Fabrication and Photocatalytic Properties of TiO2/Reduced Graphene Oxide/Ag Nanocomposites with UV/Vis Response

Article abstract of DOI:10.1002/ejic.201403238

Novel one-dimensional TiO₂ nanofibers/reduced graphene oxide/Ag nanoparticles (TiO₂/rGO/Ag) nanocomposite photocatalysts with UV/Vis response properties were fabricated by wrapping rGO sheets around TiO₂ nanofibers and then loading Ag nanoparticles onto the surfaces of TiO₂/rGO. The prepared samples were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and UV/Vis diffuse reflectance spectroscopy. The photocatalytic activities of the samples were evaluated by photocatalytic degradation of an aqueous solution of rhodamine B (RhB) under UV/Vis light irradiation. The results showed that the introduction of rGO and Ag greatly improved the photocatalytic activity of the TiO₂ nanofibers, and this enhancement was attributed to the hybridization of rGO possessing specific electronic effects and Ag nanoparticles showing surface plasmon resonance effects. As a consequence, extension of the absorption spectrum to the visible region and efficient restriction of the recombination of photogenerated carriers were achieved. TiO₂·rGO/Ag composite nanofibers are prepared by wrapping reduced graphene oxide (rGO) sheets around TiO₂ nanofibers and then loading Ag nanoparticles onto the surfaces of TiO₂/rGO. The introduction of rGO and Ag creates a marked improvement in the photocatalytic activity of the TiO₂ nanofibers.

Full text of DOI:10.1002/ejic.201403238

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DOI:10.1002/ejic.201403238
Fabrication and Photocatalytic Properties of
TiO2/Reduced Graphene Oxide/Ag Nanocomposites with
UV/Vis Response
Xueli Gou,[a,b] Yongliang Cheng,*[a] Bin Liu,[b] Bingjun Yang,*[a] and
Xingbin Yan[a]
Keywords: Photocatalysis / Nanoparticles / Nanocomposites / Silver / Graphene oxide
Novel one-dimensional TiO2 nanofibers/reduced graphene
oxide/Ag nanoparticles (TiO2/rGO/Ag) nanocomposite pho-
tocatalysts with UV/Vis response properties were fabricated
by wrapping rGO sheets around TiO2 nanofibers and then
loading Ag nanoparticles onto the surfaces of TiO2/rGO. The
prepared samples were characterized by scanning electron
microscopy, high-resolution transmission electron micro-
scopy, X-ray diffraction, X-ray photoelectron spectroscopy,
Raman spectroscopy, and UV/Vis diffuse reflectance spec-
troscopy. The photocatalytic activities of the samples were
evaluated by photocatalytic degradation of an aqueous solu-
tion of rhodamine B (RhB) under UV/Vis light irradiation.
The results showed that the introduction of rGO and Ag
greatly improved the photocatalytic activity of the TiO2 nano-
fibers, and this enhancement was attributed to the hybridiza-
tion of rGO possessing specific electronic effects and Ag
nanoparticles showing surface plasmon resonance effects. As
a consequence, extension of the absorption spectrum to the
visible region and efficient restriction of the recombination
of photogenerated carriers were achieved.
