Low-temperature synthesis of stable nanoTiO2-rGO composite colloids and their application in photoelectric films

Article abstract of DOI:10.1039/c3ra23095c

It is found that low-temperature synthesized nanoTiO₂ particles and reduced graphene oxide (rGO) would make up for each other's shortcomings: on the one hand, negatively charged nanoTiO₂ particles could obviously enhance the dispersion of rGO. The nanoTiO₂-rGO colloid synthesized at a low temperature (95°C) remained stable for at least 6 months; on the other hand, for the ultrathin and transparent nanoTiO₂-rGO films prepared using this stable colloid, rGO loading could enhance the connection of the nanoTiO₂ particles and the substrate and therefore effectively increase the photocurrent to 4.8 times more than the photocurrent generated by pure nanoTiO₂. The as-prepared transparent flexible film can be shaped into triangular calandrias which could make full use of the incident UV light, and thus increase the removal efficiency from the pass through of gas-phase acetaldehyde.

Full text of DOI:10.1039/c3ra23095c

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Low-temperature synthesis of stable nanoTiO2–rGO
composite colloids and their application in photoelectric
films3
Cite this: RSC Advances, 2013, 3,
8559
Hanyang Gao,a Wei Chen,a Jian Yuan,a Zhi Jiang,a Guoxin Hu,b
Wenfeng Shangguan,*a Yangzhou Sunc and Jiachun Suc
It is found that low-temperature synthesized nanoTiO2 particles and reduced graphene oxide (rGO) would
make up for each other’s shortcomings: on the one hand, negatively charged nanoTiO2 particles could
obviously enhance the dispersion of rGO. The nanoTiO2–rGO colloid synthesized at a low temperature (95
uC) remained stable for at least 6 months; on the other hand, for the ultrathin and transparent nanoTiO2–
rGO films prepared using this stable colloid, rGO loading could enhance the connection of the nanoTiO2
particles and the substrate and therefore effectively increase the photocurrent to 4.8 times more than the
photocurrent generated by pure nanoTiO2. The as-prepared transparent flexible film can be shaped into
triangular calandrias which could make full use of the incident UV light, and thus increase the removal
efficiency from the pass through of gas-phase acetaldehyde.
Received 29th November 2012,
Accepted 25th March 2013
DOI: 10.1039/c3ra23095c
www.rsc.org/advances
prepared by removing the hydrophilic oxygen functional
groups from carbon planar GO sheets, would easily agglom-
erate and precipitate out. This strongly agglomerating char-
acteristic becomes the biggest obstacle to the practical
applications of rGO.
1. Introduction
TiO2 films prepared at a low-temperature deposited on
polymer substrates for flexible dye-sensitized solar cells
(flexible DSSCs) usually suffer from a poor connection between
the TiO2 nanoparticles and the polymer because high-
temperature annealing is the key process to ensure the
effective connection between the nanoparticles and the
substrate.1–3 Reduced graphene oxide (rGO), a new star
material in recent years, has proved to be highly desirable
for use as a two-dimensional catalyst support because it has a
large theoretical surface area and excellent conductivity.4–7
Loading rGO onto low-temperature synthesized nanoTiO2
particles may improve the photocatalytic performance8 of the
film because rGO may act as an ‘electron bridge’ to promote
the electron transfer between the TiO2 particles and between
the TiO2 particles and the substrate.
The stable composite colloid is important for industrial
applications. The stability of the colloid will significantly
improve manufacturing efficiency and cost efficiency in large
scale industrial production of uniform films because it can be
adapted to the spraying/printing/brushing film-forming tech-
nology.11 Also, a stable colloid is particularly the key to making
ultrathin and transparent films on flexible subtrates.12,13
In the present paper, we present a simple method to
prepare a stable nanoTiO2–rGO colloid at a low temperature.
The stable colloid could be deposited on conductive-plastic
film substrates by various methods to make nanoTiO2–rGO
ultrathin and transparent films. In this system, the nanoTiO2
and rGO will reduce the limitations of one another: On the one
hand, the negatively charged nanoTiO2 particles will obviously
enhance the dispersion of rGO in solvent to obtain a stable
nanoTiO2–rGO colloid; whereas on the other hand, the rGO
loading could in turn improve the connection of nanoTiO2
particles and the connection of nanoparticles and the ITO-PET
substrate (Scheme 1).
