Synthesis of carbon quantum dot-surface modified P25 nanocomposites for photocatalytic degradation of p-nitrophenol and acid violet 43

Article abstract of DOI:10.1039/c4ra02707h

A new efficient and stable visible light driven photocatalyst, carbon quantum dots (CQDs)-surface modified P25 nanocomposite, was successfully prepared by chemical adsorption of CQDs onto the surface of TiO₂ nanoparticles and then used for water treatment. The photocatalyst was characterized by X-ray powder diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectrometry, thermogravimetric analysis, and UV-vis spectrometry. After the modification of titanium dioxide with carbon quantum dots, the UV-vis wavelength response range extended from 390 nm to 420 nm by accelerating the photogenerated electron transfer from carbon quantum dots to titanium dioxide, and the band gap decreased from 3.08 eV to 2.87 eV. In addition, a blue shift of binding energy was observed, namely, 0.6 eV for Ti_2p1/2 and 0.7 eV for Ti_2p3/2. Furthermore, compared with pure titanium dioxide, after irradiating for 2.5 h, the photodegradation ratio of acid violet 43 and p-nitrophenol have been enhanced from 60% to 93% and from 89% to 96%, respectively. Moreover, after five cycle times, the stability of the photocatalyst has been maintained due to the strong interaction between carbon quantum dots and titanium dioxide.

Full text of DOI:10.1039/c4ra02707h

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Synthesis of carbon quantum dot-surface modified
P25 nanocomposites for photocatalytic
Cite this: RSC Adv., 2014, 4, 30605
degradation of p-nitrophenol and acid violet 43
a
a
a
ab
Fengying Zheng, Zhenhua Wang, Jie Chen and ShunXing Li*
A new efficient and stable visible light driven photocatalyst, carbon quantum dots (CQDs)-surface modified
P25 nanocomposite, was successfully prepared by chemical adsorption of CQDs onto the surface of TiO
2
nanoparticles and then used for water treatment. The photocatalyst was characterized by X-ray powder
diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectrometry,
thermogravimetric analysis, and UV-vis spectrometry. After the modification of titanium dioxide with
carbon quantum dots, the UV-vis wavelength response range extended from 390 nm to 420 nm by
accelerating the photogenerated electron transfer from carbon quantum dots to titanium dioxide, and
the band gap decreased from 3.08 eV to 2.87 eV. In addition, a blue shift of binding energy was
observed, namely, 0.6 eV for Ti2p1/2 and 0.7 eV for Ti2p3/2. Furthermore, compared with pure titanium
dioxide, after irradiating for 2.5 h, the photodegradation ratio of acid violet 43 and p-nitrophenol have
been enhanced from 60% to 93% and from 89% to 96%, respectively. Moreover, after five cycle times,
the stability of the photocatalyst has been maintained due to the strong interaction between carbon
quantum dots and titanium dioxide.
Received 27th March 2014
Accepted 10th July 2014
DOI: 10.1039/c4ra02707h
www.rsc.org/advances
9
Because of serious photocorrosion, harmful heavy metal ions
1
. Introduction
10
can be released from QDs/TiO into the solutions and then the
2
The total degradation of organic pollutants (pesticides, herbi- photodegradation efficiency of these heavy metal-based QDs
cides, insecticides, fungicides, dyes, etc.) is the main eld of was decreased. Furthermore, the application of QDs/TiO to
2
1
10,14–16
water photocatalytic decontamination. The commercial pyro- water treatment is limited by its cytotoxicity.
genic titania P25 (Evonik) is routinely used as a benchmark
Carbon quantum dots (CQDs) are new functional carbona-
photocatalyst due to its unselective fairly good photoactivity ceous material. Compared to conventional dye molecules and
towards a wide spectrum of pollutants, commercial availability, QDs, CQDs are superior in terms of low toxicity, low cost,
2
and low cost. Because of the broad energy gap of TiO
2
(E
g
¼
environmentally friendly, biocompatibility, and extensive
11
3
.20–4.50 eV), the photocatalytic process should be induced by sources. Herein, a new efficient and stable visible light driven
ultraviolet light (l # 387 nm). Surface modication of TiO
been used to narrow its band gap and extend its UV-vis wave- prepared by chemical adsorption of CQDs onto the surface of
length response range. Semiconductor quantum dots (QDs), TiO
including CdS, CdSe, PbS, PbSe, InP, InAs, and CdTe, have been surface modication of TiO
assembled onto porous lms, nanotubes, nanowires, and the photoresponse of TiO to the visible-light region, accelerate
nanoparticles of TiO
and then used as photocatalysts (QDs/ the electron transfer between CQDs and TiO , and improve the
TiO
) for water splitting or the development of solar cell affinity between photocatalyst and aromatic pollutants,
devices. The unique features of QDs/TiO
3
has photocatalyst (CQDs-surface modied P25 nanocomposites) is
2
3–5
2
nanoparticles and then used for water treatment. The
2
with CQDs can effectively extend
2
2
2
2
6–8
include a high including typical dye (acid violet 43, AV 43) and p-nitrophenol
2
optical absorption coefficient, narrow band gap, and extended (PNP). Enhanced visible-light photocatalytic performance is
photostability, which underlie efficient harvesting of light therefore expected.
6
energy. In addition, QDs can form a type-II staggered band
9
interface with TiO for efficient charge separation and transfer.
2
2
. Experiments
2
.1 Materials
a
Department of Chemistry and Environmental Science, Minnan Normal University,
The P25–TiO was purchased from Degussa, with the average
2
Zhangzhou, 363000, China. E-mail: lishunxing@munu.edu.cn; shunxing_li@aliyun.
particle size of 21 nm. AV 43 (CAS no. 4430-18-6) was purchased
from the chemical reagent station in Tokyo, Japan. PNP was
obtained from Aldrich Chemical Co. (USA). All other chemicals
com; Fax: +86 596 2591395; Tel: +86 596 2591395
b
Fujian Province Key Laboratory of Modern Analytical Science and Separation
Technology, Minnan Normal University, Zhangzhou, 363000, China
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 30605–30609 | 30605

