Au/CQDs composites with and without visible light
irradiation. (b) Au L3ꢀedge XANES spectra of Au/CQDs composites with and without visible
light irradiation under in situ reaction condition.
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Figure 5. (a) The Fourier transform of the Au/CQDs composites EXAFS data in R space with
and without light. (b) The Fourier transform of the Au/CQDs composites EXAFS data in R space
with and without light under in situ reaction condition. The spectrum of Au foil is also shown for
comparison.
In the followed experiments, the photoꢀinduced electron transfer properties of CQDs irradiated
by visible light were investigated. It was found that the photoluminescence (PL) of CQDs
excited by 485 nm with emission located at 550 nm could be quenched by either electron
acceptor or electron donor molecules in solution, confirming that CQDs are excellent as both
electron donors and acceptors under visible light. As shown in Figure S4a and S4b, CQDs can
exhibit broad luminescence peak at about 550 nm with the excitation at 485 nm, and the
emission intensities were quenched by the known electron donors N, Nꢀdiethylaniline (DEA) and
electron acceptors 2, 4ꢀdinitrotoluene. Figure S4c and S4d show the luminescence decays of
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CQDs with SternꢀVolmer quenching constants (insets of Figure S4c and S4d, Ksv =τF kq) from
linear regression of 33.0 and 21.5 Mꢀ1, respectively. The above results that the PL of CQDs were
quenched highly efficiently by either electron acceptors or electron donors, clearly confirming
that CQDs are excellent as both electron acceptors and electron donors.
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Figure 6. (a) PL spectra (485 nm excitation) of CQDs, AuNPs and Au/CQDs; (b) Luminescence
decays (485 nm excitation, monitored with 550 nm narrow bandpass filter) of CQDs and
Au/CQDs.
To obtain the further insight about the interaction between AuNPs and CQDs, we examined
the PL spectra of CQDs, AuNPs, and Au/CQDs (Figure 6a). It indicated that the PL of CQDs is
effectively quenched in the Au/CQDs system. The corresponding PL life time of Au/CQDs
shown in Figure 6b decreased to 1.5 ns, from 4.0 ns for pure aqueous CQDs solution. It
demonstrates the dynamic PL quenching in the Au/CQDs system is attributed to an ultrafast
electron transfer process in the Au/CQDs system.
Proposed mechanism study. The electrocatalytic ability of Au/CQDs composites for H2O2
decomposition was examined in a conventional threeꢀelectrode electrochemical cell.20 As shown
in Figure 7a the bare glassy carbon electrode shows no obvious electrochemical response (black
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curve), while Au/CQDsꢀmodified glassy carbon electrode exhibits obvious catalytic ability for
H2O2 decomposition. Under visible light irradiation, the response of Au/CQDsꢀmodified
electrode for H2O2 decomposition is stronger (onset potential at ꢀ0.2 V) than that without light
irradiation (onset potential at ꢀ0.6 V). Further experiments show that Au/CQDs compositesꢀ
modified electrodes have the highest electrocatalytic activity for H2O2 decomposition among
various modified electrodes (CQDs, AuNPs and their mixture), which is also consistent with the
photocatalytic results in Table 1. All electrochemical experiments suggested that the visible
lightꢀpromoted catalytic activity of Au/CQDs composites for H2O2 decomposition should play
key roles for the highly catalytic activity in selective oxidation of cyclohexane.
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In general, HOꢁ produced by decomposition of H2O2 is a strong oxidant, and can easily oxidize
cyclohexane into cyclohexanone and cyclohexanol. Here, a spinꢀtrapping electron spin resonance
(ESR) test was carried out to check the active oxygen species produced by Au/CQDsꢀH2O2
system under visible light. As shown in Figure 7b, typical characteristic quartet peaks of DMPOꢀ
ꢁOH (DMPO, dimethyl pyridine Nꢀoxide) adducts appeared gradually upon light irradiation of
the aqueous Au/CQDsꢀH2O2 system, which suggested that HOꢁ is the active oxygen species in
this photocatalytic oxidation process. To further confirm whether HOꢁ is the main active oxygen
specie responsible for present selective oxidative reaction, a terephthalic acid PL (TAꢀPL)
probing assay was used to detect the amount of HOꢁ under different reaction conditions33 (see
Experimental section). The reaction between the hydroxyl radicals and terephthalate yielding one
intensely fluorescent monohydroxylated isomer is shown in Figure S5. Figure 7c confirmes that
in the buffer solution containing Au/CQDs catalyst, a larger number of HOꢁ will be produced
under visible light irradiation than that for in dark.
