Fabrication of CDs/CdS-TiO2 ternary nano-composites for photocatalytic degradation of benzene and toluene under visible light irradiation

Article abstract of DOI:10.1016/j.saa.2018.03.041

An efficient cadmium sulfide quantum-dots (CdS QDs) and carbon dots (CDs) modified TiO₂ photocatalyst (CdS/CDs-TiO₂) was successfully fabricated. The as-prepared ternary nano-composites simultaneously improved the photo-corrosion of CdS and amplified its photocatalytic activity. The introduction of CdS QDs and CDs could enhance more absorbance of light, prevent the undesirable electron/hole recombination, and promote charge separation, which was important for the continuous formation of [rad]OH and [rad]O₂ ^? radicals. When the optimal mass ratio of CdS QDs to CDs was 3:1, above 90% degradation efficiencies were achieved for benzene within 1 h and toluene in 2 h, while that of pure TiO₂ (P25), CdS QDs-TiO₂, CDs-TiO₂ nano-composites was around 15%. Owing to the symmetric structure and conjugation of methyl with benzene ring, the degradation of toluene was more difficult than benzene to carry on. The new fabricated nano-composites showed good prospective application of cleaning up refractory pollutants and the resource utilization.

Full text of DOI:10.1016/j.saa.2018.03.041

Accepted Manuscript
Fabrication of CDs/CdS-TiO2 ternary nano-composites for
photocatalytic degradation of benzene and toluene under visible
light irradiation
Meng Wang, Jianhao Hua, Yaling Yang
PII:
S1386-1425(18)30235-X
doi:10.1016/j.saa.2018.03.041
SAA 15913
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 6 January 2018
Revised date:
Accepted
date:
4 March 2018
1
2 March 2018
Please cite this article as: Meng Wang, Jianhao Hua, Yaling Yang , Fabrication of CDs/
CdS-TiO2 ternary nano-composites for photocatalytic degradation of benzene and toluene
under visible light irradiation. The address for the corresponding author was captured
as affiliation for all authors. Please check if appropriate. Saa(2017), doi:10.1016/
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ACCEPTED MANUSCRIPT
Fabrication of CDs/CdS-TiO2 ternary nano-composites for
photocatalytic degradation of benzene and toluene under visible light
irradiation
a,&
a,&
a,*
Meng Wang , Jianhao Hua , Yaling Yang
a
Faculty of Life Science and Technology, Kunming University of Science and
Technology, Yunnan Province 650500, China.
&
These authors contributed equally to this work and should be considered co-first
authors.
*
Corresponding author. E-mail address: yilyil8@163.com (Y. Yang).
Abstract: An efficient cadmium sulfide quantum-dots (CdS QDs) and carbon dots
CDs) modified TiO photocatalyst (CdS/CDs-TiO ) was successfully fabricated. The
(
2
2
as-prepared ternary nano-composites simultaneously improved the photo-corrosion of
CdS and amplified its photocatalytic activity. The introduction of CdS QDs and CDs
could enhance more absorbance of light, prevent the undesirable electron/hole
recombination, and promote charge separation, which was important for the
−
continuous formation of •OH and •O radicals. When the optimal mass ratio of CdS
2
QDs to CDs was 3:1, above 90% degradation efficiencies were achieved for benzene
within 1 h and toluene in 2 h, while that of pure TiO (P25), CdS QDs-TiO ,
2
2
CDs-TiO nano-composites was around 15%. Owing to the symmetric structure and
2
conjugation of methyl with benzene ring, the degradation of toluene was more
difficult than benzene to carry on. The new fabricated nano-composites showed good
1