Introduction
Graphene, as a monolayer of carbon atoms with a tight-
packed honeycomb lattice, is widely used in various fields
not only because of its unique electrical properties, such as
its high theoretical specific surface area (ca. 2600 m2 g–1)
and excellent mobility of charge carriers at room tempera-
ture (200000 cm2 V–1 s–1), but also because of its excellent
thermal and optical properties.[13–16] Especially, its high
electron mobility and extended π–π conjugation structure
make it an ideal electron acceptor for nanocomposites.[17]
These properties also make graphene a potential support
material for doping traditional semiconductor materials
(such as TiO2, CdS, ZnO, CuO, etc.) for photocatalysis ap-
plications.[17–27] In particular, since the first work on a TiO2/
graphene nanocomposite reported by Williams et al., TiO2/
graphene composites have gained more and more atten-
tion.[28] According to the literature, the morphology of the
nanocomposite has a significant influence on its perform-
ance.[29–37] The two-dimensional and three-dimensional
structure of graphene/TiO2, resulting from random path-
ways in these composites, can dramatically slow down the
rate of electron transfer, which thus results in unexpected
loss of electrons compared to the one-dimensional struc-
ture.[35–37] Furthermore, to investigate the effect of mor-
phology on the performance of one-dimensional graphene/
TiO2 structures, several types of morphologies including
nanoparticles, nanowires, nanofibers, and other morpho-
logies have been synthesized.[29–35] Among them, nanowires
and nanofibers show better performance owing to their
high surface to volume ratio and their effective electron-
transfer pathways.[35] For example, Pan et al. reported that
Pollution of the environment with harmful contaminants
is worsening, especially pollution from harmful organic
contaminants, and this has had an extremely negative con-
sequence in the lives of humans. Thus, ways in which pol-
lution from contaminants can be solved has become one of
the most serious problems in the world.[1] On this basis,
photocatalysis, as one of the most efficient means to de-
grade various organic contaminants, has aroused much at-
tention and is now in widespread use.[2,3] As the most com-
mon semiconductor photocatalyst, TiO2 has been widely
used and extensively studied since its first discovery by Fuji-
shima and Honda in 1972 owing to its excellent chemical
stability, low cost, and nontoxicity.[4–8] However, TiO2 can
only be excited by UV light because of its large band gap
(3.2 eV), which greatly limits its practical applications.
Hence, various methods have been developed to extend its
harvesting region, and these methods include doping with
ions (e.g., N, C, etc.),[9,10] surface deposition of noble met-
allic nanoparticles,[11] photosensitization with organic dyes,
and so on.[12]
[a] Laboratory of Clean Energy Chemistry and Materials, Lanzhou
Institute of Chemical Physics, Chinese Academy of sciences,
Lanzhou 730000, P. R. China
E-mail: yongliangcheng@licp.cas.cn
www.licp.cas.cn
[b] School of Stomatology, Lanzhou University,
Lanzhou 730000, P. R. China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201403238.
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the photocatalytic activity of graphene/TiO2 nanowires is and then the fibers were modified with 3-aminopropyltri-
higher than that of graphene/TiO2 nanoparticles and pure methoxysilane (APS) to produce a positive charge on the
nanowires or nanoparticles.[32] Dai et al. reported that re- surfaces of the fibers. In our system, if the mass ratio of
duced graphene oxide (rGO) wrapping can improve the graphene oxide (GO) to APS-modified TiO2 was 1:10, the
electron-transfer efficiency of TiO2 nanofibers through the average ζ potential of the GO sheets and the APS-modified
electron-transfer bridges in rGO and TiO2.[37] Therefore, TiO2 nanofibers were –41.3 and 36 mV, respectively (see
fabrication of a novel one-dimensional graphene/TiO2 com- Figure S1, Supporting Information). Therefore, GO-
posite to improve the photocatalytic performance of TiO2 wrapped TiO2 nanofibers were easily obtained through elec-
is very necessary.
trostatic interaction between the negatively charged GO
Notably, surface deposition of noble metallic nanopar- sheets and the positively charged TiO2 nanofibers.[43,44] Af-
ticles is one of the most effective ways to enhance the pho- ter thermal reduction to remove oxygen-containing func-
tocatalytic activity of TiO2. As we know, Ag is one of the tional groups, TG composite nanofibers were obtained. Fi-
most widely used deposited metals owing to its low cost nally, the TGA nanocomposite fibers were achieved
and the fact that it is nontoxic relative to other noble met- through photochemical reduction of Ag+ to Ag nanopar-
als.[38] Most importantly, the surface plasmon resonance ticles.
