One-step synthesis of nanohybrid carbon dots and TiO2 composites with enhanced ultraviolet light active photocatalysis

Article abstract of DOI:10.1039/c4ra14865g

Nano-hybrid composites of carbon dots (CDs) and titanium dioxide (TiO₂) were fabricated by a designed one-step solvothermal method. The crystallinity of TiO₂, morphologies and ingredients of the CDs-TiO₂ composites could be adjusted by the reaction time. Furthermore, the measurements of photo-degradation on methyl blue (MB) showed excellent photocatalytic activity of the CDs-TiO₂ composites under ultraviolet (UV) light irradiation, which was much higher than that of commercial Degussa P25. However, C-TiO₂ composites, which were produced from the CDs-TiO₂ composites by a further calcination, showed more excellent photocatalytic activity for the photo-degradation of methyl orange (MO) and rhodamine B (RB).

Full text of DOI:10.1039/c4ra14865g

View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: F. Li, F. Tian, C.
Liu, Z. Wang, Z. Du, R. Li and L. Zhang, RSC Adv., 2014, DOI: 10.1039/C4RA14865G.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

Page 1 of 7
RSC Advances
Dynamic Article Links ►
Journal Name
View Article Online
DOI: 10.1039/C4RA14865G
Cite this: DOI: 10.1039/c0xx00000x
www.rsc.org/xxxxxx
ARTICLE TYPE
One-step synthesis of nanohybrid carbon dots and TiO composites with
2
enhanced ultraviolet light active photocatalysis
Fan Li, Feng Tian,* Changjun Liu, Zheng Wang, Zhenjie Du, Ruixin Li, Li Zhang
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Nanoꢀhybrid composites of carbon dots (CDs) and titanium dioxide (TiO ) were fabricated by a designed
2
oneꢀstep solvothermal method. The crystallinity of TiO , morphologies and ingredients of the CDsꢀTiO
2
2
composites could be adjusted by the reaction time. Furthermore, the measurements of photoꢀdegradation
on methyl blue (MB) showed excellent photocatalytic activity of the CDsꢀTiO composites under
2
1
0
ultraviolet (UV) light irradiation, which was much higher than that of commercial Degussa P25. However,
CꢀTiO composites, which were produced from the CDsꢀTiO composites by a further calcination,
2
2
showed a more excellent photocatalytic activity on the photoꢀdegradation of methyl orange (MO) and
rhodamine B (RB).
26
(
CNT) had been used to suppress the recombination of photoꢀ
Introduction
generated carriers. But the limited contact between graphene (or
CNT) and TiO nanoparticles were adverse to the electrons shift
2
1
2
2
3
3
4
4
5
0
5
0
5
0
5
Due to its low cost, high chemical stability and nonꢀtoxicity,
titanium dioxide (TiO ) has been widely used as one of the most
promising photocatalysts. The photocatalytic activity of TiO2
originates from the photoꢀgenerated charge carriers (electrons and
holes) which are strong reducing and oxidizing agents. However,
the wide band gap (3.2 eV for anatase and 3.0 eV for rutile) of
27
5
5
6
6
7
7
8
0
5
0
5
0
5
0
from TiO to carbon materials. Choi et al. designed a coreꢀshell
2
2
photocatalyst of TiO @reduced graphene oxide (RGO), whereas
2
1
the RGO shell would probably hinder the degradation of organic
pollutants. The C/TiO photocatalysts with ordered nanostructures
2
and pronounced photocatalytic activity under visible light were
obtained by direct carbonization of block copolymer templates.
28
TiO limits its response to the ultraviolet (UV) light only, leaving
2
Latter the composites of amorphous carbon and TiO also showed
2
9
5ꢀ97% energy of the whole solar spectrum unusable. Great
29,30
improved photocatalytic activities.
However, the best
efforts have been made to modify TiO so as to improve its photo
response capability. A variety of effective approaches, such as
2
performance of these pholocatalysts could be achieved only when
an anneal process was carried out to etch partial carbon content.
Carbon dots (CDs), emerged as a novel and ecoꢀfriendly
carbon nanomaterial, has attracted considerable attention due to
their versatile preparation routes, better biocompatibility and
2
ꢀ4
coupling with other semiconductor materials,
doping with
5ꢀ9
10ꢀ12
metal ions, sensitization using a dye,
noble metal (deposition or encapsulation)
compounding with
and doping with
1
3ꢀ15
1
6ꢀ24
nonmetal elements,
have been explored to enhance the
31
particularly tunable photoluminescence (PL). Lee et al. firstly
photocatalytic performance of TiO2 under visible light, all of
which are based on the formation of intermediate energy levels in
the band gap. Among the doping processes using nonmetal
elements, Asahi et al. pioneered the nitrogen doping with TiO2,
which promoted visible light activity of TiO2 in
proved the availability of TiO /CDs hybrids for the efficient
2
32
photoꢀdegradation of methyl blue. Then Ho et al. found the H2
evolution rate of TiO /CDs nanocomposites was 9.7 times higher
2
33
than that of bare TiO2. Likewise, the photocatalytic rate of
alkylated graphene quantumꢀP25 nanocomposites was much
1
7
photodetoxification. Subsequently, other nonmetal elements,
34
higher than that of pure P25 under visible light. Recently, TiO
2
36
18,19
20
21
22
23,24
such as carbon,
sulfur, fluorine , boron and hydrogen
,
35
with various microstructures, such as nanosheet, nanorod,
were employed in diverse doping studies on TiO as well.