ACCEPTED MANUSCRIPT
prospective application of cleaning up refractory pollutants and the resource
utilization.
Keywords: Benzene; Toluene; CdS/CDs-TiO2 ternary nano-composites;
Photocatalytic degradation
1
. Introduction
The expanding organic environmental pollution has become the crucial issue
which not only influences human being's health, but also impedes the development of
society and economy. Volatile mono-aromatic compounds such as benzene and
toluene are a class of environmental pollutants with potentially toxic, mutagenic or
carcinogenic properties [1]. Benzene and toluene have higher water solubility than
other hydrocarbons and a lower octanol-water partition coefficient, that leads to a
slowly soil absorption and, consequently, a preferential water transport, contaminating
groundwater reservoirs [2]. The World Health Organization (WHO) has established
that the maximum permissible concentrations of benzene and toluene in potable water
are 0.01 and 0.7 mg/L, respectively [3, 4]. Various techniques have been investigated
aiming to control or remove aromatic hydrocarbons benzene and toluene from
contaminated water such as adsorption [3], bioremediation [5], biodegradation [6],
advanced oxidation [7] and photocatalytic oxidation [8, 9]. Highly efficient
heterogeneous photocatalysts for the degradation of various kinds of organic
contaminants have attached significant attention in the past two decades due to their
extensive applicability and excellent performances [10, 11].
Among the photocatalysts, the photocatalytic properties of TiO are attributed to
2
2

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the production of photogenerated electrons in the conduction band (CB) and holes in
the valence band (VB) which occur upon the radiation of ultraviolet (UV) light
(
10-400 nm). However, the photocatalytic efficiency of TiO photocatalysts have been
2
limited due to its wide band gap energy (3.0-3.2 eV) and high recombination rate,
which accounts for merely 5% of the entire solar light, thereby limiting its
photoresponse to ultraviolet light [11-17]. To improve the photocatalytic activity and
the response in the visible region, two kinds of strategies were proposed: doping of
TiO with metals and non-metals [18-21] ions to widen the solar spectrum absorption
2
window to visible radiation, or combination with narrow band gap nano-crystalline
semiconductors and quantum dots [22-25] to inhibit efficiently the recombination
−
+
between the photo-generated electrons and holes (e -h ) [26-28].
Cadmium sulfide (CdS) has been extensively studied for applications in
photocatalytic and photoelectrochemical water reduction due to its narrow band gap
energy (2.4 eV) and relatively high visible absorption coefficient [29, 30]. When CdS
was coupled with TiO , the two semiconductors were illuminated and simultaneously
2
activated under incident light owing to the synergistic effect. On the other hand, the
formation of heterojunctions wherein the contact interfaces could hamper
electron-hole recombination dominating the charge separation [31]. Nevertheless,
some drawbacks still associated with such semiconductor material, such as the rapid
charge recombination and the well-known photocorrosion induced by the
photo-induced holes, which obstructed its development and application [32, 33]. One
of the most widely concerned photocatalysts was CdS-based photocatalysts which
3

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combined CdS with other semiconductors or materials to effectively suppress
photocorrosion of CdS.
Researches indicated that when compared to fluorescent semiconductor quantum
dots like CdS, CdTe, PbS, and CuInS , carbon dots (CDs) had attracted increasing
2
attention owing to its non-toxicity, unique electron reservoir, photoinduced electron
transfer property, broad absorption spectrum, large absorption coefficient and good
chemical stability. They have been applied to couple with various semiconductors to
extend their light absorption to visible or even near-infrared (NIR) range for better
photocatalytic performance [32-34]. When CDs were loaded on the surface of
CdS-based composites, the CDs could absorb visible light and emit short wavelength
light through upconversion, which in turn excited CdS to form electron-hole pairs.
Meanwhile, CDs could also act as electron reservoirs to trap electrons emitted from
−
+
CdS, resulting in hindrance of the recombination of electron-hole (e -h ) pairs, which
−
was important for the continuous formation of •OH and •O radicals [34].
2
In the present work, we combined the advantages of introducing CDs and CdS to
design an effective photocatalyst to improve the catalysis activity of TiO . In the
2
developed system, the CDs were synthesized by one-pot hydrothermal treatment of
milk, and the ternary nano-composites of CDs/CdS-TiO were prepared by a simple
2
approach. The as-prepared photocatalysts were examined by characterizing the
structure, morphology, and optical properties. The photocatalytic activity of the
composites was investigated for the degradation of benzene and toluene under visible
light illumination. The results demonstrate that when the optimal mass ratio of CdS
4