(SPR) effect of Ag allows it to serve as a visible-light sensi-
tizer agent to extend the absorption spectrum to the visible
region and to effectively restrict the recombination of
photogenerated electron–hole pairs.[10,39–41] On this basis,
depositing Ag nanoparticles on the surface of graphene/
TiO2 could further enhance the photocatalytic activity of
the nanocomposite. Therefore, to simultaneously achieve a
large light harvest region and to facilitate separation of elec-
tron–hole pairs, the design and synthesis of a novel hybrid
structure composed of TiO2, graphene, and Ag is of great
importance.
In this paper, we report the controlled fabrication of a
novel one-dimensional TiO2/rGO/Ag nanocomposite pho-
tocatalyst with UV/Vis response properties by wrapping
rGO sheets around TiO2 nanofibers and then loading Ag
nanoparticles onto the surfaces of TiO2/rGO. In this com-
posite photocatalyst, the one-dimensional TiO2 nanofibers
in the core increase the rate of electron transfer. Following
thermal reduction, the rGO-wrapped TiO2 nanofibers (as-
sembled from strong electrostatic interactions between the
Figure 1. Schematic diagram of the synthetic route to the TGA
nanocomposite.
negatively charged GO sheets and the positively charged
The morphologies of the different samples were charac-
TiO2 nanofibers) effectively restrict the recombination of terized by scanning electron microscopy (SEM). As shown
photogenerated carriers, and the Ag nanoparticles on the in Figure 2 (a), the TiO2 nanofibers are randomly distrib-
outside of the nanocomposite extend the absorption spec- uted, and the lengths of the nanofibers reach several micro-
trum range and restrict recombination of the photo- meters. From the inset of Figure 2 (a), it is clear that all
generated electron–hole pairs. As a consequence, highly ef- fibers are about 50–200 nm in diameter with an average dia-
ficient degradation of rhodamine B (RhB) was achieved in meter about 125 nm, and they are separated from each
the presence of the prepared TiO2/rGO/Ag (TGA) nano- other. Figure 2 (b) clearly shows that the rGO sheets are
composite relative to that achieved with TiO2 nanofibers relatively uniformly wrapped around the surfaces of the
and TiO2/rGO (TG) and TiO2/Ag (TA) nanocomposites. TiO2 nanofibers. If the addition of rGO was relatively low,
The photocatalytic activity was evidently enhanced by using rGO was completely wrapped around the surfaces of the
the TiO2/rGO/Ag nanocomposite photocatalyst under UV/ TiO2 nanofibers. However, upon increasing the amount of
Vis irradiation in the decomposition of an aqueous solution GO, some GO sheets were not completely wrapped around
of RhB, and at the same time, TiO2/rGO/Ag-2 showed the the TiO2 nanofibers (Figure 2, e). Especially, at high con-
best photocatalytic activity of all samples.
centrations of GO, some rGO sheets were easily observed
by SEM and several TiO2 nanofibers are loaded on one
rGO sheet (Figure 2, e). This phenomenon can be attrib-
uted to the fact that positive charges on the surfaces of the
nanofibers could not effectively interact with the negative
charges of the GO nanosheets at high GO concentrations.
Results and Discussion
Characterization of the as-Prepared Samples
The typical process used to prepare the TGA nano- After photoreduction, Ag nanoparticles with diameters in
composite is presented in Figure 1. First, TiO2 nanofibers the range of 10 to 50 nm are observed in the SEM images
were fabricated by annealing electrospun precursor fibers, (Figure 2, c–f).
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wrapped around the TiO2 nanofibers. Lattice fringes of 0.35
and 0.235 nm can be attributed to the anatase (101) plane
of TiO2 and the d-spacing value of the Ag (111) plane,
respectively, as revealed in Figure 3 (b).[10,45] Further, the
energy-dispersive X-ray spectroscopy (EDS) mapping
clearly exhibits that rGO sheets exist around the surfaces of
the TiO2 nanofibers and that Ag nanoparticles are ran-
domly deposited over the whole TiO2/rGO/Ag composite
(Figure 3, c–f).