But another problem needs to be solved first. Although the
oxidized form of graphene (graphene oxide, GO)9,10 could
remain stable in various solvents, the rGO, which was
aResearch Center for Combustion and Environment Technology, Shanghai Jiao Tong
University, 800 Dongchuan Road, 200240 Shanghai, China.
E-mail: shangguan@sjtu.edu.cn; Fax: 86 21 34206020; Tel: 86 21 34206020
bSchool of Mechanical and Power Engineering, Shanghai Jiao Tong University, 800
Dongchuan Road, 200240 Shanghai, China
cNew Energy Research Institute, CNOOC NEI Co. Ltd, Beijing, China
Electronic supplementary information (ESI) available: Photocatalytic
3
2. Experimental
performance measurements, gas-phase acetaldehyde decomposition method,
Tyndall effect of nanoTiO2 colloid, specific surface area of nanoTiO2–rGO
composites and UV-vis absorption spectrum of the nanoTiO2–rGO films. See
DOI: 10.1039/c3ra23095c
All reagents were analytical grade and used without any further
purification. GO was synthesized by a modified Hummer’s
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Scheme 1 On one hand, nanoTiO2 could enhance the dispersion of rGO, which
is important for the storage and utilization of rGO; on the other hand, the rGO
loading could enhance the connection of nanoTiO2 particles and obviously
increases the photocurrent of the films deposited on an ITO-PET substrate.
method from natural graphite powder (99%, Sinopharm
Chemical Reagent Co., Ltd.),14 while Ti(OBu)4 (Sinopharm
Chemical Reagent Co., Ltd.) was used as the TiO2 precursor.
Different amounts of GO suspensions were used as starting
materials to prepare nanoTiO2–rGO composites with different
GO concentrations. The samples were denoted A–F respec-
Fig. 1 (a) TEM images of GO, (b, c) nanoTiO2 particles and (d) the nanoTiO2–rGO
composite.
tively as the concentration of GO was increased (see ESI for
3
details of GO concentration). After adding Ti(OBu)4 and PrOH
to the GO suspension (GO suspensions were adjusted to
different pH values previously) and keeping the mixture under
reflux conditions at 75 uC for 24 h, the nanoTiO2–GO
composite colloid was obtained. Then hydrazine was added
as reducing agent to reduce the GO to rGO. After stirring at 95
3. Results and discussion
3.1 Morphological features
The morphology of the composites can be seen from the high
resolution transmission electron microscopy (HRTEM)
images. Fig. 1a shows the winkled and flexible GO layer. The
nanoTiO2 particles in the composites are spherical particles
with an approximate diameter of 2–5 nm (Fig. 1b). The
nanoTiO2 particles are crystalline and the lattice arrangement
can be clearly seen in the image (Fig. 1c). For the composite
sample, the scattered nanoTiO2 nanoparticles outside the rGO
sheets in Fig. 1d indicate that the nanoTiO2 are not tightly
anchored on the rGO sheets.
uC for
2 h and then ultrasonicating for 20 min, the
homogeneous and stable nanoTiO2–rGO composite colloid
was obtained.
The surface morphology was examined using a JEM-2100F
high resolution-transmission electron microscope (HRTEM)
operated at 200 kV with a point-to-point resolution of 0.19 nm.
X-ray diffraction patterns (XRD) were measured on a Rigaku D/
Max-2200/PC X-ray diffractometer. Unpolarized micro-Raman
mapping was performed on a Jobin Yvon LabRam HR800 UV
micro-Raman spectrometer at room temperature. X-ray photo-
electron spectroscopy (XPS) was examined using a PHI 5000C
ESCA System (Perkin-Elmer). N2-BET was determined using a
Quantachrome Nova 1000 Surface Area Analyzer. UV-visible
absorption spectra (UV-vis) were obtained using a SHIMADZU
UV-2450 spectrophotometer. Zeta potential analysis was
examined using a Nicomp380ZLS analyzer (PSS).
3.2 Raman, XRD and XPS analysis
Raman spectroscopy can be used to study the ordered/
disordered crystal structures of carbonaceous materials.
Fig. 2a shows the Raman spectra of the nanoTiO2–GO
composite (sample F) and the corresponding nanoTiO2–rGO
composite. Both of the composites present D and G bands at
1350 cm21 and 1605 cm21 respectively.15,16 Comparing the
intensity ratios of the D and G bands (ID/IG), it can be observed
that the ID/IG of the nanoTiO2–rGO sample is smaller than that
of nanoTiO2–GO, which indicates lower defects and a higher
degree of graphitization. The reduction of ID/IG suggests that
the GO sheets are successfully reduced to rGO after the
addition of the hydrazine reducing agent.