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QDs to CDs was 3:1, the degradation efficiency for benzene and toluene within 2 h
was both above 90%. We also extensively investigated the molecular-level pathway of
benzene and toluene based on trapping experiments, and all the intermediates have
been analyzed by LC-MS.
2
. Experimental
2
.1. Materials
All reagents were analytical grade and used without further purification.
CdCl ·2.5H O (99%), Na S·9H O, sodium sulfite (Na SO ), and mercaptoacetic acid
2
2
2
2
2
3
(MAA) were purchased from J&K Chemical Ltd. (Beijing, China). Sodium
borohydride (NaBH ), disodium salt of ethylenediaminetetraacetic acid (EDTA-2Na),
4
ammonium oxalate (AO) were obtained from the Fengchuan Chemical Reagent Co.
Ltd. (Tianjin, China). Sodium hydroxide (NaOH), p-benzoquinone (p-BQ), isopropyl
alcohol (IPA), cetyltrimethylammonium bromide (CTAB) and benzene were supplied
by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Commercial TiO (P25)
2
was kindly supplied by Macklin Company (Shanghai, China). The milk was
purchased from the local supermarket of Kunming, China. All experiments were
carried out using double distilled water (DDW).
2
.2. Instruments
X-ray power diffraction (XRD, Bruker, Germany) analysis was carried out to
determine the crystal pattern of catalyst using Cu Kα radiation operated at 40 mA and
◦
◦
4
0 kV with 2θ collection range from 5 to 90 . Morphologies of the catalyst were
5

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measured using field emission scanning electron microscopy (FESEM, JEOL 7006F)
and high-resolution transmission electron microscope (HR-TEM, FEI Tecnai G2
TF30). The fluorescence spectra were recorded using a G9800A fluorescence
spectrophotometer (Agilent, USA). UV-Vis diffuse reflection spectroscopy (DRS) was
performed on a SHIMADZU UV-2550 spectrophotometer using BaSO as the
4
reference. And X-ray photoelectron spectra (XPS) were investigated on an Axis Ultra
spectrometer (Kratos) to observe the elemental compositions and the chemical
oxidation state using Mg Kα radiation excitation source
2
2
.3. Preparation of catalyst
.3.1 Synthesis of CDs
The CDs were synthesized by one-pot hydrothermal treatment of milk as the
carbon source according to follows [35, 36]. First, 50 mL of the fresh milk was added
into 50 mL of deionized water. Then, the solution was transferred into a teflon-lined
stainless steelautoclave and heated at 180 °C for 6 h. After naturally cooling to room
temperature, the CDs solution was centrifuged 10000 rpm for 10 min to remove
weight precipitate and any impurities. The product was filtered through the 0.22 μm
filter membrane.
2
.3.2. Synthesis of CdS quantum dots
MAA-stabilized CdS quantum dots (CdS QDs) were synthesized according to
-2
the literature [37]. Typically, 250 µL of MAA was added to 50 mL of 1.0×10 mol/L
CdCl aqueous solution, the pH was adjusted to 9.0 with a 1.0 mol/L NaOH solution.
2
During the above process, N was bubbled throughout the solution to remove O for
2
2
6

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3
0 min. Then, 5.0 mL of 0.1 mol/L Na S aqueous solution was injected into this
2
solution to obtain MAA-capped water-soluble CdS QDs and the reaction mixture was
refluxed under N atmosphere for 4 h. This procedure produced CdS QDs with a Cd
2
to S (Cd/S) ratio of 1.0.
2
.3.3. Synthesis of CDs/CdS-TiO ternary nanocomposites
2
The ternary nano-composite of CDs/CdS-TiO was synthesized as follows:
2
initially, 5.0 g of commercial TiO (P25) powder and 0.5 g CTAB were added into
2
1
00 mL DDW and stirred for 4 h. After that, the precipitated TiO powder was
2
separated by centrifugation, further dried in vacuum at 60 ℃. The powder was
calcined at 450 ℃ for 5 h in the muffle furnace. 1.0 g of above TiO powder was
2
added into the mixture solution of 24 mL 0.5 mg/mL CdS QDs and 1 mL 4 mg/mL
CDs, and stirred for 24 h at room temperature. The final products were centrifuged
and washed with deionized water and ethanol for several times, and finally dried in
vacuum at 60 ℃ for several hours. To investigate the effect of the content of CDs and
CdS QDs on the photocatalytic activity, different mass ratio of CDs to CdS QDs
modified TiO nano-composites were prepared. In our study, the optimal mass ratio of
2
CdS QDs to CDs was 3:1.
2
.4. Catalytic degradation experiment
The photocatalytic activity of the synthesized photocatalysts were evaluated at
ambient temperature under a commercial 300 W Xe-arc lamp (GXH-300, Shanghai,
China, Jiguang Special lamp Co. Ltd.) as a source of visible-light irradiation. In a
typical photocatalytic experiment, 40 mg the as-prepared CDs/CdS-TiO ternary
2
7