In most cases TiO could show photocatalytic activity under
visible light after doping processes. However, the recombination
of photoꢀgenerated carriers was accelerated on account of
numerous recombination centers brought by the introduction of
impurities and defects. Therefore, it is better to transfer the photoꢀ
generated electrons off TiO to retard the recombination. Taking
this into account, carbon materials have great advantages among
all candidate materials, owing to their excellent electronic
conductivity, electronꢀstorage capacity, absorbency of visible
37
38
39
2
nanowire, nanotube, hollow microspheres and nanorod
2
40
spheres , were explored to combined with CDs as well, all of
which showed enhanced photocatalytic activity. However, readyꢀ
made TiO materials or special efforts on TiO preparation were
2
2
necessary in most of previous doping studies. In fact, the Nꢀ
doped CDs themselves have showed excellent photocatalytic
property in the photoꢀdegradation of methyl orange under visible
2
41
light. Latter Kang et al. confirmed that graphiteꢀderived CDs
42
could be used for photoelectrochemical hydrogen generation. It
is indicated that CDs are promising candidates for photocatalytic
2
5
light and chemical stability. Graphene and carbon nanotubes
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

RSC Advances
Page 2 of 7
View Article Online
DOI: 10.1039/C4RA14865G
applications.
Characterizations
UVꢀVis absorption was measured on a DR 5000 UVꢀVis
spectrophotometer (HACH, America). PL emission
measurements were performed using a FLS920 fluorometer
Edinburgh Instruments, Britain). The normalized spectrum was
obtained by divided each intensity of the PL spectrum by the
maximum value of its own. The morphology and microstructure
of the samples were examined by scanning electron microscopy
In this work, we designed a oneꢀstep solvothermal process to
fabricate nanohybrid composites of CDs and TiO in situ. The
2
5
5
6
6
7
7
0
5
0
5
0
5
mixture of citric acid and tetrabutyl titanate dissolved in ethanol
acted as carbon source and TiO precursor separately. The H O
generated from the dehydration and impartial carbonization of
citric acid was immediately utilized to initiate the hydrolysis of
tetrabutyl titanate, which was turned into the earliest solꢀgel TiO2
5
0
5
2
2
(
and final TiO nanocrystal. In this way it is expected to realize a
maximum contact between the asꢀprepared CDs and TiO in the
2
(SEM) on a LEO 1530VP scanning electron microscope with an
1
2
acceleration voltage of 20 kV. The obtained composites and pure
CDs were further examined by highꢀresolution transmission
electron microscopy (HRTEM) on a Philips Tecnai G2 F20
microscope (Philips, Netherlands) with an accelerating voltage of
final composites for the benefit of an enhanced photocatalytic
performance. The possible formation process of the CDsꢀTiO2
composites was illustrated in Scheme 1. Furthermore, methyl
blue (MB), methyl orange (MO) and rhodamine B (RB) were
chosen to test the photocatalytic activity of the nanohybrid CDsꢀ
2
00 kV. The samples for HRTEM were made by dropping an
1
aqueous solution onto a 300ꢀmesh copper grid coated with a lacy
carbon film. The fourier transform infrared (FTIR) spectra of
CDs were measured on a Nicolet 380 spectrometer (Thermo,
America). The surface states and composition of the composites
were measured on a Kratos AXIS Ultra DLD Xꢀray Photoelectron
Spectroscopy (XPS, Shimadzu, Japan). XꢀRay diffraction (XRD)
profiles of the prepared samples were recorded on a Rigakuꢀ
D/MAX 2500 diffractometer (Rigaku, Japan) equipped with
graphite monochromatized CuKα (λ=0.15405 nm ) radiation at a
TiO composites under UV light irradiation.
2
ꢀ
1
scanning speed of 4° min in the range from 10° to 70°. The
BrunauerꢀEmmettꢀTeller (BET) specific surface areas of the
composites were calculated using adsorption data in the relative
pressure range from 0.04 to 0.32, while the BarretꢀJoynerꢀ
Halenda (BJH) pore size distribution was determined from the
desorption isotherms. The thermogravimetric curves were
Scheme 1 Schematic illustration of the formation process of the CDsꢀ
TiO composites in situ.
2
2
0
Experimental section
Chemicals and reagents
finished from 50 ºC to 650 ºC under N with a same heating rate
2
(
10 ºC per min) on a thermo gravimetric analyzer (Mettler).
Citric acid (CA, 99.5%) was purchased from Alfa Aesar.
Methylene blue (MB, 98.5%), methyl orange (MO, 99.7%) and
rhodamine B (RB, 99%), tetrabutyl titanate (TBT, 99%),
triethanolamine (85%) and ethanol (99.7%) were obtained from
GuangFu Technology Development CO, LTD. All other reagents
were of analytical grades and used without further purification.
Measurements of photocatalytic activity
8
8
9
0
5
0
The photocatalytic activity of all samples was tested under UV
light. MB solution with different concentrations (0ꢀ10 ppm) was
prepared in water. Typically, 10 mg powder samples were
suspended in 8 ml MB solution followed by 10 min vortex and
25
Preparation
1
20 min vibration in the dark to reach the adsorptionꢀdesorption
equilibrium between MB and photocatalysts before irradiation.