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Figure 7. (a) Cyclic voltammograms of bare glassy carbon electrode and Au/CQDsꢀmodified
glassy carbon electrode in phosphate buffer (PH = 7.4) and in the presence of 10 mM H2O2. Scan
rate, 25 mV/s. A 450 W Xe lamp was used as light source with visibleꢀCUT filter to cut off light
of wavelength < 420 nm; (b) The ESR signals of the DMPOꢀ•OH (aqueous solution) adducts for
Au/CQDsꢀH2O2 system under visible light irradiation. [Au/CQDs] = 4 mg/50 mL; [H2O2] = 10
mM; [DMPO] = 40 mM; (c) Time dependence of the fluorescence intensity of the supernatant
liquid of Au/CQDs composites for two different reaction conditions (in dark and under visible
light). A 450 W Xe lamp was used as light source with visibleꢀCUT filter to cut off light of
wavelength < 420 nm; (d) Photocatalysis activities of Au/CQDs in reactive species trapping
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experiments with three kinds of reactive species scavengers (mole ratio of H2O2/cyclohexane,
3:1).
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Figure 8 shows the PL intensities of the supernatant TAꢀPL assay solutions after removing the
Au/CQDs composites with or without visible light irradiation, and the result suggested that a
higher amount of HOꢁ was generated by the irradiated sample than that by the nonꢀirradiated one.
In the followed control experiments, to further confirm the active oxygen species with radical
characteristics, toluene was selected as the substrate in the following experiments. After 12 h
reaction, methyl group–oxygenated compounds (benzyl alcohol, benzaldehyde, benzoic acid, etc.)
were produced in high selectivity of > 92%, whereas a mixture of cresols (oꢀ : mꢀ : pꢀ isomer
ratio = 2.6 : 0.8 : 5.7) was only produced in small amounts (selectivity less than 8 % summing
the three). Generally speaking, the methyl group oxygenation requires abstraction of an
Hꢁ radical by active oxygen species with radical characteristics.34,35 The further reactive species
trapping experiments were carried out with three different scavengers, tertꢀbutanol (a ꢁOH radical
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scavenger), benzquinamide (BZQ,
a
O2−ꢁ radical scavenger) and disodium
ethylenediaminetetraacetate (Na2ꢀEDTA, a hole scavenger). Figure 7d shows the photocatalytic
results after addition of three different scavengers. The addition of tertꢀbutanol substantially
reduced the photocatalytic activity for the oxidation of cyclohexane from almost 63.8% to 3.3%
after 48 h reaction, while the introduction of BZQ and Na2ꢀEDTA barely affect the
photocatalysis results. All of above experiments confirm that that HOꢁ is the active oxygen
species in this photocatalytic oxidation process.
In our present photocatalytic system, as discussed above, the reaction happened in the interface
between Au NPs and CQDs. Although the hydroxy radicals have strong oxidation abilities, the
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synergic effect of the functions of CQDs and the SPR (surface plasmonic resonance) of AuNPs,
still can protect the products (cyclohexanone and cyclohexanol) from the over oxidation.
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Figure 8. Time dependence of the fluorescence intensity of the supernatant liquid of CQDs,
AuNPs, and Au/CQDs composites for two different reaction conditions (in dark and under
visible light). A 450 W Xe lamp was used as light source with visibleꢀCUT filter to cut off light
of wavelength < 420 nm.
Tunable-design ability of metal/CQDs photocatalysts system. The totality of the experiment
results suggests the following mechanistic processes in the photocatalytic oxidation of
cyclohexane. The surface plasma resonance of AuNPs enhances the light absorption of
Au/CQDs composites. Under visible light, H2O2 is decomposed to hydroxyl radical (HOꢁ), which
serves as a strong oxidant for conversion of cyclohexane to cyclohexanone. In the process, the
interaction between CQDs and AuNPs under visible light plays a key role in the eventual high
conversion and selectivity. Based on the proposed mechanism, Ag nanoparticles/CQDs
(Ag/CQDs) (see Supporting information Figure S6) and Cu nanoparticles/CQDs (Cu/CQDs)
composite photocatalysts (see Supporting information Figure S7) were synthesized, and used to
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catalyze cyclohexane oxidation under the same condition with Au/CQDs system. Like the
Au/CQDs system, Ag/CQDs and Cu/CQDs composites exhibited similarly good catalytic
performance (see Supporting information Table S1 and Table S2) under purple light (conversion
54.0%, selectivity 84.1% for Ag) and red light (conversion 46.7%, selectivity 75.3% for Cu),
which correspond respectively to the surface plasma resonance of Ag (see Supporting
information Figure S6b) and Cu (see Supporting information Figure S7b) nanoparticles. The
results were consistent with the proposed catalytic mechanism discussed above. It is therefore
possible to tune the wavelength response of the present photocatalyst system based on the
surface plasma resonance of metal nanoparticles to achieve highꢀefficiency and highꢀselectivity
oxidation of cyclohexane, as shown in Figure 9.