X-ray diffraction (XRD) was used to characterize the
crystallite structure of the samples. As shown in Figure 4,
six diffraction peaks in all samples located at 2θ = 25.4,
37.9, 48.0, 53.8, 54.9, and 62.8° were assigned to the (101),
(004), (200), (105), (211), and (112) planes of anatase TiO2,
respectively, and this is consistent with anatase TiO2
(JCPDS, No. 21-1272). Notably, no diffraction peaks corre-
sponding to rGO or Ag were observed, owing to the low
amount of rGO and the small size of the Ag nanopar-
ticles.[17,46,47] The existence of Ag nanoparticles was evi-
denced by TEM and was also could further evidenced by
X-ray photoelectron spectroscopy (XPS) measurements.
Figure 2. SEM images of (a) TiO2 nanofibers (inset: corresponding
magnified SEM image), (b) TG-2, and (c–f) TGA-X (inset of d is
the particle size of the Ag nanoparticles).
High-resolution transmission electron microscopy
(HRTEM) was used to further investigate the structure of
the TG and TGA composite nanofibers. As shown in Fig-
ure 3 (a), TiO2 nanofibers are uniformly covered by a thin
sheet of rGO, which indicates that rGO was successfully
Figure 4. XRD patterns of TiO2, TG-X, and TGA-X.
The surface compositions and elemental chemical states
of the TGA composite were characterized by using XPS.
As shown in Figure 5 (a), the full-scale XPS spectrum dem-
onstrates that Ti, O, C, and Ag elements exist in the TGA
sample. Figure 5 (b) displays the high-resolution XPS spec-
trum of the Ag3d doublet, for which the peaks located at
binding energies of 368.2 and 374.2 eV with a splitting of
6.0 eV correspond to Ag3d5/2 and Ag3d3/2, respectively; this
suggests that Ag+ was successfully reduced to Ag0. How-
ever, these values are higher than the binding energies of
Ag0 metal (367.9 and 373.9 eV for Ag3d5/2 and Ag3d3/2
,
respectively),[48,49] and this may be the result of electron
transfer from silver to graphene in the nanocomposite, be-
cause the work function of rGO (Ͼ4.8 eV) is higher than
that of Ag0 (4.2 eV). The high-resolution spectrum of Ti2p
exhibits two broad peaks centered at binding energies of
464.9 and 459.2 eV, which represent Ti2p1/2 and Ti2p3/2
,
respectively, in accordance with the binding energies of Ti4+
in pure anatase (Figure 5, c).[32] The high-resolution spec-
trum of rGO can be divided into four different peaks at
binding energies of 284.7, 285.5, 287.1, and 288.8 eV, which
correspond to graphitic sp2 carbon atoms (C–C) and oxy-
gen-bound C–O, C=O, and O–C=O species, respectively
Figure 3. (a) HRTEM image of TG-2 wrapped with rGO sheets,
(b) HRTEM image of TGA-2, and EDS mapping of a single TGA-
2 composite nanofiber: (c) titanium, (d) oxygen, (e) carbon, and
(f) silver.
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(Figure 5, d). The weaker peaks related to oxygenated to electron transfer between TiO2 and rGO, which suggests
groups indicate that a large proportion of the oxygenated that most of the rGO sheets were successfully reduced by
groups was removed.
thermal treatment.
Figure 6. (a) Raman spectra of TiO2, TGO-2, TG-2, and TGA-2
(the Raman spectra of TGO-2,TG-2, and TGA-2 from 800 to
1800 cm–1 are shown in the inset); (b) Raman spectra of TGA-1,
TGA-2, TGA-3, and TGA-4.