Then the above reaction system placed in a quartz cuvette was
exposed to UV light with a wavelength of 254 nm and a power of
Tetrabutyl titanate was used as the TiO source. Typically, 2 g CA
2
30
35
40
45
was dissolved in 60 ml ethanol followed by an addition of 10 ml
TBT in succession to form
a colorless, transparent and
8
W (Spectroline XLEꢀ1000/F, UV CROSSLINKER) at room
homogeneous solution. Then, a certain amount of TEA was added
dropwise into the solution under vigorous stirring for 30 min. The
mixed solution was placed into a 100 ml stainless steel autoclave
with a Teflon liner. The autoclave was incubated at 200 ºC for t h
temperature. Without stirring, the suspension was taken out at
regular intervals and centrifuged at 5000 rpm for 10 min, and
then the supernatant solution was analyzed using a UVꢀVis
absorbance spectroscopy. The photoꢀdegradation measurements
on MO and RB were executed in the same way.
(t=3, 6, 9 or 12) and then cooled to ambient temperature. After
the solvothermal reaction, the white precipitates were filtrated
and washed with ethanol and water in turns for five times. The
Results and Discussion
obtained CDsꢀTiO composites (marked as Cꢀ3, Cꢀ6, Cꢀ9 and Cꢀ
2
12 orderly) were dried at 80 ºC in a vacuum oven overnight and
then ground to fine powders with an agate mortar. Pure CDs were
prepared following the same way above except the addition of
TBT. In order to regulate the components of the obtained CDsꢀ
TiO2 composites, another 2 h calcination under different
temperature (400 ºC, 450 ºC, 500 ºC and 550 ºC, separately) in air
9
5
XRD was used to investigate the possible constituents and
crystalline phase of the samples prepared under different
conditions. As shown in Fig. 1, it is easy to find the existence of
TiO as an imperfect crystal of anatase. Meanwhile, the XRD
2
patterns differed from each other but with a regular tendency. The
1
00 diffraction peak of sample Cꢀ3 was broad, which meant an
were carried out and the final samples (CꢀTiO composites) were
2
unformed crystalline phase. But with prolonged reaction time, the
named Cꢀ400, Cꢀ450, Cꢀ500 and Cꢀ550 correspondingly.
diffraction peaks attributing to anatase TiO became visible and
2
acute gradually from sample Cꢀ6 to Cꢀ12. It should be pointed out
2
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Page 3 of 7
RSC Advances
View Article Online
DOI: 10.1039/C4RA14865G
that these peaks were comparatively weak due to the ultra small 40 joined together to form the final microspheres. While CDs were
size of the TiO crystallites which were finely doped with the
produced from incomplete carbonization of the precursor
molecules. Therefore, the basic nanoparticles were probably
2
carbon materials (CDs) in the samples. As an amorphous carbon,
the asꢀprepared pure CDs showed a very weak and broad peak
between 20° to 30° (Shown in Fig. S1), which might have been
composed of CDs, TiO nanocrystals and a very few residues of
2
5
partially reacted precursor molecules. Meanwhile it’s just the
covered by the lineshape belonging to TiO grains in the XRD 45 reason why the surface of microspheres became rough with
2
patterns of the CDsꢀTiO composites. The average grain sizes of
prolonged heating time. The remnant precursor molecules among
the nanoparticles continued to be carbonized or transformed into
TiO2 nanocrystal, bringing about a lot of mesopores or even
macropores in the microspheres and leaving a tough irregular
2
TiO measured with Scherrer equation increased from 8.1 nm of
2
sample Cꢀ6 to 11.9 nm of sample Cꢀ12. More details were listed
in the Table S1. It had been reported that carbon deposition had a
1
0
4
3
depression effect on the grain growth of TiO2, meanwhile it’s 50 surface or some broken microspheres (Fig. S3). But when a
known that crystal growth could develop gradually with
prolonged heating time. So there was a competition for the
further calcination was applied on the CDsꢀTiO composites, CDs
2
and almost all the residual precursor molecules were consumed,
the linkages among the basic nanoparticles were destroyed
completely, finally leading to the fact that a majority of
development of TiO grains in the solvothermal process. However,
2
1
5
after a further calcination merely at 400°, rutile TiO was found,
2
together with the predominant anatase TiO2 (Fig. S2). The 55 microspheres broke. It was worthwhile to note that the
proportion of rutile TiO2 increased quickly with enhanced
annealing temperature, and the ratio of anatase/rutile in sample Cꢀ
nanoparticles undergone a treatment of calcination became
smaller with a careful examination, which indirectly indicated
that CDs and TiO2 were hybridized finely and evenly in the
4
50 was very close to that of commercial Degussa P25.
microspheres of CDsꢀTiO composites.
2
60
Fig. 2 SEM micrographs of the asꢀprepared composites (a), (b) and (c):
low magnification images of sample Cꢀ3, Cꢀ12 and Cꢀ450 (scale bars:
5
00 nm); (d), (e) and (f): high magnification images of the surface
structures of sample Cꢀ3, Cꢀ12 and Cꢀ450 (scale bars: 100 nm).
2
0
6
7
7
8
8
5
0
5
0
5
HRTEM analyses were conducted to further study the
microstructure of the obtained composites. It’s confirmed that
both sample Cꢀ9 and Cꢀ450 were composed of nanoparticles (Fig.
2
Fig. 1 XRD patterns for the CDsꢀTiO composites prepared with different
heating time.
3
a and Fig. 3c). And the high magnification HRTEM images of
The microstructures and morphologies of asꢀprepared CDsꢀ
sample Cꢀ9 showed clear lattice fringes of the nanocrystals with
an interplanar spacing of 0.35 nm around, which matched well
TiO composites were probed using SEM (Shown in Fig. 2). In
2
2
3
3
5
0
5
the panoramic view, the samples were all microspheres with
diameter no more than 10 ꢁm. But the surface morphologies
differed greatly from sample Cꢀ3 (Fig. 2a) to sample Cꢀ12 (Fig.
with the (101) plane of anatase TiO . The pure CDs were
2
distributed well with an average diameter of 5 nm around, in
accordance with the poor crystallinity (Shown in Fig. S4). In Fig.
2
b). Sample Cꢀ3 possessed a rather smooth surface (Fig. 2d). But
3
b, inside the red dotted boxes were CDs, which were linked to
with prolonged heating time, the surface of microspheres became
more and more rough. The high magnification SEM images (Fig.
the TiO nanocrystals. Few CDs were found at the TiO surfaces
2
2
of sample Cꢀ450 (Fig. 3d). Besides anatase TiO , HRTEM images
2
2
e) revealed that the surface of sample Cꢀ12 was overspread with
revealed another TiO2 nanocrystal, the interplanar spacing of
which was about 0.32 nm, belonging to the (110) plane of rutile
small particles with an average diameter about 30 nm. After a
further calcination under 450 ºC, most microspheres of sample Cꢀ
TiO . The HRTEM analyses were correspondent with the XRD
2
4
50 broke (Fig. 2c), and the inner morphologies were exposed.
results. It can be concluded from Fig. 3 that sample Cꢀ9 was
composed of TiO2 nanocrystals and CDs, with few partially
reacted precursors being deposited on the surface and between
Similarly, the high magnification SEM images (Fig. 2e) showed
that the inner space of microspheres was also filled fully with
nanoparticles. Taking the surface structure into account
synchronously, it was easy to speculate that the basic unit of the
microspheres was the observed nanoparticles, which further
them. As to sample Cꢀ450, TiO nanocrystals with two structures,
2
few CDs and the carbon matrix (amorphous) produced from the
previous residues constituted the whole composites.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3

RSC Advances
Page 4 of 7
View Article Online
DOI: 10.1039/C4RA14865G
samples from Cꢀ3 to Cꢀ12 were attributed to vibrations of C=O,
NꢀH, CꢀO and TiꢀO bonds separately, and all peaks were dimmed
out with prolonged heating time. It must be pointed out that the
2
2
3
3
4
4
5
5
0
5
0
5
0
5
0
5
ꢀ
1
peaks at 1075 and 1370 cm of sample Cꢀ3 belonging to CꢀO and
ꢀ
1
CꢀN stretching vibrations shifted to 1115 and 1400 cm in the
following samples from Cꢀ6 to Cꢀ12, which probably meant
further interactions between CDs and TiO . Compared with the
2
FTIR spectrum of pure CDs, it’s easy to find that almost all the
characteristic peaks of the CDsꢀTiO composites could be found
2
in the lineshape of CDs (Fig. S6). Whereas there were two stable
ꢀ
1
characteristic peaks at 1633 and 1400 cm corresponding to C=C
and CꢀN vibrations appeared in all samples from Cꢀ400 to Cꢀ550.
Obviously, the solvothermal process was a mild approach to
obtain CDsꢀTiO composites with abundant groups on the surface
2
besides the fine hybridization. While the further calcination
realized the finely doping of inert carbon and TiO , as most of
2
surface functional groups were sacrificed in the calcination.
Further characterization studies using XPS provided
convincing evidences for the surface states and composition of
the asꢀprepared samples and the results were illustrated in Fig. 5.
Both sample Cꢀ9 and Cꢀ450 showed clear constituents of element
C, O and Ti. To be particularly, though the highꢀresolution C 1s
spectra (Fig. 5b and Fig. 5e) showed a similar distribution of
signals, the intensity of each signal differed observably. The
strongest signal of sample Cꢀ9 emerged at 286.1 eV attributing to
C=O and CꢀN, while that of Cꢀ450 was found at 284.9 eV
corresponding to CꢀO and graphite carbon. It’s easy to conclude
that the functional groups on the surface of sample Cꢀ450 were
less than that of Cꢀ9 as a result of the further calcination. In fact,
a 42.2 % and 28.9 % C atomic content were detected respectively
from Cꢀ9 and Cꢀ450, indicating a sharp decrease of C contents in
the samples. Especially to sample Cꢀ9, detection of a 4.3 % N
atomic content further confirmed the successful Nꢀdoping in the
Fig. 3 HRTEM images of sample Cꢀ9 (a and b) and Cꢀ450 (c and d).
The thermogravimetry results (Shown in Fig. S5) revealed that
the weight loss of sample Cꢀ9 was exactly similar to that of pure
CDs, while the thermogravimetric curves of sample Cꢀ450
remained nearly unchanged. This result reconfirmed that the
further calcination mainly consumed CDs and previous unreacted
precursors in sample Cꢀ9, which probably converted the CDsꢀ
5
TiO composites into CꢀTiO composites.