Fig. 2b shows the XRD patterns of the as-prepared
nanoTiO2, GO and the nanoTiO2–GO composite. As the as-
prepared nanoTiO2 mainly exhibits characteristic diffraction
peaks of anatase phases, the anatase phase is the major phase
of the low-temperature synthesized nanoTiO2 particles. The
little peak around 31u, which is ascribed to the (121) planes of
A
CHI 660D Electrochemical Analyzer was used for
photoelectrochemical performance measurements. Thin films
deposited on ITO-PET substrates were fixed in a specially
designed sample holder with a hole of 2 centimeters in
diameter to let light in for measurements. A 350 W xenon lamp
was used as a light source. The details of the photocatalytic
performance measurements and gas-phase acetaldehyde
decomposition are provided in the ESI.
3
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Fig. 2 (a) Raman spectra of GO and the nanoTiO2–rGO composite (sample F), (b) the XRD patterns of nanoTiO2, GO and sample F, (c) the C 1s core-level XPS spectra of
sample D and sample F.
the brookite phase, indicates that at such a low synthesis
temperature, the transformation to the anatase phase is not
complete. In the GO XRD pattern, a peak centered at 2h =
10.3u, which corresponds to the (002) inter-planar spacing of
0.86 nm, is shown. Consistent with the reported studies,17 no
GO characteristic peaks but all the characteristic diffraction
peaks of the nanoTiO2 could be found in the nanoTiO2–GO
composite XRD pattern, which further indicted that nanoTiO2
particles are successfully deposited on GO sheets during the
drying process and hinder the restacking of GO sheets.
XPS analysis was utilized to investigate the chemical state
variations of nanoTiO2–rGO composites prepared using
different concentration GO suspensions as the starting
materials. Fig. 2c shows XPS spectra for the C1 score level of
sample D and sample F. The morphology and distribution of
the peaks are almost the same. The C1s region is composed of
two contributions. The main contribution is the peak at a
binding energy of 285.0 eV, which is attributed to the C–C,
CLC and C–H bonds.18–20 The other contribution is assigned
to the C–O bonds. The peaks at 286.0, 287.7 and 289.2 eV were
assigned to the C–OH, CLO and OLC–OH functional groups
respectively. Slight differences in intensity exist among these
C–O peaks between sample D and sample F; this may due to
the reducing effect of the reducing agent on different types of
oxygen-containing functional groups of graphene oxide being
not fully controllable. Meanwhile, the peak at 281.9 eV, which
would indicate the C–Ti bonds, was not observed in the
spectra.18,19,21 C–C and C–O bonds existing without the
existence of C–Ti bonds indicates that the nanoTiO2 was
deposited on the surface of the rGO without the formation of
chemical bonds. The chemical bonds, which are usually
formed between the TiO2 and graphene after high temperature
annealing22–24 cannot form in the low temperature synthetic
environment used in our research. Due to the non-formation
of this tight connection, the nanoTiO2 particles can remain
suspended in the sol and their surface charges promote the
dispersion of rGO.
stability. Colloids with a high zeta potential (negative or
positive) are electrically stabilized while colloids with low zeta
potentials tend to coagulate or flocculate. The pH and solvent
type has a great influence on the zeta potentials.25 Because the
rGO would aggregate very quickly in the pH , 9 solvent, in
contrast to the other samples, the zeta potential measurement
of rGO began from pH = 9.
Fig. 3a shows the zeta potentials of nanoTiO2, GO, rGO and
nanoTiO2–rGO (sample F) with different pH values. While the
GO colloid could stay stable at different pH values, after
reduction, the stability of rGO greatly decreases owing to the
removal of the hydrophilic oxygen-containing functional
groups from the carbon plane. The addition of nanoTiO2
could obviously increase the stability of the colloid which may
result from the repulsion provided by the negatively charged
nanoTiO2 particles. The nanoTiO2–rGO composite colloid
(sample F, pH = 10) stays stable for more than three months,
which is much more stable than the rGO colloid which would
aggregate after only one day (Fig. 3b). The difference of the
colloid stability could also be seen in the photograph of the
films which were prepared from membrane filter. The film
prepared from stable nanoTiO2–rGO composite colloid is
smoother and more even than the film prepared from the rGO
colloid (Fig. 3c).
The effect of rGO concentration on the stability of the
nanoTiO2–rGO composites is shown in Fig. 3d. This suggests
that the stability of the composite samples would decrease
with an increased loading of rGO . At pH = 9, the composites
with less rGO loading (samples A, B, C and D) could stay stable
for more than one year, while for samples E and F, a certain
amount of precipitation will gradually appear after 12 months.
But the agglomeration of sample E and F is not as irreversible
as that of rGO. Simple shaking or sonication could quickly
recover the dispersed state of this colloid (Fig. 3f).
Stable composite colloids can be obtained only through the
condensation reflux of the GO and the precursor of nanoTiO2
together. If the nanoTiO2 and GO colloid were prepared
separately and then mixed together, the stability of the colloid
3.3 Dispersion enhancement
would immediately be destroyed (see Fig. S4, ESI ).
3
The zeta potentials, widely used for the quantification of the
magnitude of the electrical charge at the double layer in
colloidal systems, could be used as indicators for colloidal
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Fig. 3 (a) The zeta potential of nanoTiO2, GO, rGO and sample F colloids at different pH values. (b) The optical photograph of nanoTiO2, GO, rGO and sample F
colloids, the rGO was unstable and tended to aggregate after one day while the nanoTiO2–rGO composite (with the same rGO amount as the rGO suspension) could
be stable for months. (c) The films prepared from the rGO suspension and nanoTiO2–rGO composite colloid. (d) The effect of rGO loading amount on the zeta
potential of nanoTiO2–rGO composite colloids. (e) The optical photograph of the nanoTiO2–rGO composite colloids with different rGO loading amounts (the
concentration of nanoTiO2 in all the samples is the same while the rGO concentration increases from sample A to F). (f) The aggregation of sample E and sample F is
reversible and the colloid would return to a homogeneous state after simple shaking or sonication.
substrate even after rubbing it vigorously. In the spraying
method, we use a spray gun combined with thermal-curing for
coating films on PET substrates. The role of the rGO in the
photoelectrochemical performances of the nanoTiO2–rGO
composite films was examined. A series of films deposited
on ITO-PET with same amount of nanoTiO2 and different
amounts of rGO were prepared using the scotch-tape method.
We employed as-made films as photoanodes in the photoelec-
trochemical cell, Pt as the counter electrode and Na2SO4 as the
electrolyte to study the photocurrent generation of the
nanoTiO2–rGO films. Fig. 4c shows the photocurrent action
3.4 Photoelectrochemical performance of the films
NanoTiO2–rGO films on PET substrates were prepared by
various methods using stable composite colloids. Fig. 4a and
4b show the nanoTiO2–rGO film prepared using scotch-tape
and spraying methods. It is worth noting that the stable
colloids are also compatible with the popular screen printing
technique. In the scotch-tape method, the composite colloid
was spread onto ITO-PET substrates with the aid of micro
pipette tip and scotch tape. After being dried at room-
temperature for about 24 h, the homogeneous and compact
films were obtained. The film would not be erased off the
Fig. 4 (a) Optical photographs and optical properties of nanoTiO2–rGO films deposited on ITO-PET substrates using the scotch-tape method and (b) the spraying
method. (c) Transient photocurrents of nanoTiO2 and nanoTiO2–rGO composite films deposited on ITO-PET under the full light spectrum of a 350 W xenon lamp and
without bias introduced to the system. (d) An illustration of the nanoTiO2–rGO film and the nanoTiO2 film; the rGO acts as an ‘‘electron bridge’’ to improve the
connection between the nanoTiO2 particles and the substrate.
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not conducive to photocurrent generation. The intensity of
photocurrent decreased when the rGO loading amount was
increased. This may be because higher loading of rGO will
darken the color of the films, thus hindering the light
absorption of the TiO2.
Table 1 The photosensitivity (Iphoto/Idark) of nanoTiO2 and nanoTiO2–rGO
composite films deposited on ITO-PET under the full light spectrum of a 350 W
xenon lamp and without bias introduced to the system
A
B
C
D
E
F
nanoTiO2
Photosensitivity is defined as the ratio of the current under
photo-illumination to that in the dark state (Iphoto/Idark).28,29
From Table 1 we can find that rGO can effectively increase the
photocurrent without obviously increasing the intensity of the
dark current, thus greatly increasing the photosensitivity.
Iphoto (mA) 0.395 1.057 0.930 0.513 0.324 0.027 0.222
Idark (mA) 0.051 0.040 0.086 0.062 0.042 0.002 0.096
Iphoto/Idark 7.75 26.43 10.81 8.27 7.71 13.50 2.31
spectra of the nanoTiO2–rGO films with different rGO loading
amounts with no anodic bias introduced into the system. We
can see from the spectra that the transient photocurrent
generated from the TiO2 film is about 0.22 mA. The intensity of
the photocurrent generated from the films greatly changed as
the amount of rGO changed. For sample B, the transient
photocurrent significantly increased to about 1.06 mA, which is
about 4.8 times the photocurrent generated from nanoTiO2
film. The intensity of photocurrent depends on the amount
and mobility of photogenerated charge carriers, while the
same amount of nanoTiO2 generated same amount of
photoelectrons and photoholes in all of the films, the
increasing photocurrent is attributed to either a reduction in
the chance of electron-hole recombination or an increased
carrier mobility. Because the work function of graphene is in
the range of 4.42–4.5 eV, the electrons generated on the
nanoTiO2 surface under UV irradiation will transfer to the
rGO.26,27 The transfer of electrons from TiO2 to rGO may not
only be conducive to the separation of photogenerated
electrons and holes, but will also have benefits for the transfer
speed because graphene has remarkably high electron
mobility at room temperature. Futhermore, instead of trans-
ferring from nanoTiO2 to nanoTiO2, the electrons generated
on the upper layer of the nanoTiO2 would transfer from the
nanoTiO2 to the rGO and then to the substrate. Thus as an
‘‘electronic bridge’’, the rGO improves the connection between
nanoTiO2 and the substrate. Although adding rGO could
obviously increase the photocurrent, overloading with rGO is
3.5 Photocatalytic performance of the films
It is not convenient to use a powder catalyst to remove air
pollutants because it can easily be blown away and the
recycling is inconvenient. In order to resolve this problem, in a
lot of research the catalyst is coated on the walls of a ceramic
honeycomb to carry out the air pollution removal process. But
the incident light will obviously be attenuated in the ceramic
honeycomb which would obviously decrease the catalytic
efficiency. In our research, the as-prepared flexible film can
be shaped into triangular tubes, and then be tied up to form a
cellular structure (Fig. 5a). It can be seen from the opposite
end of the tubes (tube length = 40 mm) that the incident light
intensity loss was much less obvious than in the traditional
ceramic honeycomb and thereby this structure allows more
effective use of the incident ultraviolet light (Fig. 5b). The
cellular pipelines were used as flow-through photocatalytic
tunnel to examine the photocatalytic decomposition of the
gas-phase of acetaldehyde (Fig. 5c). The gas-phase acetalde-
hyde entered from one side of the cellular pipelines with a fuel
exiting from the other. The acetaldehyde decomposition rate
and the CO2 evolution were recorded in these experiments.
Fig. 5d shows the results of the one-pass gas-phase
acetaldehyde photocatalysis decomposition performance of
the cellular pipelines and the P25 powder. The quantity of the
P25 powder was the same as the nanoTiO2–rGO content in the
films. Although the CO2 conversion is almost the same, the
Fig. 5 (a) A photograph of the cellular structure made from combined nanoTiO2–rGO (sample B) film pipelines. (b) The incident light seen from the opposite side of
the 40 mm length transparent tubes. (c) The scheme of the flow-through the photocatalytic tunnels to achieve the photocatalytic decomposition of the gas-phase
acetaldehyde. (d) One-pass photocatalytic decomposition of acetaldehyde and CO2 selective conversion using the cellular structure made from combined nanoTiO2–
rGO (sample B) film calandrias, P25 powder and nanoTiO2 powder. The tests used a 500 W xenon lamp and 0.3 g catalyst.
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In summary, we found that the nanoTiO2 and rGO will help
overcome the shortcomings of one another. Negatively
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sion of rGO in solvents and rGO will effectively improve the
poor connection between low-temperature synthesized
nanoTiO2 particles and the substrate when it was deposited
on the substrate to make a photoelectric film.
The stable nanoTiO2–rGO colloid was successfully synthe-
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Acknowledgements
The authors are grateful to the National Natural Science
Foundation of China (20973110), the National Key Basic
Research and Development Program (2009CB220000) and the
International Cooperation Project of Shanghai Municipal
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