The optical properties of all the samples were investi-
gated by UV/Vis diffuse reflectance spectroscopy, as shown
in Figure 7 (a). All samples display similar light absorption
in the UV region owing to electron migration from the val-
ence band of TiO2 to the conduction band. It is clearly seen
that the absorption spectrum is efficiency extended from the
UV region to the visible region after introducing rGO and
that the absorption intensity clearly increases with an in-
crease in the amount of rGO added. These results are sim-
ilar to those reported in the literature.[33] An additional
band appears at around λ = 510 nm in the visible region of
the TGA sample and is due to the surface plasmon reso-
nance effect of the Ag nanoparticles.[39,40] Photolumines-
cence measurements were further undertaken to determine
the charge recombination behavior of the different samples.
As we known, photoluminescence emission intensity is re-
lated to the rate of recombination of the photogenerated
electron–hole pairs. High emission intensity corresponds to
a faster recombination rate, and low emission intensity im-
plies that more photogenerated electrons can be transfer-
red.[53] As shown in Figure 7 (b), the emission intensity of
TiO2 displays a clear decrease after modification by rGO
and the Ag nanoparticles, which suggests that both rGO
and Ag can effectivity improve the transformation rate of
the electrons to suppress recombination of electron–hole
pairs.[38]
Figure 5. (a) Full-scale XPS spectrum and high-resolution XPS
spectra of the (b) Ag3d, (c) Ti2p, and (d) C1s regions of the TGA-
2 sample.
Raman spectroscopy was used to further confirm the
structures of the prepared samples. Four typical peaks at
147 (Eg), 398 (B1g), 514 (A1g + B1g), and 638 cm–1 (Eg) are
ascribed to anatase TiO2 (Figure 6, a).[37,50,51] The Raman
spectra of the TiO2/GO-2 (TGO-2), TG-2, and TGA-2 sam-
ples are shown in the inset of Figure 6 (a). There are two
prominent signals corresponding to the D band and
G band of graphene in the range of 800 to 1800 cm–1. The
positions of the D and G bands show some shift in the
composite and are located at 1346 or 1351 cm–1 for the
D band and at 1593 or 1596 cm–1 for the G band. The ratio
of the intensities of these bands (ID/IG) offers information
about the degree of crystallinity of the graphene sheets. An
increased ID/IG ratio indicates an increased amount of de-
fects in graphite comprising sp2 hybridization.[52] The ID/IG
intensity ratio of the TG-2 sample shows a larger increase
after thermal treatment than TGO-2, and consequently, it
has fewer defects. Furthermore, the intensities of all signals
of TGA-2 are clearly enhanced relative to the intensities of
the signals of TG, which can be attributed to the surface-
enhanced Raman scattering effect of the Ag nanopar-
ticles.[38] This further demonstrates that Ag nanoparticles
were successfully reduced and deposited on the surfaces of
the rGO sheets. Figure 6 (b) presents the Raman spectra of
TGA; two main signals exist in all samples that represent
the D band and G band. Upon increasing the amount of
rGO added, there is a clear enhancement in the intensities
of the D band and G band among the samples. Moreover,
the Raman signals of the D band and G band are slightly
Figure 7. (a) UV/Vis diffuse reflectance spectra of the samples;
blueshifted with the increase of the amount of rGO owing (b) photoluminescence spectra of TiO2, TA, TG, and TGA.