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nano-composites were suspended in 50 mL of aqueous solution containing benzene
(30 μg/mL) and toluene (30 μg/mL), separately. Prior to photocatalytic experiments,
the suspension was stirred for 2 h to achieve adsorption-desorption equilibrium state
of the solution, which was kept in the dark before light irradiation. The reaction
mixture was irradiated with stirring under the Xe-arc lamp while the system was
cooled by fan to maintain the reaction mixture at room temperature. After the
specified time interval 5 mL of solution was drawn and centrifuged at 5000 rpm for 5
min to remove the catalyst. The concentrations of benzene and toluene in the solution
were measured using UV-Vis spectrophotometer (UV-2550, SHIMADZU, Japan) at
wavelength 254 and 260 nm, respectively. The same experiments were done for bare
TiO , CdS QDs-TiO , and CDs-TiO for comparison purpose. The photodegradation
2
2
2
percentage was calculated using the following equation: D(%) = (C - C )/C × 100,
0
t
0
where C and C are the concentrations of target analytes at time 0 and t (s),
0
t
respectively, and t is the irradiation time.
2
.5. Detection of active species
On the basis of the previously reported method [38], in order to determine and
reveal the role of various kinds of photocatalytic-dependent radicals and reaction
mechanisms, a series of scavenger test experiments were conducted. Herein, isopropyl
alcohol (IPA), ammonium oxalate (AO), p-benzoquinone (p-BQ) and Na EDTA were
2
+
−
used for hydroxyl radicals (•OH), holes (h ), superoxide radicals (•O ) and H O
2
2
2
scavengers, respectively.
.6. Identification of degradation pathway
2
8

ACCEPTED MANUSCRIPT
The degradation intermediates of benzene and toluene were detected by an
ACQUITY UHPLC/MS (Waters, America). The mixture solution of 90% acetonitrile
and 10% formic acid (0.1% by volume) was used as the mobile phase. The flow rate
was 0.2 mL/min. The mass spectrometry analysis was conducted by electrospray
ionization (ESI) in positive ionization mode. The mass spectra with a scan range m/z
1
0-110 were recorded. The temperature of drying gas heater was 500 °C. The drying
gas (N ) flow rate was 1000 L/h, and the capillary voltage was 600 V. The data were
2
analyzed by Waters Masslynx™ 4.1 System.
3
3
3
Results and discussion
.1 Structural characterization and surface composition
.1.1 XRD analysis
A systematic X-ray diffraction measurement (XRD) study was carried out to
investigate the crystalline phase and the structure. Fig. 1 showed the XRD pattern of
the pure TiO (P25), CdS QDs-TiO , CDs-TiO and CDs/CdS-TiO nano-composites,
2
2
2
2
respectively. It was found that the samples were all well-crystallized. The XRD
◦
◦
◦
patterns displayed diffraction peaks centered at around 2θ values of 25.3 37.8 , 48.0 ,
◦
◦
◦
5
3.9 , 55.1 and 62.7 could be assigned to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1)
and (2 0 4) crystallographic planes of anatase TiO (JCPDS 21-1272) [39,40]. The
2
◦
peak at around 2θ value of 27.4 could be attributed to the (1 1 0) crystallographic
plane of rutile TiO (JCPDS 21-1276). CdS QDs-TiO , CDs-TiO and CDs/CdS-TiO
2
2
2
2
nano-composites also exhibited the similar characteristic diffraction peaks of anatase
TiO , suggesting that the existence of CdS QDs and CDs could hardly affect the
2
9