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Figure 9. Based on the surface plasma resonance of metal nanoparticles, Ag/CQDs, Au/CQDs
and Cu/CQDs are catalytically active under purple, green and red light, respectively.
EXPERIMENTAL SECTION
General information. Unless otherwise noted, all materials including dry solvents were
obtained from commercial suppliers and used without further purification. All chemicals were
purchased from Sigma–Aldrich. Transmission electron microscopy (TEM) images were recorded
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using a FEI/ Philips Techal 12 BioTWIN TEM operated at 200 kV with EDX analyses. HRTEM
images were obtained with a CM200 FEG TEM. UVꢀvisible spectra were measured with an
Agilent 8453 UVꢀVIS Diode Array Spectrophotometer, while the PL study was conducted with a
Horiba Jobin Yvon (FluoroMax 4) Luminescence Spectrometer. We also collected Raman
spectra on an HR 800 Raman spectroscope (J Y, France) equipped with a synapse CCD detector
and a confocal Olympus microscope. The spectrograph used 600 g mmꢀ1 gratings and a 633 nm
HeꢀNe laser. The crystal structure of the resultant products was characterized by Xꢀray powder
diffraction (XRD) by using an8/ X‘PertꢀProMPD (Holand) D/maxꢀγAXꢀray diffractometer with
Cu Kα radiation (λ=0.154178 nm). The GC were performed in a Varian 3400 GC column with a
crossꢀlinked 5% PhMe silicone column (25 m × 0.20 mm × 0.33 ꢂm) and a FID detector under
the following conditions: carrier gas (N2) at 140 K; temperature program 60 ℃, 1 min, 15 ℃
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/min, 180 ℃, 15 min; split ratio, 6 : 1; injector, 280 ℃; detector, 280 ℃. Xꢀray Absorption Near
Edge Structure (XANES) and Extended Xꢀray Absorption Fine Structure (EXAFS) data were
collected on beamline 14W at the Shanghai Synchrotron Radiation Facility. The electrocatalysis
action of CQDs was test by a CHI 660C workstation (CH Instruments, Chenhua, Shanghai,
China), while the fluorescence lifetimes of the CQDs were measured by FluoroLog 3ꢀ211ꢀ
TCSPC.
General Procedure for Preparation of CQDs. The CQDs were synthesized through a
method of electrochemical ablation of graphite. In the synthetic process, two identical graphite
rods placed parallel (99.99%, Alfa Aesar Co. Ltd., 13 cm in length and 0.6 cm in diameter) were
inserted into the electrolyte at 3 cm above the liquid surface and used as both cathode and anode
in a separation distance of 7.5 cm. The electrolyte was ultrapure water (18.4 Mꢃ cmꢀ1, 600 mL)
without any other additives. Static potentials of 15–60 V were applied to the two electrodes using
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a direct current (DC) power supply for ten days with intense stirring. Subsequently, a darkꢀ
yellow solution appeared gradually in the reactor. The solution was filtered with slowꢀspeed
quantitative filter paper, and the resultant solution was centrifuged at 22000 rpm for 30 min to
remove the precipitated graphite oxide and graphite particles. Finally, the obtained solution
contained water soluble CQDs.
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General Procedure for Preparation of AuNPs. AuNPs were prepared by chemical reduction
method. A aqueous solution (20 mL) containing HAuCl4 (2.5 × 10−4 molꢁLꢀ1) and trisodium
citrate (2.5 × 10−4 molꢁLꢀ1 ) were prepared in a conical flask under continuous vigorous stirring.
Then iceꢀcold freshly NaBH4 solution (0.6 mL, 0.1 molꢁLꢀ1) was added into the above solution.
The resultant solution turned pink immediately after adding NaBH4, indicating the formation of
AuNPs.
General Procedure for Preparation of Au/CQDs Composites. The Au/CQDs composite
was obtained by dropwise addition of HAuCl4 solution (400 ꢄL, 25 mmolꢁLꢀ1) to CQDs solution
(20 mL) with vigorous magnetic stirring in dark environment at room temperature. The resultant
mixture was under constant magnetic stirring at room temperature for 2 h, aged overnight, the
solution turned red, implying the Au/CQDs composites were obtained. Then, the Au/CQDs
composites were centrifuged at 12000 rpm for 15 min, washed three times with deionized water
and ethanol respectively, and finally the obtained sample were dried 6 h at 25ꢀ60℃.
Photo-induced electron transfer properties of CQDs. The PL and photoꢀinduced electron
transfer properties of CQDs irradiated by visible light were investigated. In this experiment, PL
spectra of the CQDs in organic solutions were recorded with luminescence emission intensities
(485 nm excitation, laser source), which were quenched by the known electron acceptors 2,4ꢀ
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dinitrotoluene and electron donors N,Nꢀdiethylaniline in toluene solution (different
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concentration). The Stern–Volmer quenching constants (KSV =
τF0kq) were obtained from linear
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regression. The luminescence decays were tested by TCSPC method. For the CQDs sample,
when the excitation wavelength is 485 nm, we monitored the sample with 550 nm narrow
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bandpass filter, with the observed Stern–Volmer quenching constants (KSV =
τF0kq) from linear
regression.
The Electrocatalytic Activity of Au/CQDs Composites. The electrocatalytic activity of
Au/CQDs composites for H2O2 decomposition was tested in a conventional threeꢀelectrode
electrochemical cell with a platinum wire as the auxiliary electrode and an Ag/AgCl (saturated
KCl) as the reference electrode. The working electrodes were Au/CQDs compositesꢀmodified
glassy carbon electrode, The Au/CQDs compositesꢀmodified glassy carbon electrode was
prepared by spreading aqueous Au/CQDs composites over glassy carbon electrode. A 450 W Xe
lamp was used as light source with visibleꢀCUT filter to cut off light of wavelength < 420 nm to
get visible light, and was positioned 3 cm away from the photoelectrochemical cell. The cyclic
voltammograms (CVs) of the bare glassy carbon electrode and Au/CQDs composites over glassy
carbon electrode were obtained in 0.05 M phosphate buffer solution (PBS, pH 7.4) in the
presence of different H2O2 concentration at a scan rate of 25 mV/s. The photocurrent signal was
recorded with a CHI 660C workstation (CH Instruments, Chenhua, Shanghai, China) connected
to a personal computer. All electrochemical experiments were carried out at room temperature.
Terephthalic Acid Photoluminescence Probing Assay (TA–PL) to Detect the HO•.
Terephthalic acid (THA, 0.415 g), sodium phosphate (Na3PO4, 2.2 g), and deionizer water were
added into 250 mL volumetric flask to form a buffer at pH 7.5. After adding a suitable amount of
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catalyst into the buffer, the mixture was irradiated by visible light or no light. Product
fluorescence (THAꢀOH) was measured with a fluorescent spectrophotometry (Horiba Jobin
Yvon, FluoroMax 4) with 350 nm excitation and 429 nm emission at pH 7.5.
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CONCLUSIONS
In conclusion, we report that, the Au/CQDs composite shows unprecedentedly high
photocatalytic activity for the selective oxidation of cyclohexane to cyclohexanone with
conversion efficiency of 63.8% and selectivity of over 99.9% using H2O2 as oxidant without any
solvents. The mechanism of the catalyst system is proposed to involve: enhancement of light
absorption by surface plasma resonance of metal nanoparticles, production of active oxygen
species (HOꢁ) by H2O2 decomposition, and interaction between CQDs and AuNPs under visible
light; these processes collectively lead to the eventual high conversion and selectivity. The
present work suggests a new route for the design of highꢀperformance photocatalytic systems for
selective hydrocarbon oxidation for green chemical industry.
ASSOCIATED CONTENT
Supporting Information., UVꢀvis spectra and (HR)TEM images of metal nanoparticles/CQDs
composites. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* Eꢀmail: zhkang@suda.edu.cn; yangl@suda.edu.cn.
ACKNOWLEDGMENT
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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 Excelent Doctoral Dissertation of
China (200929), and a project funded by the Priority Academic Program Development of Jiansu
Higher Education Institutions.
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