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Photocatalytic Activity
fore, rGO not only acts as an electron acceptor but also as
an electron-transport pathway, which results in the inhibi-
tion of the recombination of the photogenerated electron–
hole pairs and an enhancement in the photocatalytic degra-
dation rate of RhB. On the other hand, under UV-light irra-
diation (Figure 9, a), the photogenerated electrons pro-
duced from the TiO2 nanofibers migrate from the conduc-
tion band of TiO2 to the Ag nanoparticles through rGO.[17]
In this system, the Ag nanoparticles act as an electron tank
and inhibit the recombination of the photogenerated elec-
tron–holes by transporting and capturing the electrons.[17,56]
Simultaneously, once the transferred electrons contact
active oxygen molecules on the surface of graphene and the
Ag nanoparticles, the electrons react to produce superoxide
anion radicals (O2·–), and the photogenerated holes react
with water (H2O) to produce hydroxyl radicals (·OH). Fi-
nally, the RhB molecules are degraded into CO2, H2O, and
other intermediates by the superoxide anion radicals and
the hydroxyl radicals.[57] Under visible-light irradiation,
TiO2 cannot be excited owing to its wide band gap; there-
fore, only Ag nanoparticles are excited and hot electrons
are generated through local surface plasmon resonance, and
then the hot electrons are transferred to rGO or injected
into the empty conduction band of TiO2 through the rGO
sheets[17,58] (Figure 9, b). As the photocatalytic experiment
progresses, some of the Ag nanoparticles can be oxidized
to form Ag2O. As a kind of P-type semiconductor with a
direct band gap of 1.46 eV, Ag2O can be excited under vis-
ible light, Therefore, Ag2O can couple with TiO2 to extend
the absorption of the solar spectrum. Although Ag2O alone
is unstable under visible light, Ag/Ag2O can form stable
photocatalysts.[59] This phenomenon is very similar to that
of silver halides, which can easily form a self-stabilized pho-
tocatalyst with Ag.[60] With the formation of Ag2O, type II
p–n nano-heterojunctions are formed at the interface and
electron transfer occurs from Ag2O to TiO2 through Ag
and rGO until their Fermi levels align, and the recombina-
tion of electron–hole pairs is then reduced (Figure 9, c).
Moreover, hot electrons generated by Ag owing to its strong
absorption by surface plasmon resonance in the visible re-
gion are also injected into the conduction band of TiO2
through rGO. The electrons on the surface of TiO2 or the
rGO sheets can react with active oxygen molecules to form
The photocatalytic activity of the TGA nanocomposites
was evaluated by photocatalytic degradation of a solution
of RhB under visible-light irradiation at room temperature.
For comparison, TiO2 nanofibers and TA and TG compos-
ite nanofibers were also fabricated as reference. As shown
in Figure 8 (a), TGA-2, TG-2, and TA display better photo-
catalytic activity than the pure TiO2 nanofibers. The TGA-
2 sample exhibits the best photocatalytic activity, and RhB
was completely degraded over a 50 min irradiation period.
In contrast, about 92, 91, and 44% of RhB was decolorized
by the TG-2, TA, and TiO2 samples, respectively. This indi-
cates that the introduction of rGO sheets and Ag nanopar-
ticles increases the photocatalytic activity of TiO2.[17] From
Figure 8 (b) it can be seen that the amount of rGO has a
clear influence on the photocatalytic activity of the compos-
ite photocatalyst. With an increase in the amount of rGO
added, the photocatalytic activity increased first and then
gradually decreased. This phenomenon is attributed to the
shielding effect of graphene. An increase in the amount of
rGO could effectively limit the recombination rate of the
photogenerated electron–hole pairs to improve the photo-
catalytic activity of the nanocomposite. However, the active
sites on the TiO2 surface would be shielded by rGO with
an increase in the amount of rGO added, and this would
result in a decrease in the intensity of light by the depth
of the reactants, which is called the “shielding effect”.[54,55]
Moreover, the TGA composite shows no marked decrease
after three successive cycles (Figure S2a). Although some
Ag was oxidized to Ag2O (Figure S2c), no clear structure
change in TGA was observed (Figure S2b) after running
through three cycles, which suggests that it has good cycling
stability.
·–
O2 radicals, which are highly reactive species that are re-
sponsible for the degradation of pollutants.[58] In addition,
Figure 8. Photocatalytic degradation of RhB with (a) TiO2, TA,
TG-2, and TGA-2 and (b) TG-X and TGA-X; comparison of RhB RhB itself can also absorb the visible light in the range of
degradation reaction rate kinetic constants of (c) TiO2, TA, TG-2,
and TGA-2 and (d) TG-X and TGA-X.