2
2
CDsꢀTiO composites. However, most Nꢀdoping were probably
2
10
accompanied with CDs, for the N1s peak (400 eV around) of
sample Cꢀ450 almost vanished, leaving a very unapparent
lineshape, which meant a very low N content. Whether or not, the
highꢀresolution Ti 2p spectra (Fig. 5c and Fig. 5f) of sample Cꢀ9
and Cꢀ450 held same signals all through at 458 eV and 464 eV
around, attributing to the atomic state of TiꢀO.
2
Fig. 4 FTIR spectra of the asꢀprepared CDsꢀTiO composites (left) and Cꢀ
TiO composites (right).
2
The FTIR spectra were shown in Fig. 4, the trend of
characteristic peaks for CDsꢀTiO composites that only suffered a
2
15
solvothermal process was evident with prolonged heating time,
while there were nearly no difference for the CꢀTiO composites.
2
ꢀ
1
The peaks at 1735, 1641, 1194 and 1023 cm that appeared in all
4
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Page 5 of 7
RSC Advances
View Article Online
DOI: 10.1039/C4RA14865G
Fig. 5 XPS spectra of sample Cꢀ9 (a, b c) and Cꢀ450 (d, e, f).
The detailed PL properties of pure CDs were studied by
changing excitation wavelengths ranging from 360 to 500 nm
concentration was no more than 10 ppm. Fig. 7 plotted the UVꢀ
vis absorption curves of the reaction solutions at regular intervals,
representing the photocatalytic ability of different catalysts. We
50 can see that when a 10 min irradiation of UV light was carried
out on the 10 ppm MB solution containing sample Cꢀ9,
decontamination of MB was almost complete (ca. 95%). In fact,
(Fig. 6). With the increase of the excitation wavelength, the
5
maximum emission peak position shifted to longer wavelength
gradually, accompanied with remarkable decrease of PL intensity.
34
This phenomenon was accordant with that observed by Zhang ,
which meant the photoexcited charge carriers could be generated
by the visible light, attributing to the complex surface states of
CDs. And the droplet containing CDs exhibited blue, green and
red photoluminescence, respectively, under ultraviolet (330 – 385
nm), blue (460 – 495 nm) and green (530 – 550 nm) light
excitation. Likewise, powders of sample Cꢀ9 showed clear blue,
green and red photoluminescence compared with Cꢀ450 and pure
all the CDsꢀTiO composites were efficient on photoꢀdegradation
2
of MB under UV light (Fig. S11), and sample Cꢀ9 was the best
55 one. In the control experiment, using sample Cꢀ450 as catalysts,
the degradation rate of MB was about 37% after 10 min
irradiation. And a complete photoꢀdegradation of MB (ca. 96%)
for Cꢀ450 required 40 min of UV light irradiation. Compared
with the other CꢀTiO2 composites, Cꢀ450 showed the best
1
1
2
0
5
0
TiO under the same photograph conditions (Fig. S7), indicating 60 photocatalytic ability, following by sample Cꢀ500 (Fig. S12).
2
that CDs were abundant and wellꢀdistributed in the CDsꢀTiO2
composites. However, the upconverted PL property of CDs,
which was regarded as an important causation to improve the
While under the same conditions, reduction of MB was only 72%
even after 40 min irradiation to the pure TiO . These results
2
indicated that CDsꢀTiO2 composites could be served as an
efficient photocatalyst for MB. Moreover, in order to check the
65 recyclability of the catalysts, we repeated the above catalytic tests
but using a timesaving concentration of MB (5 ppm). The
intensity ratios of the absorbance at 664 nm after and prior to the
3
2
35
photocatalytic activity of TiO mentioned by Lee and Liu , was
2
not found here.
UV light irradiation for a certain time (A/A ) were plotted as a
0
function of the reaction time. It was found that there was no
7
0
significant drop of the photocatalytic efficiency even after ten
cycles for sample Cꢀ9, and all the photoꢀdegradation processes
were finished in 10 min, which meant that the CDsꢀTiO2
composites had great potential for practical application. While in
the control experiment, as shown in Fig. 7d, though there was a
10% around drop of the photocatalytic efficiency after ten cycles
for sample Cꢀ450 with a 20 min irradiation, the photoꢀ
degradation processes were completed after 40 min of UV light
75
irradiation. As to the pure TiO , it was too difficult to collect all
the powders of P25 after the photoꢀdegradation trials, let alone
80 the recycling for next photocatalysis.
2
Fig. 6 PL emission spectra of the pure CDs (left) and fluorescent
microscopy images of diluted aqueous solution containing CDs under
white, ultraviolet (330 – 385 nm), blue (460 – 495 nm) and green (530 –
550 nm) light excitation (right).
2
5
To further determine the possible constituents of the asꢀ
prepared composites, concentrated sulfuric acid was used to
dissolve the samples. As shown in Fig. S8, both sample Cꢀ9 and
P25 could be dissolved completely, forming a transparent
solution separately but with different colors. However, we could
only get a suspension for sample Cꢀ450 even using the hot
concentrated sulfuric acid. Furthermore, these obtained solutions
were examined under a UV lamp with emission wavelengths
centered at 365 nm (Fig. S9). Obvious fluorescence could be
found from the solution in which sample Cꢀ9 was dissolved.
Then the suspension of sample Cꢀ450 emitted a relatively weak
fluorescent light too. As for P25, no fluorescence was observed at
all. These results reconfirmed that sample Cꢀ9 was a nanohybrid
3
3
4
4
0
5
0
5
of CDs and TiO , and most CDs were converted into inert and
2
insoluble carbon materials in sample Cꢀ450 after the calcination.
In order to investigate the special chemical properties of the asꢀ
prepared composites, we firstly evaluated their photocatalytic
ability for the photoꢀdegradation of MB under UV light (254 nm),
Fig. 7 UVꢀvis absorption spectra monitoring the degradation of MB (a, b,
c) and cycling tests of photocatalytic activity of sample Cꢀ450 (d).
To further explore the photocatalytic activity of the asꢀprepared
85
composites, MO and RB were also used as the mimetic pollutants.
As shown in Fig. S13, after a 60 min irradiation under UV light,
comparing with the pure TiO (Degussa P25). It had been proved
2
that the UVꢀvis absorbance at 664 nm was liner with the
concentration of MB solutions (Fig. S10), especially when the
5
4
6% and 82% of MO were degraded by the sample Cꢀ9 and Cꢀ
50, separately. It was interesting that this time sample Cꢀ450 is
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5

RSC Advances
Page 6 of 7
View Article Online
DOI: 10.1039/C4RA14865G
more efficient on photoꢀdegradation of MO. Furthermore, lower
photocatalytic efficiency (ca. 34%) was achieved by sample Cꢀ9
to degrade RB, comparing to 88% of Cꢀ450. That is to say, the
and reactions in the CDsꢀTiO composites brought forth by the
2
calcination. It’s known that the BET specific surface area of
2
ꢀ1
commercial pure TiO (P25) was 50.0 m g around, which was
2
CDsꢀTiO composites were good at the photoꢀdegradation of MB. 55 close to that of sample Cꢀ9. Combined with the above
2
5
0
5
0
5
0
As to MO and RB, the CꢀTiO2 composites exhibited more
advantages.
photcatalytic results, it could be speculated that the superior
adsorption of MB for the CDsꢀTiO composites did not only stem
2
During the photocatalysis, three factors are crucial, including
the adsorption of reactants, the light absorption, and the charge
separation and transportation. The adsorption capacity of sample
Cꢀ9 and Cꢀ450 was roughly tested upon MB, MO and RB. The
absorbance measured at the adsorptionꢀdesorption equilibrium
was contrasted with that of the original solution (Shown in Fig.
S14). Sample Cꢀ450 showed a slight adsorption upon all the
pollutants. It was similar for sample Cꢀ9 upon MO and RB.
However, MB was seriously absorbed by sample Cꢀ9. The effect
of this absorption on the photocatalytic ability of sample Cꢀ9 was
further explored. As shown in Fig. S15, the absorbance at the
adsorptionꢀdesorption equilibrium in each cycle was measured
from their big specific surface areas, but also probably from the
microporous structure and aboundant surface functional groups
60 introduced by CDs. However, it seemed that the superior
1
1
2
2
3
adsorption of CDsꢀTiO composites functioned on MB only. As
2
to MO and RB, the big specific surface areas and proper
proportion of anatase/rutile content of the CꢀTiO2 composites
were more advantageous.
65
Here, we conjectured that the introduction of CDs, together
with fine nanoꢀhybrid of CDs and TiO , may be responsible for
2
their excellent photocatalytic performance on MB. And the
underlying causes for the enhanced photocatalytic property of the
CDsꢀTiO2 composites were presumed as follows. Firstly, the
without irradiation. The MB concentration increased along with 70 programmed preparation process induced microporous structure
the number of equilibrium cycles. Several cycles were chosen to
test the photocatalytic ability of sample Cꢀ9 after absorption of
considerable amounts of MB. Though 48% MB was absorbed by
sample Cꢀ9 at the adsorptionꢀdesorption equilibrium of sixth
cycle, photoꢀdegradation of MB was still up to 91% after 10 min
irradiation. The figure dropped to 64% in test of eighth cycle.
While in the eighteenth cycle, the photocatalytic efficiency was
only 46% after 20 min irradiation. It was reconfirmed that the
and abundant functional groups were beneficial to adsorption of
reactants as soon and much as possible. Secondly, as a carbon
material at the nanoscale, CDs could well accept the
photogenerated electrons from TiO and promote the separation
2
75 of photogenerated charge carriers. Lastly, the nanoꢀhybrid and
tight integration of CDs and TiO could help for the transfer of
2
photogenerated electrons efficiently, and then reduce the
recombination rate of electronꢀhole pairs greatly. As to the Cꢀ
CDsꢀTiO composites were efficient on photoꢀdegradation of MB,
TiO composites, though most CDs lost in the calcination, the
2
2
besides a special absorption capability for MB. However, the 80 rest CDs and new amorphous carbon, the bigger specific surface
photocatalytic activity would be weakened if too many MB were
absorbed.
areas and proper proportion of anatase/rutile content, all together
contributed to the final excellent photocatalytic activity.
Conclusion
The CDsꢀTiO composites were successfully synthesized in situ
2
85
via a oneꢀstep solvothermal route. And the doping of CDs and
TiO2 particles was achieved at the nanoscale under our
programmed experiments. Moreover, the final CDsꢀTiO2
composites were of
advantaged in separation off the reaction system after
photocatalysis. The crystallinity of ingredients,
a “microsphere” shape, which was
Fig. 8 Nitrogen adsorptionꢀdesorption isotherms (a) and pore size
distribution curves (b) of sample Cꢀ9 and Cꢀ450.
90
TiO2,
microstructure and surface state of CDsꢀTiO composites could
2
35
40
45
50
As the adsorption of reactants usually was dependent on
structural or surface characteristics of the catalysts, the structural
characteristics of prepared samples here were further analyzed by
nitrogen adsorptionꢀdesorption isotherm, as shown in Fig. 8a. 95 MB showed excellent photocatalytic activity of the CDsꢀTiO2
Both of the isotherms were of type IV (BDDT classification). At
be adjusted by the reaction time. Though CDs here didn’t show
the upconverted PL property, it was sensitive to light in a quite
broad range, especially to UV light. The photoꢀdegradation of
composites, which was much higher than that of commercial
Degussa P25. It’s believed that nanohybird of CDs and TiO2,
together with the chemical and optical properties introduced by
CDs, made most contributions to the enhanced photocatalytic
high relative pressures from 0.6 and 1.0, the isotherms exhibited
hysteresis loops of type H , indicating that the two samples
3
contained mesopores (2 ꢀ 50 nm). But evidently, the specific
volume of adsorbed nitrogen molecules increased sharply for 100 activity of the CDsꢀTiO2 composites. Furthermore, CꢀTiO2
sample Cꢀ450 with enhanced relative pressure of nitrogen. And
composites were produced from the CDsꢀTiO composites by a
2
the BET specific surface areas calculated for sample Cꢀ450 was
further calcination, which had shown
a more excellent
2
ꢀ1
2
ꢀ1
9
2.7 m g , comparing to a value of 55.5 m g for sample Cꢀ9.
photocatalytic activity on the photoꢀdegradation of MO and RB.
Besides the effects of CDs, the newly generated amorphous
Fig. 8b showed the corresponding pore size distribution curves.
The pore diameters of sample Cꢀ9 and Cꢀ450 were 7.0 nm and 105 carbon that finely doped with TiO , bigger specific surface areas
2
1
0.5 nm, respectively. In other words, sample Cꢀ450 possessed
and proper proportion of anatase/rutile content, probably
accounted for the enhanced photocatalytic activity of the CꢀTiO2
composites.
higher BET specific surface area and bigger pore diameter than
that of Cꢀ9. This might be caused by the further carbonization
6
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Page 7 of 7
RSC Advances
View Article Online
DOI: 10.1039/C4RA14865G
3
1
S. N. Baker and G. A. Baker, Angew. Chem. Int. Ed., 2010, 49, 6726ꢀ
744.
Acknowledgements
6
The authors gratefully acknowledge the support for this work
from the National Natural Science Foundation of China (Grant
70 32 H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, S. Lian, C. H. A.
Tsang, X. Yang and S.ꢀT. Lee, Angew. Chem., 2010, 49, 4430ꢀ4434.
3
3
3
4
J. Wang, M. Gao and G. W. Ho, J. Mater. Chem. A, 2014, 2, 5703ꢀ
5
1303210 and 51202292) and National Science and Technology
5
709.
5
Major Project of China (Grant 2012ZX10004801ꢀ003).
L. Feng, X. Y. Tang, Y. X. Zhong, Y. W. Liu, X. H. Song, S. L. Deng,
S. Y. Xie, J. W. Yan and L. S. Zheng, Nanoscale, 2014, 6, 12635ꢀ
75
1
2643.
Notes and references
3
5
X. Yu, J. Liu, Y. Yu, S. Zuo, B. Li. Carbon, 2014, 68, 718ꢀ724.
Institute of Medical Equipment, Academy of Military Medical Sciences,
Tianjin 300161, PR China. Email: tianfeng62037@163.com.
36 J. Bian, C. Huang, L. Wang, T.F. Hung, W. A. Daoud, and R. Zhang,
ACS Appl. Mater. Interfaces, DOI: dx.doi.org/10.1021/am4059183.
†
Electronic Supplementary Information (ESI) available: Experimental
80 37 S. Xie, H. Su, W. Wei, M. Li, Y. Tong and Z. Mao, J. Mater. Chem. A,
2014, 2, 16365ꢀ16368.
1
1
2
2
3
3
4
4
5
5
6
6
0
5
0
5
0
5
0
5
0
5
0
5
details and supplementary data. See DOI: 10.1039/b000000x/
1
2
3
4
X. B. Chen, S. H. Shen, L. J. Guo and S. S. Mao, Chem. Rev., 2010,
10, 6503ꢀ6570.
G. Li, L. Wu, F. Li, P. Xu, D. Zhang and H. Li, Nanoscale, 2013, 5,
118ꢀ2125.
Z. X. Li, Y. L. Xie, H. Xu, Y. M. Wang, Z. G. Xu and H. L. Zhang, J.
Photochem. Photobiol., A, 2011, 224, 25ꢀ30.
38 M. Sun, X. Ma, X. Chen, Y. Sun, X. Cui and Y. Lin, RSC Adv., 2014,
4, 1120ꢀ1127.
39 Z. Jiang, W. Wei, D. Mao, C. Chen, Y. Shi, X. Lv and J. Xie,
1
2
85
90
Nanoscale, 2015, 7, 784ꢀ797.
40 J. Liu, W. Zhu, S. Yu, X. Yan, Carbon, 2014, 79, 369ꢀ379.
41 Z. Ma, H. Ming, H. Huang, Y. Liu and Z. Kang, New J. Chem., 2012,
36, 861ꢀ864.
42 X. Zhang, F. Wang, H. Huang, H. Li, X. Han, Y. Liu and Z. Kang,
Nanoscale, 2013, 5, 2274ꢀ2278.
H. Kim, J. Kim, W. Kim and W. Choi, J. Phys. Chem. C, 2011, 115,
9
797ꢀ9805.
5
6
S. Liu, E. Guo and L. Yin, J. Mater. Chem., 2012, 22, 5031ꢀ5041.
P. Xu, T. Xu, J. Lu, S. Gao, N. S. Hosmane, B. Huang, Y. Dai and Y.
Wang, Energy Environ. Sci., 2010, 3, 1128ꢀ1134.
43 J. Zhong, F. Chen and J. Zhang, J. Phys. Chem. C, 2010, 114, 933ꢀ
939.
7
8
M. Yang, H. Jha, N. Liu and P. Schmuki, J. Mater. Chem., 2011, 21,
1
5205ꢀ15208.
K. E. deKra, C. Wang and W. B. Lin, Adv. Mater., 2012, 24, 2014ꢀ
2018.
G. Halasi, G. Schubert and F. Solymosi, J. Catal., 2012, 294, 199ꢀ206.
J. Wang and Z. Lin, Chem. Mater., 2010, 22, 579ꢀ584.
Y. Park, S. H. Lee, S. O. Kang and W. Choi, Chem. Commun., 2010,
9
1
11
0
4
6, 2477ꢀ2479.
12 T. T. Le, M. S. Akhtar, D. M. Park, J. C. Lee and O. B. Yang, Appl.
Catal., B, 2012, 111-112, 397ꢀ401.
1
3
P. A. DeSario, J. J. Pietron, D. E. DeVantier, T. H. Brintlinger, R. M.
Stroud and D. R. Rolison, Nanoscale, 2013, 5, 8073ꢀ8083.
B. Liu, Q. Wang, S. Yu, P. Jing, L. Liu, G. Xua and J. Zhang,
Nanoscale, 2014, 6, 11887ꢀ11897.
N. Zhang, S. Liu and Y. J. Xu, Nanoscale, 2012, 4, 2227ꢀ2238.
G. S. Wu, J. P. Wang, D. F. Thomas, A. C. Chen, Langmuir, 2008, 24,
1
4
1
1
5
6
3
503ꢀ3509.
1
1
1
7
8
9
R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science,
2001, 293, 269ꢀ271.
S. U. M. Khan, M. A. Shashry and W. B. Ingler Jr, Science, 2002,
2
97(5590), 2243ꢀ2245.
S. Sakthivel and H. Kisch, Angew. Chem., Int. Ed., 2003, 42, 4908ꢀ
911.
4
20 T. Ohno, M. Akiyoshi, T. Umebayashi, K. Asai, T. Mitsui and M.
Matsumura, Appl. Catal., A, 2004, 265, 115ꢀ121.
J. C. Yu, J. Yu, W. Ho, Z. Jiang, L. Zhang, Chem. Mater., 2002, 14,
2
2
1
2
3
808ꢀ3816.
S. In, A. Orlov, R. Berg, F. Garcı´a, S. PedrosaꢀJimenez, M. S.
Tikhov, D. S. Wright, R. M. Lambert, J. Am. Chem. Soc., 2007, 129,
1
3790ꢀ13791.
2
2
3
4
X. Chen, L. Liu, P. Y. Yu and S. S. Mao, Science, 2011, 331, 746ꢀ750.
Y. Yan, M. Han, A. Konkin, T. Koppe, D. Wang, T. Andreu, G. Chen,
U. Vetter, J. R. Morante and P. Schaaf, J. Mater. Chem. A, 2014, 2,
12708ꢀ12716.
G. C. Xie, K. Zhang, B. D. Guo, Q. Liu, L. Fang and J. R. Gong, Adv.
Mater., 2013, 25, 3820ꢀ3839.
N. Zhang, Y. H. Zhang and Y. J. Xu, Nanoscale, 2012, 4, 5792ꢀ5813.
H. I. Kim, G. H. Moon, D. M. Satoca, Y. Park and W. Choi, J. Phys.
Chem. C, 2012, 116, 1535ꢀ1543.
S. T. Kochuveedu, Y. J. Jang, Y. H. Jang, W. J. Lee, M. A. Cha, H.
Shin, S. Yoon, S. S. Lee, S. O. Kim, K. Shin, M. Steinhart and D. H.
Kim, Green Chem., 2011, 13, 3397ꢀ3405.
2
5
2
2
6
7
2
8
2
3
9
0
G. Zhang, F. Teng, Y. Wang, P. Zhang, C. Gong, L. Chen, C. Zhao
and E. Xie, RSC Adv., 2013, 3, 24644.
S. Wang, L. Zhao, L. Bai, J. Yan, Q. Jiang and J. Lian, J. Mater.
Chem. A, 2014, 2, 7439ꢀ7445.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 7