Decoration of TiO2/g-C3N4 Z-scheme by carbon dots as a novel photocatalyst with improved visible-light photocatalytic performance for the degradation of enrofloxacin

Article abstract of DOI:10.1039/c7ra05485h

A novel visible-light-driven carbon dot (CDs)/TiO₂/g-C₃N₄ photocatalyst was successfully synthesized by doping CDs in TiO₂ nanoparticles and the surface of g-C₃N₄ nanosheets via a facile hydrothermal process and was confirmed by characterization methods. UV-vis diffuse reflectance spectra (DRS) revealed that CDs/TiO₂/g-C₃N₄ showed obvious additional absorption in the 370-450 nm region. DMPO spin-trapping ESR spectra demonstrated the existence of O₂^- and ·OH. The photocatalytic activity of the CDs (1.0 wt%)/TiO₂/g-C₃N₄ was remarkably enhanced as compared to that of the single components (TiO₂ and g-C₃N₄) and double component (TiO₂/g-C₃N₄) towards the degradation of enrofloxacin (ENX) under visible-light irradiation. About 91.6% of ENX was decomposed by CDs (1.0 wt%)/TiO₂/g-C₃N₄ in 1 h, which is nearly 3.95 times, 4.82 times, and 1.69 times that for TiO₂, g-C₃N₄, and TiO₂ (90.0 wt%)/g-C₃N₄, respectively. Scavenging experiments revealed that O₂^- and ·OH played key roles during the photocatalytic degradation of ENX. This study provides a simple and convenient method to modify materials with enhanced photocatalytic performance, and the CDs/TiO₂/g-C₃N₄ catalyst is efficient, stable, and reusable for environmental practical applications.

Full text of DOI:10.1039/c7ra05485h

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Decoration of TiO2/g-C3N4 Z-scheme by carbon
dots as a novel photocatalyst with improved
visible-light photocatalytic performance for the
degradation of enrofloxacin†
Cite this: RSC Adv., 2017, 7, 34096
Yuehan Su,
Ping Chen, Fengliang Wang, Qianxin Zhang, Tiansheng Chen,
*
Yingfei Wang, Kun Yao, Wenying Lv and Guoguang Liu
A novel visible-light-driven carbon dot (CDs)/TiO2/g-C3N4 photocatalyst was successfully synthesized by
doping CDs in TiO2 nanoparticles and the surface of g-C3N4 nanosheets via a facile hydrothermal
process and was confirmed by characterization methods. UV-vis diffuse reflectance spectra (DRS)
revealed that CDs/TiO2/g-C3N4 showed obvious additional absorption in the 370–450 nm region. DMPO
spin-trapping ESR spectra demonstrated the existence of O2cꢀ and $OH. The photocatalytic activity of
the CDs (1.0 wt%)/TiO2/g-C3N4 was remarkably enhanced as compared to that of the single
components (TiO2 and g-C3N4) and double component (TiO2/g-C3N4) towards the degradation of
enrofloxacin (ENX) under visible-light irradiation. About 91.6% of ENX was decomposed by CDs (1.0
wt%)/TiO2/g-C3N4 in 1 h, which is nearly 3.95 times, 4.82 times, and 1.69 times that for TiO2, g-C3N4,
and TiO2 (90.0 wt%)/g-C3N4, respectively. Scavenging experiments revealed that O2cꢀ and $OH played
key roles during the photocatalytic degradation of ENX. This study provides a simple and convenient
method to modify materials with enhanced photocatalytic performance, and the CDs/TiO2/g-C3N4
catalyst is efficient, stable, and reusable for environmental practical applications.
Received 15th May 2017
Accepted 17th June 2017
DOI: 10.1039/c7ra05485h
rsc.li/rsc-advances
recombination.10 Furthermore, the low band gap (2.70 eV)
capacitates g-C3N4 to efficiently harvest sunlight within the
Introduction
visible-light region. Although g-C3N4 possesses better photo-
catalytic activity as compared to TiO2, it is far from enough to
meet the demands in practical use. Various attempts including
surface sensitization, chemical modication, and coupling with
other semiconductor materials have been made to increase the
photocatalytic efficiency of TiO2 and g-C3N4.9,11,12 Much interest
has recently been concentrated on the fabrication of TiO2/g-
C3N4 heterostructures, which extend the absorption to the
visible-light region.13–15 However, to the best of our knowledge,
h+ from EVB of g-C3N4 is incapable of generating $OH under
visible light; in addition to the abovementioned shortcomings
of TiO2, the photocatalytic performance of TiO2/g-C3N4 is still
unsatisfactory.
Carbon dots (CDs), with an appealing group of zero-
dimensional nanostructures, have attracted extensive research
interest in the photocatalytic eld due to their low-cost, aqueous
dispersibility, chemical stability, unique photoinduced electron
transfer, and electron reservoir properties.16 Hence, photo-
catalysts with the incorporation of CDs have achieved excellent
effects in the past few years. For example, Guo et al. synthesized
a carbon quantum dot/carbon nitride hybrid photocatalyst to
improve the photocatalytic degradation of methyl orange.17 Wu
et al. used CDs as a solid-state electron mediator for the
In the past decade, use of solar-energy-driven semiconductors
as photocatalysts to remove aquatic contaminants became one
of the most promising approaches.1–3 As is known, semi-
conductor titanium dioxide (TiO2) plays a signicant role in the
eld of photocatalysis due to its nontoxicity, inexpensiveness,
high activity, etc.4 However, TiO2 has a band gap of approxi-
mately 3.2 eV, which requires ultraviolet irradiation with
wavelengths less than 385 nm for activation. That is, this exci-
tation wavelength can activate electron transfer from the TiO2
valence band (VB) to the conduction band (CB). Consequently,
the photocatalyst utilizes less than 5% of the energy available in
sunlight, which extremely restricts its application in practice.5–7
More recently, a metal-free semiconductor, graphitic carbon
nitride (g-C3N4), has been successfully synthesized through
a facile thermal polycondensation method.8,9 It is reported that
g-C3N4 exhibits high photocatalytic activity due to its delo-
calized conjugated p structures, leading to relatively rapid
photoinduced charge separation and slow charge
School of Environmental Science and Engineering, Institute of Environmental Health
and Pollution Control, Guangdong University of Technology, Guangzhou, 510006,
China. E-mail: liugg615@163.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra05485h
34096 | RSC Adv., 2017, 7, 34096–34103
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
preparation of BiVO4/CD/CdS Z-scheme photocatalyst, which g) urea was mixed with 10 mL DI water, which was placed into
showed excellent photocatalytic activity and stability for overall a Teon autoclave for 5 h at 180 ꢁC. Aer being cooled to room
water splitting under visible light.18 Recently, Yu et al. reported temperature, the obtained brown water solution was centri-
a novel CD/ZnFe2O4 composite photocatalyst with enhanced fuged at 10 000 rpm for 15 min to remove large particles.
photocatalytic performance for the removal of NOx, demon- Finally, the remaining solution was transferred to the vacuum
strating that CDs acted as an electron reservoir and transporter drier for 3 h at 80 ꢁC to obtain a brownish black solid (C-dots).
as well as a powerful energy-transfer component.19 Due to the
Preparation of g-C3N4. g-C3N4 was prepared by directly
outstanding photocatalytic properties, in this study, CDs were heating dicyandiamide.28 In brief, 5.0 g of dicyandiamide was
rst used for the decoration of the TiO2/g-C3N4 Z-scheme placed in an alumina crucible with a cover and then heated to
through a facile hydrothermal-polymerized method.
550 ꢁC for 3 h at a heating rate of 2.8 ꢁC minꢀ1. Aer being
Pharmaceuticals and personal care products (PPCPs), as cooled down to room temperature, the obtained bulk g-C3N4
emerging contaminants in recent years, have received unprec- was milled into powders for further use.
edented concern because of their potential hazard to the
Preparation of the TIO2/g-C3N4 binarys. The TiO2/g-C3N4
ecological environment and human health.20 According to the composites were prepared using a typical method.29 Briey, an
previous study, PPCPs have been conrmed to be ubiquitous appropriate amount of TiO2 was mixed with 20 mL of methanol
throughout the world,21–24 and even worse, the drinking water in a beaker and sonicated until it was completely dispersed.
treatment plants are also included.25,26 Consequently, it is Aer this, a calculated amount of g-C3N4 was mixed with the
extremely urgent to develop efficient methods for the elimina- TiO2 suspension and stirred in a fume hood for 15 h. Subse-
tion of PPCPs in aquatic environments.
quently, the obtained materials were transferred to a muffle
In this study, the visible-light-driven CD/TiO2/g-C3N4 ternary furnace and heated to 550 ꢁC at a heating rate of 2.8 ꢁC minꢀ1
,
composites were rst synthesized and used as photocatalysts and the temperature was maintained at 550 ꢁC for 3 h. Through
for the degradation of PPCPs. Herein, enrooxacin (ENX), this method, a series of TiO2 (wt%)/g-C3N4 photocatalysts (wt ¼
largely used as veterinary bactericides, was taken as targeted 10.0, 30.0, 60.0, and 90.0) were synthesized and labelled as TCN-
PPCPs under the effects of the as-prepared materials. The x (x ¼ 10, 30, 60, and 90, respectively).
structural and optical properties of the composites were inves-
Decoration of the CDs onto TiO2/g-C3N4 composites. The
tigated in detail by TEM, XRD, XPS, PL, etc. In addition, the CDs/TiO2/g-C3N4 ternary photocatalysts with different doping
inuence of the TiO2 : g-C3N4 molar ratio and doping amount of amounts of CDs (0.5, 1, 2, and 3 wt%) were synthesized as
CDs on the photocatalytic performance of these composite follows: rst, the optimal mass ratio of the as-prepared TiO2/g-
photocatalysts was studied in an attempt to illuminate the C3N4 that exhibited the best performance for the photocatalytic
potential mechanisms. The CD/TiO2/g-C3N4 ternary composites degradation of ENX was placed in an alumina crucible with
were efficient towards the decomposition of refractory pollut- a cover. Aer this, a series of different mass ratiosꢁof CDs were
ants because of the synergic effect of accelerated charge sepa- added to the alumina crucible and heated to 550 C for 3 h at
ration and an extended light harvesting region. It was a heating rate of 2.8 ꢁC minꢀ1. Aer being cooled down to room
anticipated that this study could be instrumental in studying temperature, the nal CD/TiO2/g-C3N4 composite photo-
the removal of PPCPs and fabrication of novel ternary catalysts were obtained and denoted as CTCN-0.5, CTCN-1,
composite photocatalysts.
CTCN-2, and CTCN-3.
Experimental
Characterization
Materials
Thermogravimetric (TG) analysis was conducted using STA-
449C Jupiter (NETZSCH Corporation, Germany). The inter-
layer structure of the samples was characterized by a trans-
mission electron microscope (TEM, JEM-2100HR). The crystal
structure and phase purity of the obtained materials were
tested by X-ray diffraction (XRD, BRUKER D8 ADVANCE)
equipped with Cu Ka radiation (l ¼ 0.154178 nm). The UV-vis
diffuse reection spectra (DRS) of the as-prepared samples
were analyzed via a UV2450 UV-vis spectrophotometer (Shi-
madzu) using BaSO4 as a reectance standard. X-ray photo-
electron spectroscopy (XPS) was used to analyze the ionic
characteristics, which was conducted using a Thermo VG
ESCALAB 250 spectrometer with Al Ka radiation at 1486.6 eV.
Fourier transform infrared (FT-IR) spectra were obtained
using a Nicolet 6700 spectrophotometer (Thermosher).
Enrooxacin (ENX, 98% purity) was acquired from TCI Reagent
Co. Ltd. (China). Degussa P25 (commercial TiO2) was purchased
from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China).
Dicyandiamide was commercially purchased from Aladdin
(China). Acetonitrile and methanol were obtained from CNW
Technologies GmbH (Germany, HPLC-grade). Other reagents
(e.g., citric acid, urea, acetic acid, sulfuric acid, sodium
hydroxide, benzoquinone, isopropanol, potassium iodide, and
sodium azide) were of analytical grade and used without further
purication. Deionized (DI) water from a Milli-Q apparatus
(Germany) was used in the whole experiment.
Synthetic procedures
Preparation of the CDs. C-Dots were prepared via the Photoluminescence (PL) spectra were obtained using a Fluo-
hydrothermal method and modied according to a previous roMax-4 uorescence spectrophotometer (HORIBA Jobin
report.27 Briey, a weighed quantity (3.0 g) of citric acid and (3.0 Yvon).
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 34096–34103 | 34097

View Article Online
RSC Advances
Paper
Fig. 2 TG analysis of the as-prepared samples.
Fig. 1 Rotary photochemical reactor with a 350 W xenon lamp and
420 nm cut-off filter.
report.34 As shown in Fig. 2, the real contents of g-C3N4 can be
easily calculated from the weight remained aer combustion.
Fig. 3 shows the TEM and HRTEM images of the obtained
materials. It can be clearly observed that the g-C3N4 displays
a graphite-like layered structure with the agglomeration of TiO2
on the surface (Fig. 3A and B). CDs, as the decoration of TiO2/g-
C3N4 heterojunction, are embedded in TiO2 nanoparticles and
deposited onto the surface of g-C3N4 nanosheets (Fig. 3C and
D). The HRTEM images of the CDs/TiO2/g-C3N4 samples
(Fig. 3E) clearly reveal an interplanar spacing of 0.35 nm, which
authenticates the existence of TiO2 (anatase 101).35 HRTEM
images of CDs and the inset in Fig. 3F show CDs (average size:
Photocatalytic experiments
Photocatalytic activity tests of the photocatalysts were carried
out in a XPA-7 rotary photochemical reactor (Fig. 1, Nanjing
Xujiang machine plant). A 350 W xenon lamp with a 420 nm cut-
off lter was used as the visible-light irradiation source. For
each experiment, 50 mL of a 4 mg Lꢀ1 ENX aqueous solution
with 1.0 g Lꢀ1 catalysts was introduced into a 50 mL quartz tube,
which was adjusted to a neutral pH via 1% NaOH and H2SO4
solution. Before the light irradiation, the reaction solution was
stirred in the dark for 30 min to reach adsorption equilibrium
for ENX on the photocatalyst. The samples were ltered through
0.22 mm millipore lters to remove the nanophotocatalyst. ENX
was analyzed by high-performance liquid chromatography
(HPLC), and the chromatographic conditions of ENX are listed
as follows: column: Zorbax Eclipse XDB-C18, 4.6 ꢂ 150 mm, 5
mm; temperature: 40 ꢁC; mobile phase: methanol/Milli-Q water
containing 0.2% formic acid (65 : 25 v/v); ow rates: 1 mL
minꢀ1; injection volume: 20 mL; and detection wavelength:
278 nm.
The quenching experiments were employed to prove the
existence of reactive species in the ENX photodegradation
process. Herein, 0.0001 mol Lꢀ1 benzoquinone (BQ, O2cꢀ scav-
enger), 0.01 mol Lꢀ1 isopropanol (IPA, $OH scavenger), 0.01 mol
Lꢀ1 potassium iodide (KI, $OH and h+ scavenger), and 0.075 mol
Lꢀ1 sodium azide (NaN3, 1O2 scavenger) were used as quenchers
in the experiments.30–33
Results and discussion
Characterization
To determine the real contents of g-C3N4 in TiO2/g-C3N4, ther-
ꢁ
mogravimetric analysis was performed from 30 C to 800 ꢁC at
ꢀ1
ꢁ
a heating rate of 10 C min under air conditions. It can be
observed from Fig. 2 that the burning of g-C3N4 occurs in the
Fig. 3 (A and B) TEM images of TCN-90; (C and D) TEM images of
CTCN-1; (E) HRTEM image of CTCN-1; and (F) HRTEM image of CDs.
ꢁ
temperature range of 500–650 C, and the pure TiO2 presents
almost no weight loss, which is in accordance with the previous Inset: HRTEM image of CDs with obvious crystal lattice.
34098 | RSC Adv., 2017, 7, 34096–34103
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
ꢃ5 nm) with a lattice spacing of 0.22 nm corresponding to the
(100) facet of graphite.36
XRD was carried out to reveal the phase composition of the
samples (Fig. 4). As shown in Fig. 4A, no impurity diffraction
peaks are observed in the XRD patterns of TiO2 and g-C3N4,
which can be indexed to anatase (JCPDS 21-1272), rutile phases
(JCPDS 21-1276) of TiO2, and hexagonal phase of g-C3N4 (JCPDS
87-1526). TCN-90 samples show relatively strong peaks at the
position of R(100), indicating the introduction of g-C3N4. Aer
decoration of CDs, the characteristic diffraction peaks of TiO2/g-
C3N4 remain unchanged with the increasing mass contents of
CDs. It can be concluded that CDs have a negligible effect on the
crystalline structure of the TiO2/g-C3N4 Z-scheme, decorated via
a facile hydrothermal process. For others, no new diffraction
peaks appear due to the low doping content of CDs.
To study the surface chemical composition and chemical
status of CD/TiO2/g-C3N4, XPS analysis was carried out, and the
results are shown in Fig. 5. It can be concluded that the ternary
composites primarily consist of C, N, Ti, and O elements (see
Fig. 5A). High-resolution spectra of C 1s at 288.88 eV and
284.78 eV are shown in Fig. 5B. The former can be attributed to
the N–C]N bonds, whereas the latter is assigned to the C–C
coordination including sp2-hybridized C atoms present in g-
C3N4.37 High-resolution spectra of C 1s of TCN-90 are provided
as a contrast. It can be observed that C–C at 288.88 eV of CTCN-1
occupies relatively larger percentage than TCN-90 due to the
doping of CDs. The N 1s high-resolution spectrum in Fig. 5C
shows three individual peaks with the binding energies of
400.98 eV, 399.98 eV, and 398.78 eV, which can correspond to C–
N–H, N–(C)3, and C]N–C, respectively.38 From Fig. 5D, due to
the phenomena of spin–orbit separation, two peaks at the
Fig. 5 X-ray photoelectron spectra (XPS) survey spectrum of CTCN-1
and TCN-90 (A); high-resolution spectra of the C 1s (B), N 1s (C), Ti 2p
(D), and O 1s (E) of CTCN-1 and TCN-90.
N–H.43 For g-C3N4, the peak at 1630 cmꢀ1 corresponds to the
C–N heterocycle stretching vibration modes, whereas the peaks
at 1450 cmꢀ1 and 1250 cmꢀ1 can be assigned to aromatic C–N
stretching vibration modes.9 The peaks at 808 cmꢀ1 and 3170
cmꢀ1 are in accordance with the characteristic breathing mode
of triazine units and N–H.15 A peak at 1409 cmꢀ1 is observed in
the FTIR spectra of both TCN-90 and CTCN-1, which can be
interpreted as the introduction of g-C3N4 and CDs.
binding energies of 458.88 eV (Ti 2p3/2) and 464.58 eV (Ti 2p1/2
)
suggest the existence of Ti4+ in the composite.39 The O 1s high-
resolution spectrum in Fig. 5E is observed at the binding
energies of 532.08 eV and 530.48 eV, which are ascribed to the
O–H bond and Ti–O bond, respectively.40,41 The former can be
connected to the presence of a hydroxyl group on the surface of
the CDs/TiO2/g-C3N4 composite, which is in line with the FT-IR
analysis in Fig. 6.
The optical properties of the as-prepared samples were
characterized by the DRS technique, and the results are shown
in Fig. 7A. TiO2 and g-C3N4 each revealed a basal absorption
The FT-IR spectra of the TiO2 microspheres, g-C3N4, TCN-90
composite, and CTCN-1 composite are shown in Fig. 6. For
TiO2, a wide absorption band can be observed at 500–700 cmꢀ1
,
which is attributed to Ti–O stretching.42 The peaks at 1640 cmꢀ1
and 3420 cmꢀ1 are related to the bending vibration of O–H and
Fig. 4 (A) XRD patterns of TiO2, g-C3N4, TCN-90, CTCN-0.5, CTCN-1,
CTCN-2, and CTCN-3; (B) XRD patterns of CDs.
Fig. 6 FT-IR spectra of the as-prepared samples.
RSC Adv., 2017, 7, 34096–34103 | 34099
This journal is © The Royal Society of Chemistry 2017

View Article Online
RSC Advances
Paper
PL spectra were obtained (Fig. 7B) to detect the recombina-
tion of electron–hole pairs. Generally, lower PL intensity
corresponds to lower recombination of electron–hole pairs,
contributing to higher photocatalytic activity.47 The g-C3N4 has
an intensive emission peak at around 460 nm, which can be
attributed to the fast recombination of the photoinduced elec-
tron–hole pairs of g-C3N4. The intensities of the peaks for
CTCN-0.5, CTCN-1, CTCN-2, and CTCN-3 are much lower than
those for TCN-90, revealing that CDs conduce to slow the
recombination rate of electron–hole pairs. Further, the emis-
sion peak of CDs/TiO2/g-C3N4 shows an obvious red-shi from
455 nm to 470 nm aer doping of CDs, which demonstrates the
existence of CDs from another perspective. The PL results
conrm the importance of CDs/TiO2/g-C3N4 in blocking the
recombination of electrons and holes.
PL spectra of CDs are shown in Fig. 8, which demonstrate
superior up-converted photoluminescence property of CDs.
When CDs were excited by long-wavelength light (from 600 nm
to 900 nm), the up-converted emissions were signicantly
located in the range from 350 nm to 600 nm. These results
imply that CDs might enhance the photocatalytic efficiency via
the conversion of near-infrared wavelength to visible light,
which markedly promotes the light utilization of the
photocatalysts.
Fig. 7 UV-vis diffuse reflectance spectra. Inset: band gap of the as-
prepared photocatalyst determined from the (ahv)1/2 versus photon-
energy (A) and photoluminescence spectra under 350 nm excitation
(B) of the photocatalysts.
Photocatalytic performance of composites
Fig. 9 displays the photocatalytic degradation of ENX over
various photocatalysts. As can be seen, the self-photolysis of
ENX could be neglected in the blank experiment. For TCN-90
(see Fig. 9A), the concentration of ENX is reduced to 45.95%
in 1 h. This means TCN-90 performs better than other doping
ratios of g-C3N4, which conrms that the small doping ratio of
g-C3N4 is in favour of ENX degradation. Aer doping of CDs (1
wt%), enhanced photocatalytic activity was exhibited by both
TiO2 and g-C3N4, which could be attributed to the up-converted
PL property. In Fig. 9B, we drew a comparison between TiO2,
TiO2/CDs, g-C3N4, g-C3N4/CDs, TCN-90, CTCN-0.5, CTCN-1,
CTCN-2, and CTCN-3 in the photocatalytic degradation of
ENX. CTCN-1 was found to be the most optimal photocatalyst
among all the photocatalysts as it reduced ENX to 8.34% in 1 h,
band at ꢃ390 nm and ꢃ475 nm, respectively, which was
consistent with previous reports.31,44,45 g-C3N4/TiO2 possesses
the absorption features of single TiO2 and g-C3N4, which indi-
cates the existence of g-C3N4. As expected, CD/TiO2/g-C3N4
shows obvious additional absorption in the 370–450 nm region
when doped with different amounts of CDs. The absorption
edge of CTCN-1 ternary composite shows a remarkable red shi
towards a higher wavelength at ꢃ450 nm; this suggests that the
ternary composite is able to utilize the visible-light region
better. Herein, Kubelka–Munk transformation was used to
estimate the band gap energies of the semiconductors accord-
ing to eqn (1).
(ahv)(1/2) ¼ A(hv ꢀ Eg)
(1)
where a is the absorption coefficient, v is the light frequency, Eg
is the band gap energy, and A is a constant.46 The potentials of
VB and CB of the catalyst can be estimated according to eqn (2)
and (3), respectively.
EVB ¼ X ꢀ Ec + 0.5Eg
ECB ¼ EVB ꢀ Eg
(2)
(3)
where X is the electronegativity of the semiconductor (X values
are 5.81 eV for TiO2 and 4.72 eV for g-C3N4) and Ec is the energy
of free electrons on the hydrogen scale (about 4.5 eV). The
energy band structure parameters are listed in Table S1.†
Fig. 8 Up-converted PL spectra of CDs.
34100 | RSC Adv., 2017, 7, 34096–34103
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
Fig. 10 The repeated photocatalytic experiments for CTCN-1.
$OH/H2O (+2.73 eV), h+ of g-C3N4 fails to provide $OH by
oxidizing OHꢀ or H2O.50 The signals of $OH radicals appear in g-
C3N4 due to the reaction of O2cꢀ and h+.51 Compared with TCN-
90, the signals of O2cꢀ radicals are greatly enhanced for CTCN-1,
indicating that CDs play a signicant role as electron mediator
to improve the photocatalytic activity of CD/TiO2/g-C3N4.
Simultaneously, the signals of O2cꢀ radicals are also improved,
demonstrating the increased photocatalytic activity of CD-
modied TiO2/g-C3N4.
Fig. 9 Photocatalytic activity of TiO2/g-C3N4 binarys (A) and CDs/
TiO2/g-C3N4 ternarys (B) based on the photocatalytic degradation of
ENX under visible-light irradiation (l > 420 nm).
which corresponds to the DRS results in Fig. 7A. Compared with
the single-component (TiO2 and g-C3N4) samples and two-
component samples (TiO2/CDs, g-C3N4/CDs, TCN-90), CD/
TiO2/g-C3N4 exhibits remarkably enhanced photocatalytic
activity, owing to the introduction of CDs as an electron reser-
voir and transporter. However, further CD loading (>1.0 wt%)
leads to decreased degradation rates, which can be attributed to
the inner lter effect of CDs via scrambling for the absorption of
photons and reduction of the formation of reactive species.48
Reusability is a critical factor for practical utilization. To
investigate the stability of CD/TiO2/g-C3N4, ve photocatalytic
experiment cycle runs were completed, and the results are
shown in Fig. 10. As a result, 88.7% of the ENX was decomposed
in 60 min in the h cycle, which was slightly reduced as
compared to that in the rst round (91.6%). Thus, the CTCN-1
can be identied as a stable photocatalyst.
To study the generation of radicals in the photocatalytic
system under visible-light irradiation (l > 420 nm), the ESR
spin-trap with DMPO technique was used. As shown in Fig. 11,
the signals of O2cꢀ and $OH increase with time elapsing in 10
minutes, whereas no signals appear in the dark. The weak
signals of O2cꢀ and $OH radicals in TiO2 can be attributed to
fast recombination of photogenerated electron–hole pairs.49
Because the VB position of g-C3N4 (+1.36 eV) is more negative
than the standard redox potential of $OH/OHꢀ (+1.99 eV) and
Fig. 11 DMPO spin-trapping ESR spectra under visible-light irradiation
(l > 420 nm) in aqueous solutions (A) and ethanol solutions (B).
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 34096–34103 | 34101

View Article Online
RSC Advances
Paper
(+1.51 eV), which is less than the standard redox potential of
$OH/OHꢀ (+1.99 eV) and $OH/H2O (+2.73 eV). The eꢀ from TiO2
may be captured by molecular oxygen to generate O2cꢀ radi-
cals,55 which can directly attack pollutants. If TiO2/g-C3N4
follows this mechanism, h+ should play a major role in the
photocatalytic system, whereas IPA has minor effects. However,
the quenching experiments and ESR spectra results have con-
tradicted this hypothesis. Moreover, the results of the 1O2
quenching experiment are inconsistent with this mechanism.
Thus, the TiO2/g-C3N4 photocatalyst follows the direct Z-scheme
type mechanism.
For the TiO2/g-C3N4 Z-scheme with the introduction of CDs,
the near-infrared light wavelength could be converted to visible
light due to the up-converted PL properties of CDs53 Herein, the
presence of CDs can facilitate a Z-scheme transfer mechanism,
which differs from the general heterostructured mechanism.
The general heterostructured mechanism can hardly interpret
the formation of $OH, and the transfer speed of electrons and
holes might be very slow.56 The active species, h+, O2cꢀ, 1O2, and
$OH can effectively degrade ENX into small molecule
compounds or directly into nal products such as CO2 and H2O.
Fig. 12 Kinetic constants of the as-prepared photocatalysts with
different quenchers.
Fig. 12 shows the kinetic constants of TiO2, g-C3N4, TCN-90,
and CTCN-1 in the presence of different quenchers. For the
addition of isopropanol (IPA, $OH scavenger) and KI ($OH and
h+ scavenger), a slight decrease can be observed in the degra-
dation of ENX by CTCN-1, indicating the minor role of h+. For
the TCN-90 composite, similar results were obtained, whereas
a slight decrease of k was observed aer the addition of NaN3
(1O2 scavenger) for the TCN-90 and CTCN-1 composites.
Although benzoquinone (BQ, O2cꢀ scavenger) has a slight effect
in the presence of TCN-90, it plays a quite important role in
CTCN-1 probably because CDs can act as electron mediators
and generate more O2cꢀ.
Based on the abovementioned results and the previous
study,43,52–54 the potential photocatalytic mechanisms are
proposed in Fig. 13. For the conventional TiO2/g-C3N4 hetero-
junctions, electron–hole pairs are formed at the ECB and EVB of
TiO2 and g-C3N4 under visible light. Subsequently, h+ generates
at the EVB of TiO2 and transfers to the EVB of g-C3N4, and the eꢀ
from ECB of g-C3N4 can transfer to the ECB of TiO2. However, the
h+ from EVB of g-C3N4 cannot oxidize OHꢀ or H2O to generate
$OH because of the negative valence band position of g-C3N4
Conclusions
In this study, CDs/TiO2/g-C3N4 was synthetized via a facile
hydrothermal method, which exhibited high photocatalytic
performance towards the degradation of ENX under visible-
light irradiation. The results of the characterization veried
the formation of CD/TiO2/g-C3N4, in which the CDs were
embedded in TiO2 nanoparticles and deposited onto the surface
of the g-C3N4 nanosheets. The enhanced photocatalytic perfor-
mance of CDs/TiO2/g-C3N4 might be attributed to the unique
up-converted PL property, efficient charge separation, and effi-
cient electron transportation of CDs. The synergistic effects
could be concluded as follows: (1) the extension of visible-light
response scope; (2) efficient photogenerated electron–hole pair
separation; and (3) retardation of charge recombination. The
1
active species h+, O2cꢀ, O2, and $OH were produced in the Z-
scheme system. The present study conrms that the CD/TiO2/
g-C3N4 catalyst is efficient, stable, and reusable for environ-
mental applications.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21377031 and 21677040) and the
Scientic and Technical Projects of Guangdong Province (No.
2013B020800009).
Notes and references
1 S. M. Lam, J. C. Sin and A. R. Mohamed, Mater. Sci. Semicond.
Process., 2016, 47, 62–84.
2 O. Mehraj, N. A. Mir, B. M. Pirzada, S. Sabir and M. Muneer,
J. Mol. Catal. A: Chem., 2014, 395, 16–24.
Fig. 13 Photocatalytic mechanism scheme in conventional TiO2/g-
C3N4 heterojunctions and TiO2/g-C3N4 Z-scheme with the intro-
duction of CDs.
34102 | RSC Adv., 2017, 7, 34096–34103
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
3 M. J. Nalbandian, M. Zhang, J. Sanchez, Y. H. Choa, 30 G. Li, X. Nie, J. Chen, Q. Jiang, T. An, P. K. Wong, H. Zhang,
D. M. Cwiertny and N. V. Myung, J. Mol. Catal. A: Chem.,
2015, 404, 18–26.
4 G. W. Cui, W. L. Wang, M. Y. Ma, M. Zhang, X. Y. Xia,
H. Zhao and H. Yamashita, Water Res., 2015, 86, 17.
31 H. Xiao, J. Zhu and A. Thomas, RSC Adv., 2015, 5, 105731–
105734.
F. Y. Han, X. F. Shi, Y. Q. Zhao, Y. B. Dong and B. Tang, 32 G. Li, K. H. Wong, X. Zhang, C. Hu, J. C. Yu, R. C. Chan and
Chem. Commun., 2013, 49, 6415–6417. P. K. Wong, Chemosphere, 2009, 76, 1185–1191.
5 T. H. Tan, J. Scott, H. N. Yun, R. A. Taylor, K. F. Agueyzinsou 33 J. Cao, B. Xu, B. Luo, H. Lin and S. Chen, Appl. Surf. Sci.,
and R. Amal, ACS Catal., 2016, 6(3), 1870–1879. 2011, 257, 7083–7089.
6 L. Pan, S. Wang, J. Xie, L. Wang, X. Zhang and J. J. Zou, Nano 34 Y. He, L. Zhang, X. Wang, Y. Wu, H. Lin, L. Zhao, W. Weng,
Energy, 2016, 28, 296–303. H. Wan and M. Fan, RSC Adv., 2014, 4, 13610–13619.
7 L. Pan, S. Wang, J. J. Zou, Z. F. Huang, L. Wang and X. Zhang, 35 X. Sun, W. Dai, G. Wu, L. Li, N. Guan and M. Hunger, Chem.
Chem. Commun., 2014, 50, 988–990. Commun., 2015, 51, 13779.
8 X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, 36 X. Zhai, P. Zhang, C. Liu, T. Bai, W. Li, L. Dai and W. Liu,
J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater.,
2009, 8, 76.
9 L. Ge, F. Zuo, J. Liu, Q. Ma, C. Wang, D. Sun, L. Bartels and
P. Feng, J. Phys. Chem. C, 2012, 116, 13708–13714.
10 B. Chai, J. Yan, C. Wang, Z. Ren and Y. Zhu, Appl. Surf. Sci.,
2016, 391, 376–383.
11 S. Li, F. Zheng, S. Cai, W. Liang and Y. Li, Sens. Actuators, B,
2013, 188, 280–285.
12 D. R. Baker and P. V. Kamat, Adv. Funct. Mater., 2010, 19,
805–811.
Chem. Commun., 2012, 48, 7955.
37 J. A. Singh, S. H. Overbury, N. J. Dudney, M. Li and
G. M. Veith, ACS Catal., 2012, 2, 1138–1146.
38 S. J. Yang, J. H. Cho, G. H. Oh, K. S. Nahm and C. R. Park,
Carbon, 2009, 47, 1585–1591.
39 X. Yang, J. Qin, J. Yan, K. Chen, X. Yan, Z. Du, L. Rong and
T. Hua, Appl. Catal., B, 2015, 166–167, 231–240.
40 Y. Chen, W. Huang, D. He, S. Yue and H. Hong, ACS Appl.
Mater. Interfaces, 2014, 6, 14405.
41 L. A. D. Silva, V. A. Alves, S. C. D. Castro and J. F. C. Boodts,
Colloids Surf., A, 2000, 170, 119–126.
13 X. Zhong, M. Jin, H. Dong, L. Liu, L. Wang, H. Yu, S. Leng,
G. Zhuang, X. Li and J. G. Wang, J. Solid State Chem., 2014, 42 J. Yu, S. Wang, J. Low and W. Xiao, Phys. Chem. Chem. Phys.,
220, 54–59. 2013, 15, 16883.
14 H. Tang, S. Chang, L. Jiang, G. Tang and W. Liang, Ceram. 43 Y. Chen, W. Huang, D. He, Y. Situ and H. Huang, ACS Appl.
Int., 2016, 42, 18443–18452.
Mater. Interfaces, 2014, 6, 14405.
ˇ
´
´
ˇ
15 M. Reli, P. Huo, M. Sihor, N. Ambrozova, I. Troppova, 44 N. Boonprakob, N. Wetchakun, S. Phanichphant, D. Waxler,
ˇ
´
´
L. Matejova, J. Lang, L. Svoboda, P. Kustrowski and
P. Sherrell, A. Nattestad, J. Chen and B. Inceesungvorn, J.
Colloid Interface Sci., 2014, 417, 402–409.
M. Ritz, J. Phys. Chem. A, 2016, 120, 8564–8573.
16 X. Xu, Z. Bao, Z. Gang, H. Zeng and J. Hu, ACS Appl. Mater. 45 Z. F. Huang, J. Song, L. Pan, Z. Wang, X. Zhang, J. J. Zou,
Interfaces, 2016, 8, 14118–14124.
17 Y. Guo, P. Yao, D. Zhu and G. Cheng, J. Mater. Chem. A, 2015,
3, 13189–13192.
W. Mi, X. Zhang and L. Wang, Nano Energy, 2015, 12, 646–
656.
46 J. Tauc, Mater. Res. Bull., 1970, 5, 721–729.
18 X. Wu, J. Zhao, L. Wang, M. Han, M. Zhang, H. Wang, 47 X. Zhou, B. Jin, L. Li, F. Peng, H. Wang, H. Yu and Y. Fang, J.
H. Huang, Y. Liu and Z. Kang, Appl. Catal., B, 2017, 206,
501–509.
19 H. Yu, Y. Liang, Y. Rao, D. Zhu, J. Cao, Z. Shen, W. K. Ho and
S. C. Lee, Environ. Sci. Technol., 2017, 51, 2924–2933.
20 J. L. Liu and M. H. Wong, Environ. Int., 2013, 59, 208–224.
Mater. Chem., 2012, 22, 17900–17905.
48 J. Pan, Y. Sheng, J. Zhang, J. Wei, P. Huang, X. Zhang and
B. Feng, J. Mater. Chem. A, 2014, 2, 18082–18086.
49 K. Lv, X. Li, K. Deng, J. Sun, X. Li and M. Li, Appl. Catal., B,
2010, 95, 383–392.
21 G. R. Boyd, H. Reemtsma, D. A. Grimm and S. Mitra, Sci. 50 D. Wu, B. Wang, W. Wang, T. An, G. Li, T. W. Ng, H. Y. Yip,
Total Environ., 2003, 311, 135.
22 B. J. Richardson, P. K. Lam and M. Martin, Mar. Pollut. Bull.,
2005, 50, 913.
23 B. Kasprzykhordern, R. M. Dinsdale and A. J. Guwy, Water
Res., 2009, 43, 363–380.
24 R. Shen and S. A. Andrews, Water Res., 2011, 45, 944–952.
25 T. Lin, S. Yu and W. Chen, Chemosphere, 2016, 152, 1.
C. Xiong, H. K. Lee and P. K. Wong, J. Mater. Chem. A, 2015,
3, 15148–15155.
51 J. Ng, X. Wang and D. D. Sun, Appl. Catal., B, 2011, 110, 260–
272.
52 Y. Li, K. Lv, W. Ho, F. Dong, X. Wu and Y. Xia, Appl. Catal., B,
2017, 202, 611–619.
53 F. Wang, P. Chen, Y. Feng, Z. Xie, Y. Liu, Y. Su, Q. Zhang,
Y. Wang, K. Yao and W. Lv, Appl. Catal., B, 2017, 207, 103–113.
54 Y. He, L. Zhang, M. Fan, X. Wang, M. L. Walbridge, Q. Nong,
Y. Wu and L. Zhao, Sol. Energy Mater. Sol. Cells, 2015, 137,
175–184.
´
´
´
26 J. Radjenovic, M. Petrovic, F. Ventura and D. Barcelo, Water
Res., 2008, 42, 3601–3610.
27 S. Qu, X. Wang, Q. Lu, X. Liu and L. Wang, Angew. Chem., Int.
Ed., 2012, 51, 12215–12218.
28 F. Dong, L. Wu, Y. Sun, M. Fu, Z. Wu and S. C. Lee, J. Mater. 55 J. Liu, Y. Yang, N. Liu, Y. Liu, H. Huang and Z. Kang, Green
Chem., 2011, 21, 15171–15174. Chem., 2014, 16, 4559–4565.
29 J. Yu, S. Wang, J. Low and W. Xiao, Phys. Chem. Chem. Phys., 56 Y. He, L. Zhang, B. Teng and M. Fan, Environ. Sci. Technol.,
2013, 15, 16883–16890.
2015, 49, 649.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 34096–34103 | 34103