λ = 420–600 nm resulting from the ground state and the
excited state of RhB (RhB*). Upon irradiation with visible
light, the intramolecular π–π* transition causes the transfer
On the basis of the characterization and experimental of the photogenerated electron to the excited state. Then,
results, the excellent photodegradation performance of the photogenerated electron is immediately injected into the
TGA can be explained as follows. On the one hand, once conduction band of TiO2 through rGO and is captured by
·–
the RhB molecules come into contact with rGO in solution, O2 to form O2 radicals to degrade pollutants.[61,62] To
the RhB molecules stick to the surfaces of the rGO sheets eliminate this influence, methyl orange was also employed
through strong π–π interactions.[40] This increases the de- to further investigate the photocatalytic activity of the dif-
gradation rate of RhB. Furthermore, graphene can greatly ferent samples. These experiments also demonstrated that
enhance the transfer and separation of photogenerated elec- the introduction of rGO and Ag clearly enhances the pho-
trons owing to its excellent charge-carrier mobility. There- tocatalytic activity of TiO2 (Figure S3); the lower photo-
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To prepare rGO wrapped-TiO2 nanofibers (denoted TG-X), the as-
prepared TiO2 nanofibers (100 mg) were dispersed in ethanol
(150 mL) containing (3-aminopropyl)trimethoxysilane (APS, 10 g;
AR, Shanghai Kaifeng chemical reagent, Ltd.). The mixture was
heated at reflux at 80 °C in an oil bath for 8 h. After filtration and
drying, the APS-modified TiO2 nanofibers were dispersed in water
(300 mL), and then the GO content in the TiO2 nanofibers was
adjusted to 5, 10, 15, and 20 wt.-% (samples denoted TG-1, TG-2,
TG-3, and TG-4, respectively). Afterwards, the solution was soni-
cated for 2 h to obtain the TGO composite. Finally, the TG com-
posite was obtained by annealing TGO at 500 °C for 1 h under a
mixture of H2 and Ar (5% of Ar).
catalytic activity for methyl orange degradation relative to
that for RhB degradation can be attributed to the absence
of the self-sensitization of RhB.
The TiO2/rGO/Ag nanocomposite (denoted TGA-X; TGA repre-
sents generic terms of TiO2/rGO/Ag with different rGO amounts,
and X represents different amounts of rGO) was prepared by dis-
persing the TiO2/rGO nanocomposite into an aqueous solution of
AgNO3 (the Ag content was 5 wt.-%), and then the mixture was
stirred and photoreduced by a 300 W Xe lamp at room temperature
for 5 h. The resulting samples were filtered, washed, and dried at
60 °C. For comparison, TiO2/Ag nanocomposite nanofibers (de-
noted TA) were also prepared under the same conditions.
Figure 9. Illustration of the photocatalytic mechanism of TGA:
(a) UV part and (b,c) visible part.
Characterization: The morphologies and microstructures of sam-
ples were analyzed by scanning electron microscopy (SEM, JSM
6701F) and transmission electron microscopy (TEM, Tecnai G20).
The structures of the as-synthesized samples were characterized by
powder X-ray diffraction (XRD, Cu-Kα radiation, Panalytical
X’Pert Pro). X-ray photoelectron spectroscopy (XPS) measure-
ments were performed with a VG Microtech Multilab electron
spectrometer by using a Mg-Kα (1253.6 eV) radiation source. Ra-
man spectrum was recorded by using micro-Raman spectroscopy
(JY-HR800, excitation at λ = 532 nm). UV/Vis diffuse reflectance
spectra of the samples were obtained with an UV/Vis spectropho-
tometer (U-3010) by using BaSO4 as the reference. Photolumines-
cence spectra were recorded by using an F-4500 spectrophotometer
at λ = 325 nm.