ACCEPTED MANUSCRIPT
crystalline structure of TiO nano-particles.
2
3
.1.2 TEM analysis
The morphology and detailed microstructures features of prepared catalysts were
further explored using TEM and high resolution TEM (HRTEM). As was shown in
Fig. 2a, it could be seen that the average diameters of those spherical shape particles
CdS QDs and CDs both were about 2-5 nm along, which were uniformly dispersed on
the surface of larger TiO (with an average diameter of about 20-50 nm). It confirmed
2
that these samples were both composed by a mass of irregular nano-particles, which
meant that the addition of CdS QDs and CDs could not change the morphologies of
TiO nano-particles obviously. The corresponding high magnification TEM image
2
reflected that most of the particles possessed well-resolved lattice fringes. As
displayed in the HRTEM images (Fig. 2b), the lattice distances shown that the inter
planar spacing of 0.35 nm and 0.308 nm were corresponded well with (1 0 1) and (2 2
0
) plane of anatase TiO and CdS QDs, while the lattice spacing of CDs was 0.24 nm
2
which well agreed with the plane (0 0 4) spacing of carbon. Again, the structures of
CdS QDs-TiO , CDs-TiO and CDs/CdS-TiO nano-composites were confirmed,
2
2
2
which was also consistent with the XRD results.
The textural properties of as-prepared bare, binary and ternary samples also were
investigated using Brunauer-Emmett-Teller (BET) surface analysis. The BET specific
surface areas of the as-prepared samples calculated from the linear part of the BET
plot are summarized in Table 1. Compared with the as-prepared samples, the ternary
2
CDs/CdS-TiO has a BET surface area of 160.76 m /g, which is higher than those of
2
10

ACCEPTED MANUSCRIPT
pure CdS, CDs and TiO . The higher specific surface areas may beneficial for the
2
photocatalytic degradation because there being more reactive radical species and
oxidative intermediates for the oxidation and mineralization of pollutants, ease of
transportation of them through the interconnected networks, and enhanced harvesting
of light [41].
3
.1.3 XPS analysis
To investigate surface chemical composition and bonding configuration of the
as-prepared pure TiO
2
,
CdS QDs-TiO
2
,
CDs-TiO2 and CDs/CdS-TiO2
nano-composites, the X-ray photoelectron spectroscopy (XPS) analysis was employed
and the results were shown in Fig. 3. The full-scale XPS pattern observed in Fig. 3a
showed the coexistence elements of Ti, O, C, Cd and S in the as-prepared pure TiO
2
,
CdS QDs-TiO , CDs-TiO and CDs/CdS-TiO nano-composites. Fig. 3b-f illustrated
2
2
2
the high-resolution XPS spectra of Ti 2p, Cd 3d, S 2p, C 1s and O 1s, respectively.
The Cd 3d and S 2p signals came from CdS and the presence of C 1s and O 1s signals
again confirmed the existence of CDs in the composite nano-particles. Ti 2p XPS
peaks were located at 458.8 eV and 464.4 eV (Fig. 3b) with a spin-orbital doublet
splitting (Ti 2p -Ti 2p ) of 6.6 eV in as-prepared composite sample, which implied
3
/2
1/2
4
+
an oxidation state of Ti . As presented in Fig. 3c, the deconvoluted peaks of the Cd
2
+
3
d5/2 and Cd 3d3/2 located at 405.4 and 412.2 eV finely indicate the Cd in CdS. The
core level S 2p spectrum was shown in Fig. 3d, which could be fitted into two
individual peaks S 2p3/2 and S 2p at 162.0 and 164.1 eV with a splitting energy of
1/2
2
-
2
.1 eV, corresponding to the valence state of S in CdS. The C 1s XPS spectrum in
11

ACCEPTED MANUSCRIPT
Fig. 3e could be fitted by two main peaks at 284.5 and 285.6 eV, which could be
attributed to the C–C/C=C and C–O groups, respectively. To further investigate the
oxide state, O 1s was measured (Fig. 3f). The main peak were observed at the binding
energie of 530.2 eV, which might be assigned to C–Ti–O, indicating that
photo-generated electrons should transfer from valence band (VB) of the TiO to CDs
2
through C–Ti–O bond formed between TiO microsphere interface and CDs.
2
3
.2 UV-vis diffuse reflectance spectra analysis
The optical properties of as-prepared binary composites pure TiO , CdS
2
QDs-TiO
UV-vis diffuse reflectance spectroscopy (UV-vis DRS) and the results were shown in
Fig.4a. It was clearly that the spectrum of the pure TiO (P25) showed a significantly
2
, CDs-TiO and CDs/CdS-TiO nano-composites were investigated using
2 2
2
absorption edge at wavelength shorter than 380 nm, which could be assigned to its
intrinsic band gap absorption. For CDs/CdS-TiO nano-composites, the absorption
2
edge slightly shifted to the visible region with enhanced absorption intensity. The
band gap narrowing of CDs/CdS-TiO nano-composites was mainly attributed to the
2
C atoms occupying the interstitial sites in the TiO lattice, which led to a decrease in
2
optical band gap energy [34]. It could be also found that the deposition of CdS
improved obviously the absorption intensity of TiO narrow band gaps in visible light
2
region (λ > 420 nm) and the absorption edge appeared an obvious red shift which
might be ascribed to the narrow band gap of CdS QDs. It was a direct proof that the
loading of CdS QDs and CDs on the surface of TiO both would enhance the
2
absorption intensity in visible light region and improved the light harvest and was also
12

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beneficial to its photocatalytic performance.
The energy band gap (Eg) of the as-synthesized photocatalysts was calculated
according to the UV-vis absorption spectra. In semiconductor physics, the formula
m
(
αhυ) = A(hυ - Eg) was used to relate the absorption coefficient to the energy band
gap, where α, h, υ, A, and Eg corresponded to the absorption coefficient, Planck
constant, light vibration frequency, proportional constant and band gap energy,
respectively. The m value depended on the nature of the electronic transition, when m
=
2, the semiconductor had a direct band gap, while when m = 1/2, the semiconductor
had an indirect band gap [32, 42]. TiO was an indirect semiconductor, the switched
2
1
/2
tauc plots of the (αhυ) versus photon energy (hυ) were displayed in Fig. 4b, where
the two intersections of the tangent lines and the baseline denoted the band gap
energies. The band gap energies of CDs/CdS-TiO , CdS QDs-TiO
2
, CDs-TiO
nano-composites determined from the intercepts were approximately 2.28,
.56, 2.79 and 3.10 eV, respectively. Obviously, the modification of CdS QDs and
2
and
2
pure TiO
2
2
CDs with TiO could reduce the corresponding band gap energy, indicating an easier
2
trend for CDs/CdS-TiO nano-composites to generate the electron-hole pairs by
2
absorbing wider wavelength of light.
3
.3 Photoluminescence spectra analysis
It is well known that photoluminescence (PL) emission is important to disclose
the fate of electron-hole pairs in semiconductor because PL emission is resulted from
the recombination of photogenerated electrons and holes for some semiconductors
[
41]. PL spectra were taken to understand the recombination rate of electron/hole of
13

ACCEPTED MANUSCRIPT
charge carrier trapping, migration and transfer. The higher intensity corresponds to the
faster recombination. Fig. 5 showed the typical PL spectra of pure TiO (P25), CdS
2
QDs-TiO , CDs-TiO , CDs/CdS-TiO nano-composites excited in 300 nm. The main
2
2
2
emission peak of these composites was centered at 602 nm, which could be
contributed to the band-band PL phenomenon with the energy of light approximately
equal to the band energy of TiO . The results showed that the modification of CdS
2
QDs and CDs onto TiO drastically decreased the PL intensity of pristine TiO . When
2
2
both of CdS QDs and CDs were decorated onto the surface of TiO , the peak intensity
2
further decreased for CDs/CdS-TiO ternary nano-composites comparing with the
2
binary composites of CdS QDs-TiO and CDs-TiO . These results suggest that the
2
2
hetero structure among TiO , CdS QDs and CDs can effectively diminish the
2
electron/hole recombination rate. The PL intensity has strong effect on the
photocatalytic activity of the corresponding photocatalysts.
3
.4 Photocatalytic activity evaluations
The degradation efficiencies of benzene and toluene over pure TiO (P25), CdS
2
QDs-TiO , CDs-TiO , CDs/CdS-TiO nano-composites under simulated sunlight
2
2
2
irradiation within 3 h were displayed in Fig. 6a. All CDs/CdS-TiO nano-composites,
2
which were prepared with different mass ratios mixture of CdS QDs to CDs, exhibited
much higher photocatalytic activities compared to the as-prepared photocatalysts like
pure TiO (P25), CdS QDs-TiO and CDs-TiO . The maximum photocatalytic
2
2
2
degradation efficiency of benzene and toluene was obtained with a 3:1 mass ratio of
CDs to CdS QDs modified TiO nano-composites (90%), while that of pure TiO
2
2
14

ACCEPTED MANUSCRIPT
(
P25), CdS QDs-TiO , CDs-TiO nano-composites was around 15%. When the
2
2
proportion of CdS QDs arrived to a large amount, it would lead to stability concerns
raised by photocorrosion and the CdS electron transfer center was likely to be the
electron recombination center in the process of photocatalytic reaction, and the
efficiency of the electron hole separation was decreased [43]. Besides, both CdS QDs
and CDs modified TiO nano-composites could effectively hinder the electron/hole
2
−
+
(
e -h ) recombination and enhance more absorbance of light, which was important for
−
the continuous formation of •OH and •O radicals. CDs/CdS-TiO nano-composites
2
2
improved the photocatalytic property owing to the synergistic effect between CDs and
CdS QDs. It should be noted that the best-performing system CDs/CdS-TiO2
nano-composites, which were prepared with 3:1 mass ratios mixture of CdS QDs to
CDs, was selected as a model for the follow-up study.
In addition, photocatalytic degradations of benzene and toluene were studied by
as-prepared CDs/CdS-TiO nanocomposites under simulated sunlight irradiation. The
2
curves of degradation efficiency versus reaction time for samples were shown in Fig.
6
b. From these figures, it was clear that the time taken for 90% degradation of the
benzene was 1 h, whereas 90% degradation of the toluene was 2 h. As a contrast on
the correctness of the results, the benzene and toluene concentrations of the same
samples before and after degradation were analyzed simultaneously by the HPLC
method. The results showed that photocatalytic degradation rates of benzene and
toluene were fairly close to the results of HPLC-VWD. Besides, photocatalytic
degradation of p-xylene was also studied for contradistinction, and the results showed
15

ACCEPTED MANUSCRIPT
only 33% degradation of the p-xylene was obtained even after 3 h. Those results
indicated that CDs/CdS-TiO nano-composites showed better photocatalytic activity
2
towards benzene and toluene compared to p-xylene. It can be explained as follows: in
−
this system, the degradation is started by addition of superoxide radicals (•O ),
2
+
hydroxyl radicals (•OH) and positive holes (h ) to benzene and toluene, which yield
some by-products such as phenol, benzaldehyde and benzoic acid. Eventually, the
reaction mineralizes these benzene and toluene to end products such as carbon dioxide
and H O. From molecular structure, owing to the symmetric structure and conjugation
2
of methyl with benzene ring of toluene and p-xylene [44], the more superoxide
−
+
radicals (•O ), hydroxyl radicals (•OH) and positive holes (h ) they need, the more
2
difficult it is to photocatalytic degrade, especially p-xylene.
3
.5 Photocatalysis degradation mechanism analysis
To speculate on the photocatalytic mechanism and reveal the contribution of
each active radical species in the photo-degradation system, a series of the radical
trapping experiments were carried out further, including p-benzoquinone (p-BQ) as a
−
superoxide radical (•O ) scavenger, isopropyl (IPA) as a hydroxyl radical (•OH)
2
+
scavenger, Na EDTA as an H O scavenger and ammonium oxalate (AO) as hole (h )
2
2
2
scavenger [45-48]. As displayed in Fig. 7, without the addition of scavengers, the
photocatalytic conversion ratio of benzene and toluene degradations were 93.8% and
9
0.2% after 3 h of visible light irradiation, respectively. The degradation ratio of
benzene and toluene decreased slightly when Na EDTA was added into the
2
photoreaction system, while p-BQ, IPA and AO both decreased obviously. Based on
16

ACCEPTED MANUSCRIPT
−
+
the above analysis, it clearly suggested that the •O , h and •OH should be the major
2
radicals for benzene and toluene degradation in the photodegradation process.
Meanwhile, H O also promoted the process of photocatalytic reaction.
2
2
According to the discussion above, the possible reaction mechanism schematic
for the photocatalytic degradation of benzene and toluene has been illustrated in
scheme 1. The CDs could absorb visible light and emit short wavelength light through
upconversion, which in turn excited CdS to form electron-hole pairs. Meanwhile, CDs
could also act as electron reservoirs to trap electrons emitted from CdS, resulting in
−
+
hindrance of the recombination of the undesirable electron-hole (e -h ) pairs, and
promote charge separation, resulting in a higher utilization ratio of energy [32].
Owing to their synergistic effect, the electrons trapped from those two paths could be
transferred effectively to the adsorbed oxygen molecules to form superoxide radical
−
−
(
•O ). Simultaneously, superoxide radical (•O ) could further react with two protons
2
2
to form hydrogen peroxide (H O ), which undergoes reaction with electrons to form
2
2
more •OH radicals. Meanwhile, the holes could be consumed quickly by adsorbed
hydroxyl ions or water molecule to produce more hydroxyl radicals (•OH). More
importantly, the electron-hole pairs transferred to CDs could react with H O instead of
2
CdS itself, especially the holes, hence making CDs/CdS-TiO a stable photocatalyst
2
without requirement for any sacrificial agents.
3
.6 Degradation reaction pathway and intermediate products analysis
LC-MS was used to identify the intermediates and final degradation products of
benzene and toluene. In this system, the main characteristic peaks of benzene (m/z =
17

ACCEPTED MANUSCRIPT
7
8) and toluene (m/z = 91) were detected before light illumination and its intensity
decreased gradually after visible light irradiation, respectively. Upon photocatalytic
reaction, a reaction product such as CO was detected and intermediate products such
2
as phenol and benzoquinone (m/z = 93), benzaldehyde (m/z = 106), benzoic acid (m/z
=
121), malonic acid (m/z = 104), oxalic acid (m/z = 89), glycolic acid (m/z = 76),
acetaldehyde (m/z = 44), acetic acid (m/z = 59) and formaldehyde (m/z = 29) were
identified. Based on the results of mass to charge ratio (m/z) and preliminary
structures of intermediates products, the proposed reaction pathway describing the
photodegradation mechanism can be suggested as shown in scheme 2. The
−
degradation is started by addition of superoxide radicals (•O ), hydroxyl radicals
2
+
(
•OH) and positive holes (h ) to benzene and toluene, which yield phenol or
benzaldehyde. The subsequent cleavage of the phenol or benzaldehyde would result in
the formation of malonic acid. The malonic acid is converted to lower molecular
weight organic compounds such as formaldehyde and acetic acid, which will then be
converted to carbon dioxide and H O as the end products through the subsequent
2
reaction steps. Eventually, the reaction mineralizes benzene and toluene to end
products such as carbon dioxide and H O.
2
4
. Conclusion
In summary, an efficient CDs/CdS-TiO ternary nano-composite was fabricated
2
as a photocatalyst for visible light organic pollutants splitting without requirement for
any sacrificial agents. These charge carriers generated on CdS would transfer to CDs
due to a dual synergistic effect of sensitization between CDs and CdS QDs, leading to
18

ACCEPTED MANUSCRIPT
−
+
a rapid separation of the electron-hole pairs (e -h ). Besides, CDs, as efficient electron
reservoirs, enhanced the absorption towards visible light and trapped electrons
emitted, resulting in a higher utilization ratio of energy and significantly enhancing
the photocatalytic activity of the system. The experimental results showed that when
the optimal mass ratio of CdS QDs to CDs was 3:1, above 90% degradation
efficiencies were achieved both benzene within 1 h and toluene within 2 h,
respectively. The proposed system is a promising photocatalytic degradation
pathway that has good potential for application to pollutant purification.
Acknowledgments
The work was greatly supported by the Analysis and Testing Foundation
(No.2017M20152118084) of Kunming University of Science and Technology, Yunnan
Province, China.
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Table 1 BET surface area of the as-prepared catalysts.
Sample
CdS
CDs
TiO2
CdS-TiO2
165.62
CDs-TiO2
198.09
CDs/CdS-TiO2
160.76
2
BET (m /g)
23.70
57.12
130.14
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Highlights:
. An efficient cadmium sulfide quantum-dots and carbon dots modified TiO2
photocatalyst (CdS/CDs-TiO ) was successfully fabricated.
1
2
2
. A dual synergistic effect of sensitization between CDs and CdS QDs effectively
−
+
hinder the electron/hole (e -h ) recombination and enhance more absorbance of light.
3
2
. Above 90% degradation efficiencies were achieved for benzene and toluene within
h while the optimal mass ratio of CdS QDs to CDs was 3:1.
23

Graphics Abstract

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5