View Article Online
RSC Advances
Paper
were of the highest purity commercially available. De-ionized 2.5 Data processing
water was puried with a Milli-Q water ion-exchange system
The photocatalytic degradation ratio was calculated by (A ꢁ A )/
0
t
(Millipore Co., USA) for a resistivity of 18 MU cm and used
A
0
ꢂ 100. The value of A was proportional to the concentration
of AV 43 and PNP based on Beer–Lambert law. A was the initial
absorbancy of the specimen, and A was the absorbancy of the
specimen when the photocatalytic degradation time is t.
throughout the experime.
0
t
2.2 Preparation of CQDs and CQDs/TiO
2
composites
An electrolytic denudation method was used for the synthesis of
12
CQDs and its procedures were reported by Li et al. In a beaker,
the electrolyte of the electrochemical process was prepared by
3
. Results and discuss
mixing ethanol–H
2
O (100 mL; volume ratio ¼ 99.5 : 0.5) with 0.4 3.1 TEM characterization
g of NaOH. By using graphite rods (diameter about 0.5 cm) as
both anode and cathode, CQDs was synthesized with a current
intensity 80 mA for 4 hours. The raw CQDs solution was treated
As shown in Fig. 1a, the size distribution of CQDs was narrow in
the range of 2–10 nm. The dispersibility of CQDs in water was
good, i.e., without larger aggregates, indicating that CQDs were
protected by some function groups. From the HRTEM image
(seen in Fig. 1b), the crystal lattice of graphite was clearly
observed and similar results were reported by Zhou et al. Aer
mixing of CQDs and P25, CQDs were found onto the surface of
P25 in some area (seen in Fig. 1c and d).
by adding 7.0 g of MgSO , stirred for 20 min, and then stored for
4
2
4 h to remove the water and salt. The residue of MgSO was
4
detached by the ltration. Aerwards, the puried CQDs solu-
tion was rotary evaporated to remove the ethanol, carbon
quantum dots were redispersed in water.
13
Aer dispersing of P25 powder into 45 mL of water, 5 mL of
puried CQDs solution was added with stirring for 4 h, vacuum
3.2 TGA data
ꢀ

ltered, dried at 60 C overnight, and then CQDs/TiO2 was
The thermogravimetric analysis (TGA) was conducted to
conrm whether CQDs was combined with P25 in the CQDs/P25
sample and the results for P25 and CQDs/P25 were shown in
Fig. 2. TGA of P25 showed a 3.3 wt% total weight loss in two
obtained. The loading of CQDs was calculated by thermogravi-
metric analysis (TGA).
2.3 Photocatalytic test
ꢀ
apparent steps on heating to 600 C. The mass loss between 205
The photocatalytic activity of the photocatalysts (P25 and CQDs/
P25) were evaluated by the photodegradation of AV and PNP. A
ꢀ
ꢀ
C and 265 C was due to the release of surface hydroxylated
species. The TGA curve of CQDs/P25 could be divided into three
apparent steps with a 8.1 wt% total weight loss. The rst step
was similar as P25, mass loss (3.3 wt%) and its reason was same,
60 W high-pressure mercury lamp placed 10 cm away from the
reaction vessel was used as the light source. The illumination
ꢁ
2
intensity was 53 mW cm . A 100 mL beaker was used as a
ꢀ
but its temperature range was decreased from 60 C (i.e., 205–
ꢁ
1
photoreactor, into which 100 mL 0.15 mmol L of AV 43 (or 30
ꢀ ꢀ ꢀ
2
65 C) to 35 C (i.e., 185–220 C). The 4.8 wt% weight loss
ꢁ1
mg L of PNP) with a certain pH value and the designed
concentration of CQDs/P25 (or P25) were added. The photo-
reactor was equipped with an electromagnetic stirrer. Aer
being irradiated for a period of time, 3 mL of the reaction
solution were taken out and centrifuged at 10 000 rpm to
remove the photocatalyst. The concentration of AV 43 in the

ltrates were determined spectrophotometrically at l ¼ 560 nm.
Aer adjusting the pH value of the ltrates as 9.0, the concen-
trations of PNP were determined by measuring the absorbance
at 400 nm.
2.4 Characterization
The crystalline phase and crystal size of the photocatalysts (P25
and CQDs/P25) were determined by X-ray diffraction (XRD, Cu
Ka, 40 kV, 100 mA, D/max2400 Rigaku, Japan MAC Science). The
morphologies of the photocatalysts were observed with trans-
mission electron microscope (TEM, Hitachi-600-2). Using an
Aglient 8453 UV-vis diode array spectrophometer, the diffuse
reectance UV-visible spectra were obtained for dry pressed
disk samples, which were prepared by mixing the photo-
4
catalysts and BaSO and its scans range was 300–700 nm. Aer
being Infrared absorptions were studied by a Nicolet 360 IR
spectrometer for judging the functional groups of P25 and
CQDs/P25. Samples were mixed with potassium bromide, and
the content of the samples were kept around 1.0%.
Fig. 1 TEM images of carbon quantum dots (CQDs) and CQDs/TiO
a) TEM image of CQSs; (b) HRTEM image of CQDs; (c) TEM image of
CQDs/P25 and (d) magnified TEM images of CQDs/P25.
2
:
(
30606 | RSC Adv., 2014, 4, 30605–30609
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
2
Fig. 2 TGA datum of P25 (a),CQDs/TiO (b) sample and the corresponding calculated loss mass percentage.
ꢀ
ꢀ
between 210 C and 510 C was due to the slow combustion of the surface of CQDs and TiO . So, the combination between
2
carbon in the CQDs, so the CQDs loading was about 4.8%.
CQDs and TiO
2
was chemical adsorption and it promoted the
electronic conduction and connective stability.
3.3 XRD analysis
The XRD patterns of P25 and CQDs/TiO were shown in Fig. 3. 3.5 UV-vis diffuse reection absorption
2
The phase structure in P25 samples were the coexistence of
Fig. 5 gave the UV-vis diffuse reection absorption spectra of
P25, CQDs and CQDs/P25. Compared with the pure P25, its UV-
vis wavelength response range was expanded from 390 nm to
anatase and rutile. However, no carbon diffraction peaks could
be observed in the CQDs/TiO sample, because the size of CQDs
2
was small (2–10 nm) and the loading of CQDs was only
about 4.8%.
420 nm, which might be contributed by the visible light
absorption of CQDs. The E (band gap energy) value of P25
g
could be narrowed by the surface modication with CQDs and
its band gap was changed from 3.08 eV to 2.87 eV. This
3.4 FT-IR analysis
2
To discuss the combination mode between CQDs and P25, the absorption feature suggested that this CQDs/TiO catalyst could
functional groups in P25 and CQDs/P25 were characterized and be activated by visible light and the utilization of sunlight were
compared in Fig. 4. The hydroxyl groups and carboxyl groups improved.
were reported on the surface of CQDs under alkaline environ-
1
2
ꢁ1
ment. The peak around 3380 cm was overlapped by the
3.6 XPS analysis
hydroxyls on the surface of P25 and CQDs. Aer surface modi-
The XPS spectral of Ti2p region were shown in Fig. 6. The bind
energy of Ti2p1/2 and Ti2p3/2 positioned around 464 eV and 458
eV. Compared to the binding energy of 464.6 eV and 458.7 eV in
P25 sample, there was a blue-shi of 0.6 eV and 0.7 eV for the
CQDs/P25 sample. Hence, the photogenerated electron could be
injected easily and rapidly into the CQDs and following

cation with CQDs, the obvious change on the functional
ꢁ1
groups of P25 was the appearance of a peak around 1380 cm
.
This peak was conjectured to be –COOTi- group, which origi-
nated from the esterication between the carboxyl groups from
CQDs and the hydroxyls from P25. In addition, the strong
hydrogen bonds could form between the hydroxyl groups from
Fig. 3 XRD patterns of P25 (a) and CQDs/TiO
2
(b) samples.
Fig. 4 FT-IR spectrums of P25 (a) and CQDs/P25 samples (b).
RSC Adv., 2014, 4, 30605–30609 | 30607
This journal is © The Royal Society of Chemistry 2014

View Article Online
RSC Advances
Paper
Fig. 7 Photodegradation ratio of acid violet 43 and p-nitrophenol with
CQDs/TiO
2
catalysts under different pH value. Conditions: tempera-
ꢀ
ꢁ1
ture 25 C; catalyst dosage 1.0 g L ; initial concentrations of acid
ꢁ1
ꢁ1
,
violet 43 and p-nitrophenol were 0.15 mmol L and 30 mg L
respectively; irradiation time for acid violet 43 and p-nitrophenol were
.5 h and 4.0 h, respectively.
Fig. 5 UV-vis diffuse reflection absorption spectra of P25 (a), CQDs/
P25 (b) and CQDs (c) samples.
2
decomposition of AV 43 and PNP was used as probe photore-
actions and the results were shown in Fig. 9. The photo-
degradation ratio of AV 43 and PNP could be increased by the
increasing of irradiation time. When the irradiation time was
more than 150 min for AV 43 and 240 min for PNP, the increase
of the photodegradation ratio was not obvious. The optimal
ꢁ
1
photodegradation conditions were: pH 3.0, CQDs/P25 1.0 g L
ꢁ1
for AV 43 and 2.0 g L for PNP, and irradiation time 150 min for
AV 43 and 240 min for PNP. Compared with the pure P25, the
ꢁ
1
ꢁ1
degradation ratio of AV 43 (0.15 mmol L ) and PNP (30 mg L
)
could be enhanced from 60% to 93% and from 89% to 96%,
respectively. Aer ve times circles, the photocatalytical
performance of CQDs/P25 was same as its rst use, because
surface modication of TiO by –COOTi- group could be stable
2
Fig. 6 The XPS data of Ti2p in P25 and CQDs/P25 samples.
3–5
for the photodegradation of aromatic pollutants.
The
enhanced photodegradation activity should be attributed to the
existence of CQDs, because: (a) CQDs narrowed the band gap of
the CQDs/TiO nanocomposites, (b) the electron–hole separa-
2
tion of CQDs/P25 could be excited by lower energy than that of
P25, (c) the light absorption range of P25 was expanded into
departed away to the oxygen in water through the strong
interaction between CQDs and TiO2. The CQDs offered a high-
speed channel for the photogenerated electron and reduced
the probability of electron–hole recombination.
3.7 Photodegradation test
Using CQDs/P25 as the photocatalyst, the effect of pH value and
catalyst concentration on the degradation ratio of AV 43 and
PNP was shown in Fig. 7 and 8. The inuence trends of pH value
on the photodegradation of AV 43 and PNP were similar and the
optimum pH value was same as 3.0. With the increasing of the
ꢁ
1
concentration of CQDs/P25 from 0.10 to 3.00 g L , the active
sites for the production of cOH radicals were increased, the
photodegradation ratio of AV 43 and PNP was increased, the
increasing level were more signicant in the range of 0.1–1.0 g
ꢁ
1
ꢁ1
Fig. 8 Photodegradation ratio of acid violet 43 (a) and p-nitrophenol
L
for AV 43 and 0.1–2.0 g L
for PNP. So the adopted
ꢁ
1
ꢁ1
(b) with different concentration of CQDs/TiO
2
. Conditions: tempera-
concentration of CQDs/P25 was 1.0 g L for AV 43 and 2.0 g L
for PNP in the photocatalytic experiments.
ꢀ
ture 25 C; pH 3.0; initial concentrations of acid violet 43 and
ꢁ1
ꢁ1
p-nitrophenol were 0.15 mmol L and 30 mg L , respectively; irra-
diation time for acid violet 43 and p-nitrophenol were 2.5 h and 4.0 h,
To evaluate and compare the photocatalytic performance of
P25 and CQDs/P25 in aqueous contaminants, the respectively.
30608 | RSC Adv., 2014, 4, 30605–30609
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
2
S. J ˜o ks, D. Klauson, M. Krichevskaya, S. Preis, F. Qi,
A. Weber, A. Moiseev and J. Deubener, Appl. Catal., B,
2
012, 111–112, 1–9.
S. X. Li, S. J. Cai and F. Y. Zheng, Dyes Pigm., 2012, 95, 188–
93.
S. X. Li, F. Y. Zheng, W. L. Cai, A. Q. Han and Y. K. Xie, J.
Hazard. Mater., 2006, 135, 431–436.
3
4
5
1
S. X. Li, F. Y. Zheng, X. L. Liu, F. Wu, N. S. Deng and
J. H. Yang, Chemosphere, 2005, 61, 589–594.
6 B. R. Hyun, Y. W. Zhong, A. C. Bartnik, L. Sun, H. D. Abru nˇ a,
F. W. Wise, J. D. Goodreau, J. R. Mattews, T. M. Leslie and
N. F. Borrelli, ACS Nano, 2008, 11, 2206–2212.
Fig. 9 Photodegradation ratio of acid violet 43 (a) and p-nitrophenol
(
b) with P25 and CQDs/TiO
2
ꢀ
catalysts for different irradiation time.
ꢁ1
Conditions: temperature 25 C; pH 3.0; catalyst dosage of 1.0 g L
and 2.0 g L
ꢁ1
were used for acid violet 43 and p-nitrophenol,
7
8
9
G. S. Wu, M. Tian and A. C. Chen, J. Photochem. Photobiol., A,
2012, 233, 65–71.
N. Guijarro, T. Lana-Villarreal, I. Mora-Ser ´o , J. Bisquert and
R. G ´o mez, J. Phys. Chem. C, 2009, 113, 4208–4214.
Q. Wang, X. Yang, L. Chi and M. Cui, Electrochim. Acta, 2013,
respectively; initial concentration of acid violet 43 and p-nitrophenol
ꢁ1
ꢁ1
were 0.15 mmol L and 30 mg L , respectively.
visible light by accelerating the photogenerated electron trans-
fer from CQDs to P25, and (d) the separation process of the
photoexcited electron–hole pairs could be improved by CQDs as
an electron pool. This visible light degradation mechanism was
demonstrated in the UV-vis diffuse reection absorption
spectra and XPS analysis.
9
1, 330–336.
0 A. M. Derfus, W. C. W. Chan and S. N. Bhatia, Nano Lett.,
004, 1, 11–18.
1 S. N. Baker and G. A. Baker, Angew. Chem., Int. Ed., 2010, 49,
726–6744.
2 H. T. Li, X. D. He, Z. H. Kang, Y. Liu, Y. S. Lian,
C. H. A. Tsang, X. B. Yang and S. T. Lee, Angew. Chem., Int.
Ed., 2010, 49, 4430–4434.
1
1
1
2
6
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (20977074 and 21175115), the Program for
New Century Excellent Talents in University (NCET-11 0904),
Outstanding Youth Science Foundation of Fujian Province,
China (2010J06005), and the Science & Technology Committee
of Fujian Province, China (2012Y0065).
13 J. G. Zhou, C. Booker, R. Y. Li, X. T. Zhou, T. K. Sham,
X. L. Sun and Z. F. Ding, J. Am. Chem. Soc., 2007, 129, 744–
745.
14 S. X. Li, J. Z. Zheng, D. J. Chen, Y. j. Wu, W. X. Zhang,
F. Y. Zheng, J. Cao and Y. L. Liu, Nanoscale, 2013, 5,
11718–11724.
1
1
5 J. Z. Zheng, S. F. Li, W. X. Zhang, J. Cao, S. X. Li and
Z. M. Rao, Analyst, 2013, 138, 916–920.
6 J. Z. Zheng, Z. J. Xue, S. F. Li, S. X. Li and Z. M. Rao, Anal.
Methods, 2012, 4, 2791–2796.
References
1
J. M. Herrmann, C. Duchamp, M. Karkmaz, B. T. Hoai,
H. Lachheb, E. Puzenat and C. Guillard, J. Hazard. Mater.,
2007, 146, 624–629.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 30605–30609 | 30609