Conclusions
In summary, novel one-dimensional TiO2/rGO/Ag nano-
composite photocatalysts with different reduced graphene
oxide (rGO) contents were successfully fabricated by wrap-
ping rGO sheets around TiO2 nanofibers and then loading
Ag onto the TiO2/rGO nanocomposite fibers. The introduc-
tion of rGO and Ag significantly enhanced the photocata-
lytic degradation rate of rhodamine B relative to that of
TiO2 alone. This was attributed to hybridization of rGO
possessing specific electronic effects with Ag nanoparticles
showing a surface plasmon resonance effect, which thus re-
sulted in the extension of the absorption spectrum to the
visible region and restricted the recombination of photo-
generated carriers. At the same time, the rGO content also
played an important role in the photocatalytic properties.
Photocatalytic Experiments: Photocatalytic experiments were per-
formed in a homemade reactor, which was surrounded with a cool-
ing system. A 300 W Xe-illuminator was used as the light source.
The photocatalyst (0.1 g; TiO2, TA, TG-X, TGA-X) was suspended
in a solution of RhB or methyl orange (10 mgL–1, 100 mL). The
suspension was sonicated for at least 30 min before use. After that,
the solution was magnetically stirred in the dark for 30 min to ob-
tain equilibrium adsorption on the catalyst surface, and it was then
exposed to light. During the irradiation, an aliquot (5 mL) of the
mixture was withdrawn, and the catalyst was separated by centrifu-
gation. Photocatalytic activity was evaluated by monitoring the de-
colorization of RhB at the maximum absorption around λ =
553 nm with a UV/Vis spectrophotometer.
Experimental Section
Preparation of TiO2/rGO/Ag Nanocomposite Photocatalysts:
Graphene oxide (GO) sheets were prepared by a modified Hum-
mer’s method.[42] A homogeneous GO aqueous suspension with a
concentration of 1.8 mgmL–1 was formed by dispersing certain GO
powders in water.
Acknowledgments
TiO2 nanofibers were prepared by a simple electrospinning method
followed by calcination under an air atmosphere. In a typical pro-
cedure, Ti(OBu)4 (2.00 g; AR, Tianjin Chemical Reagent Co., Ltd.)
and polyvinyl pyrrolidone (PVP, Mw = 1300000, 0.45 g; Aladdin
reagent Co., Ltd.) were added to solvent (10 mL) containing abso-
lute ethanol (AR, Sinopharm Chemicals) and acetic acid (AR, Ti-
anjin Chemical Reagent Co., Ltd.). The mixture was vigorously
stirred for 4 h and then ejected from a stainless steel capillary with
a voltage of 15 kV. The distance between the collector and the cap-
illary was 15 cm. Then, the collected precursor film was calcined
in air from room temperature up to 500 °C at a rate of 2 °Cmin–1,
followed by holding at this temperature for 4 h.
The authors acknowledge financial support by the Chinese Acad-
emy of Sciences (Top Hundred Talents Program) and by the
National Nature Science Foundation of China (grant number
21201171).
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Received: December 29, 2014
Published Online: ᭿
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Date: 30-03-15 12:39:22
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Nanocomposite Photocatalysts
X. Gou, Y. Cheng,* B. Liu, B. Yang,*
X. Yan ............................................... 1–8
TiO2·rGO/Ag composite nanofibers are
prepared by wrapping reduced graphene
oxide (rGO) sheets around TiO2 nanofibers
and then loading Ag nanoparticles onto
the surfaces of TiO2/rGO. The introduction
of rGO and Ag creates a marked improve-
ment in the photocatalytic activity of the
TiO2 nanofibers.
Fabrication and Photocatalytic Properties
of TiO2/Reduced Graphene Oxide/Ag
Nanocomposites with UV/Vis Response
Keywords: Photocatalysis / Nanoparticles /
Nanocomposites / Silver / Graphene oxide
Eur. J. Inorg. Chem. 0000, 0–0
8
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim