Synthesis of 3D flower-like structured Gd/TiO2?rGO nanocomposites: Via a hydrothermal method with enhanced visible-light photocatalytic activity

Article abstract of DOI:10.1039/c9ra06045f

In this study, novel Gd/TiO₂?rGO (GTR) nanocomposites with high photocatalytic performance were fabricated via a one-pot solvothermal approach. During the preparation step, graphene oxide (GO) was reduced to reduced graphene oxide (rGO), and subsequently, on the surfaces of which anatase TiO₂ doped with Gd metal was grown in situ with a 3D petal-like structure. Gd doping into the classical TiO₂?rGO system efficiently expands the absorption range of light, improves the separation of photogenerated electrons, and increases the photocatalytic reaction sites. The specific surface areas, morphological structures, and valence and conduction bands of the obtained GTR nanocomposites were analyzed and correlated with their enhanced photocatalytic performances for the degradation of an aqueous RhB solution. The experimental results indicated that the best performance was achieved with the 3% GTR composite, which exhibited the highest photoelectrocatalytic activity because of two aspects: the rapid separation of electrons and holes, and improvement in adsorption capacity. As compared with pure TiO₂, the GTR composites demonstrated enhanced photoactivity due to synergetic effects between the effective photo-induced electron transfer from TiO₂ to the surface of the rGO acceptor through interfacial interactions and the variation of structure and electrons under the adoption of Gd.

Full text of DOI:10.1039/c9ra06045f

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Synthesis of 3D flower-like structured Gd/
TiO2@rGO nanocomposites via a hydrothermal
method with enhanced visible-light photocatalytic
activity
Cite this: RSC Adv., 2019, 9, 31177
a
ab
ab
Lijuan Du,a Yulin Zhang,a
*
*
Shuaiqiang Jia,
Jinlong Li,
Guozhe Sui,
Yan Zhuanga and Boxin Lia
In this study, novel Gd/TiO2@rGO (GTR) nanocomposites with high photocatalytic performance were
fabricated via a one-pot solvothermal approach. During the preparation step, graphene oxide (GO) was
reduced to reduced graphene oxide (rGO), and subsequently, on the surfaces of which anatase TiO2
doped with Gd metal was grown in situ with a 3D petal-like structure. Gd doping into the classical
TiO2@rGO system efficiently expands the absorption range of light, improves the separation of
photogenerated electrons, and increases the photocatalytic reaction sites. The specific surface areas,
morphological structures, and valence and conduction bands of the obtained GTR nanocomposites
were analyzed and correlated with their enhanced photocatalytic performances for the degradation of
an aqueous RhB solution. The experimental results indicated that the best performance was achieved
with the 3% GTR composite, which exhibited the highest photoelectrocatalytic activity because of two
aspects: the rapid separation of electrons and holes, and improvement in adsorption capacity. As
compared with pure TiO2, the GTR composites demonstrated enhanced photoactivity due to synergetic
effects between the effective photo-induced electron transfer from TiO2 to the surface of the rGO
acceptor through interfacial interactions and the variation of structure and electrons under the adoption
of Gd.
Received 4th August 2019
Accepted 23rd September 2019
DOI: 10.1039/c9ra06045f
rsc.li/rsc-advances
carbon nitride (g-C3N4),14–17 and graphene,18–21 which have high
carrier mobilities and large specic surface areas; this strategy
1. Introduction
is considered as an effective way to further strengthen the
photocatalytic activities of TiO2 catalysts.
In recent years, photocatalysis technology using semi-
conductors has drawn signicant attention from researchers
because it is one of the most effective ones for the decomposi-
tion of pollutants present in aqueous media.1,2 Among various
semiconductors, TiO2 has been considered one of the most
representative photocatalysts owing to its strong oxidizing
power,3 photostability,4 and relative non-toxicity.5 Nevertheless,
anatase TiO2 possesses a wide band gap energy (ꢀ3.2 eV) that
limits its utilization of natural sunlight and photoactivity under
UV light (<387 nm) due to fast electron–hole recombination. UV
radiation comprises around 3 to 5% of the sunlight at Earth's
surface.6,7 Thus, researchers are focusing on extending the
absorption spectrum of TiO2 from UV to the visible-light region
to exploit the visible light occupying 42–43% of sunlight.8,9
Recently, researchers have modied TiO2 with carbon-
containing materials such as carbon nanotubes,10–13 graphitic
Compared with other carbon materials, graphene is a crys-
talline allotrope of carbon with a two-dimensional structure,22,23
and is both the thinnest and the strongest material measured so
far in the world.24,25 Graphene can be used in many applications
such as new energy materials, energy conversion, and tissue
engineering. Owing to its high electron mobility,26 lower resis-
tivity,27 extended p–p conjugation structure,28 and faster elec-
tron transfer ratio,29,30 graphene has attracted increasing
attention recently. Because of this, graphene can be easily
combined with nanostructured materials to form some
semiconductor-supported compounds.31 Therefore, several
methods have been used to combine TiO2 and graphene to
improve the photodegradation efficiency of TiO2, including
hydrothermal, physical mixing, sol–gel, and electrochemical
deposition.32 A large number of TiO2/graphene composites with
different structure have been designed and prepared to improve
the photocatalytic activity of TiO2; the combination of graphene
with TiO2 not only reduces the electron–hole recombination by
transferring the photoexcited electron to the graphene surface,
aCollege of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar 161006,
China. E-mail: jinlong141@163.com; suiguozhe@163.com; Fax: +86-452-2738205;
Tel: +86-452-2738205
bHeilongjiang Provincial Key Laboratory of Catalytic Synthesis for Fine Chemicals,
Qiqihar 161006, China
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but also enlarges the absorption band of pure TiO2 to the visible were poured into a round-bottom ask and 46 mL of concen-
region and enhances the surface area of TiO2.33–35
trated H2SO4 was slowly added to it. The mixture was kept
Additionally, to enhance the photocatalytic activity of TiO2, stirring for 1 h with magnetic stirrers. Subsequently, 6.0 g
many researchers have focused on metal-loaded TiO2 materials KMnO4 was slowly added to the mixture and stirred for 2 h for
to expand to visible-light-response region via the induction of uniform stir. During the above synthetic processing, an ice bath
impurity levels. Liu et al.36 synthesized a small particle size of apparatus was used to prevent temperature from exceeding
Ga–TiO2 composite powders with high specic surface area by 10 ꢁC. Thereaer, the reaction temperature was adjusted to
a mild hydrothermal method; the synthesized photocatalyst 35 ꢁC and continuously stirred for another 0.5 h. Then, 160 mL
exhibits better photocatalytic activity than that of commercial of distilled water ꢁwas carefully added and the resulting mixture
P25 TiO2 or undoped TiO2. Chen et al.37 investigated the higher was stirred at 98 C for 0.5 h. During the reaction, the color of
photoactivity of a TiO2/diatomite with co-doped Ce/N granule the mixture changed from snuff color to bright yellow. Aer-
than that of a single N-doped granule through the sol–gel way by ward, 30 mL H2O2 was slowly fed into the mixture with stirring
evaluating the degradation efficiency of oxytetracycline under for 0.5 h, during which plenty of bubbles evolved. The resulting
visible-light illumination. Muhammad et al.38 prepared Gd/ bright yellow mixture was centrifuged, and the precipitate was
SnO2–TiO2 nanoparticles by using ultrasonic and hydrothermal washed with a 5% HCl aqueous solution until no metal ions
methods, and investigated the effect of Gd concentration on remained and then washed thoroughly with deionized water to
band gap, which revealed a red shi from 5.3 to 2.0 eV for Gd/ neutral pH. The obtained GO mud was dried in a vacuum oven
ꢁ
SnO2–TiO2. All these methods provide notable improvements in at 60 C.
the photocatalytic property of TiO2. However, there is no report
on the investigation of Gd-loaded TiO2 on rGO (Gd/TiO2@rGO;
GTR) photocatalysts.
2.3 Synthesis of Gd/TiO2@rGO
GTR was synthesized through an efficient one-step sol-
vothermal route. A solution of GO nanosheets was obtained
aer that 0.1 g of GO was dissolved in 30 mL of isopropyl
alcohol by sonication for 30 min. Then, the mixture added by
1 mL of TBT was sonicated for another 30 min to form
a homogeneous solution. Next, 0.5 mL of HF (40 wt%) was
dropped into the solution under continuous stirring. Mean-
while, 1.8 mL of a Gd(NO3)3 aqueous solution (0.5 mol Lꢂ1) was
dispersed into the above solution and magnetically stirred for
20 min to ensure uniform distribution. Finally, the mixed
solution was put into a Teon-lined stainless steel autoclave
In this work, we selected graphene as the support material to
enhance the synergistic effect between TiO2 and Gd, and
synthesized GTR photocatalysts via a convenient and fast one-
pot method for synthesizing photocatalytic materials. Gra-
phene was used not only to improve the adsorbability of organic
contaminants, but also to induce a positive relationship
between Gd and Ti species. In order to investigate the photo-
catalytic activities of the as-synthesized GTR composites, the
photodegradation of Rhodamine B (RhB) was performed in
aqueous solution. And the reusability of GTR composites was
also examined in this work.
ꢁ
and then treated at 180 C for 10 h. At the end of this process,
the obtained product from the autoclave was washed with
deionized water and absolute ethanol several times, and dried
in a vacuum oven at 60 ꢁC for 24 h. A series of GTR composites
was synthesized by varying the volumes of Gd(NO3)3 and TiO2.
For comparison, TiO2@rGO nanosheets were prepared without
Gd by the above mentioned solvothermal method.
2. Experimental
2.1 Reagents
Graphite powder (purity > 99.9%), sulfuric acid (H2SO4,
98.0 wt%), potassium permanganate (KMnO4), hydrogen
peroxide (H2O2, 30.0 wt%), sodium nitrate (NaNO3), hydro-
chloric acid (HCl, 36.5 wt%), hydrouoric acid (HF, 40 wt%),
and absolute ethanol (CH3CH2OH) were analytical-grade
reagents. Tetrabutyl titanate (TBT, C16H36O4Ti, 98.0%) was
used as a titanium precursor to synthesize the TiO2 nano-
crystals. Rhodamine B (RhB, C28H31N2O3Cl) was selected to
evaluate the photocatalytic degradation efficiency of dyestuff.
All the above chemicals were obtained from Tianjin Kemiou
Chemical Reagent Co. (Tianjin, China). Gadolinium(III) nitrate
hexahydrate (GdN3O9$6H2O, 99.0%) was purchased form
Aladdin Chemistry Co. Ltd. The used deionized (DI) water in all
experiments was generated from a UPE-60 water purication
system (Beijing Sanda Technology Co., Ltd). All reagents were
used without further purication.
2.4 Characterization
Scanning electron microscopy (SEM) images were taken with
a Hitachi S-3400 system (Tokyo, Japan) operated at 100 kV, and
transmission electron microscopy (TEM) images were taken
with a Hitachi system (Tokyo, Japan) operated at 200 kV. X-ray
diffraction (XRD) analysis was performed on a Bruker D8
system (Germany) to determine the crystal structure and the
phase compositions of the samples equipped with Cu Ka radi-
ation under 40 kV and 30 mA. And the patterns were recorded in
the range of 2q ¼ 10–80ꢁ with a speed of 4.00ꢁ minꢂ1. UV-visible
diffused reectance spectra were obtained for the dry-pressed
disk samples using a UV-visible spectrophotometer (Du800,
Beckman Co.) with BaSO4 as a reectance standard. The Bru-
nauer–Emmett–Teller (BET) analysis were carried out to deter-
mine the specic surface areas of the samples using a N2
2.2 Synthesis of GO
GO was prepared through the modied Hummers' method. In adsorption–desorption apparatus (Nova 2000e, Quantachrome
a typical synthesis, 2 g of graphite powder and 1.0 g of NaNO3 Co.) with the relative pressure ranging from 0.05–0.35, and the
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samples were previously degassed at 300 ꢁC for 3 h under
vacuum. The Barrett–Joyner–Halenda (BJH) approach was used
to determine the pore size distribution of the photocatalysts.
Fourier transform infrared (FT-IR) spectra were measured to
determine the functional groups with a Nexus 670 (Nicolet Co.)
FT-IR spectrophotometer using the KBr pellet technique. X-ray
photoelectron spectroscopy (XPS) was performed on an Axis
Ultra DLD (Krotos Co., UK). The photoelectron take-off angle
was kept at 45ꢁ, and the binding energy was carefully referring
to the C 1s peak by setting at 285.0 eV.
2.5 Evaluation of photocatalytic activity
RhB as one of the most important representative organic
dyestuff has been well applied in industrial production, which
serioursly contaminates the surrounding environment. Hence,
we chose it as a representative pollutant to evaluate the photo-
catalytic activities of the GTR composites by photo-degradation
of an aqueous solution of RhB dye under visible-light illumi-
nation. A 500 W Xe lamp was employed as the light source, and
Fig. 1 TEM images of (a) GO nanosheets and (b) 3% GTR sample. (c)
and (d) SEM images of 3% GTR composite.
the visible wavelength was adjusted using a UV-cut lter (420 surface of rGO sheets. This implies that TiO2 nanoparticles were
nm). The experiments were performed with 50 mg of the porous successfully deposited onto the GO surface by the hydrothermal
TiO2 photocatalyst and a solution of RhB (50 mL; 10 mg Lꢂ1) in method, resulting in the formation of large GTR composites.
a tube reactor. The suspension of the RhB solution and pho- The diameters of the TiO2 nanoparticles deposited onto the GO
tocatalyst was magnetically stirred in the dark for 60 min to surface ranged from 150 to 250 nm. All the precursors exhibited
ensure establish an adsorption/desorption equilibrium before very similar and uniform spherical morphologies. Fig. 1(d)
irradiation. Then, visible light was directed towards the shows that the morphology of TiO2 nanoparticles is petaloid
suspension. Aliquots of samples were withdrawn from the ambulacra, and many TiO2 nanosheets assemble into a 3D
suspension at regular time intervals (30 min) and centrifuged to petalage-like structure.
separate the catalyst powder using a high-speed rotation of
The elemental maps of C, Ti, and Gd in the 3% GTR sample
centrifuge rotor (RG-TGL-16C, Ruijiang Co.). The absorbance of are shown in Fig. 2(b–d), respectively. It is observed that
collected ltrate was measured to calculate the RhB degradation massive C and Ti nanoparticles are dispersed uniformly among
efficiency by UV-vis spectrophotometry (SP 1900, Shanghai the rGO sheets in the composite. Besides, a small amount of Gd
Spectrum Instruments Co.) using the representative peak of nanoparticles can be observed in Fig. 2(d). The small amount of
RhB at 552.0 nm.
2.6 Recycling test
Aer each photocatalytic experiment, the synthesized GTR
composites were recycled by repeated centrifuging and washing
with ethanol. The recycled GTR composites were used as pho-
tocatalysts to degrade RhB (10 mg Lꢂ1) solution under visible-
light illumination, respectively. In this recycling test, the reus-
ability of 3% GTR composite was investigated by repeating the
experiment ve times.
3. Results and discussion
3.1 Characterization
The TEM and SEM images clearly show the morphology,
structure and size of the as-synthesized composites (Fig. 1). The
TEM image of GO shown in Fig. 1(a) clearly illustrates the
typical ake-like shapes of GO with wrinkles, which is attributed
to the thin and large sheets of GO. Here, the 3% GTR sample
was chosen as a representative composite as it demonstrated
the best photocatalytic performance toward RhB photo-
degradation. As seen from the TEM image in Fig. 1(b), well-
dispersed Gd/TiO2 nanoparticles are wrapped tightly onto the the same area of 3% GTR composite shown in (a).
Fig. 2 Elemental maps of (b) C atoms, (c) Ti atoms, and (d) Gd atoms in
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Gd nanoparticles plays an important role in the photocatalytic desorption isotherms of the samples were considered to be type
performance of the GTR, which will be discussed in a later IV curves with a clear H1 hysteresis loop, which reect the
section.
mesoporous structures existing in the synthesized GTR mate-
The XRD proles of GO, pure TiO2, and GTR composites rials as well as facile connectivity between the pores. The high
containing different amounts of Gd are shown in Fig. 3. The adsorption amount in the initial step of the hysteresis loop
peak at 12.6ꢁ in the prole of the GO sample corresponds to the corresponding to the capillary condensation of Gd inside the
(002) crystal plane, indicating that graphite has considerably mesopores reveals a narrow pore-size distribution of the GTR
oxidized into GO. Aer the solvothermal treatment, this strong materials.39 The specic surface area, pore volume, and pore
characteristic diffraction peak of GO almost disappeared but size of different samples were calculated, and the data are
only a weak diffraction peak appeared at 24.0ꢁ, suggesting that summarized in Table 1. For pure TiO2, GO@TiO2, 2% GTR, 3%
most of GO was reduced, which is a clear indication of the GTR, and 4% GTR, the specic surface areas are determined as
conversion of GO to rGO. In the hydrothermal experimental 49.0, 119.6, 122.7, 125.6, and 112.7 m2 gꢂ1, the total pore
conditions, the synthesized anatase TiO2 has a perfect crystal volumes are 0.25, 0.31, 0.34, 0.35, and 0.37 cm3 gꢂ1, and the
structure, as observed from the XRD results. The GTR pore sizes are 7.22, 4.61, 3.82, 3.84, and 4.32 nm, respectively. It
composites show peaks at 2q ¼ 25.1ꢁ, 38.2ꢁ, 48.0ꢁ, 53.9ꢁ, 54.9ꢁ, is interesting to observe that when GO was combined, the
62.8ꢁ, 68.9ꢁ, 70.1ꢁ, and 75.0ꢁ, corresponding to the (101), (004), specic surface area and pore volume increased as compared
(200), (105), (211), (204), (116), (220), and (215) crystal planes, with those of pure TiO2, respectively. Furthermore, the BET
which can be well indexed to anatase TiO2 (JCPDS card no. 21- characteristic results suggested that the increased Gd content
1272), as shown in Fig. 3. The result demonstrates that the results in a slight improved surface area of the GTR composite,
obtained nanocomposites exhibit similar XRD proles as that of which can facilitate photocatalysis owing to the improved
the pure TiO2. The average nanocrystal sizes (D in nm) of the adsorbability. With the content of metal Gd increasing, it may
GTR samples were evaluated by the Scherrer formula based on affect the folding degree of carrier material aer hydrothermal
the (101) XRD peak broadening, D ¼ kl/b cos q, where k is the reaction. This result can be ascribed to the Gd formation on the
Scherrer constant that depends on the particle shape (k ¼ 0.89), carbon (GO) edge, which leads to the volumetric expansion of
l is the X-ray radiation wavelength (0.154056 nm), b is the full pore.40 Therefore, it should be mentioned that the surface area,
width half-maximum (FWHM) of the most intense peak, and q pore volume, and pore size have a denite effect on the pho-
is the Bragg diffraction angle. The average particle diameter of tocatalytic activity.
the TiO2 nanostructures is given in Table 1. We can clearly see
To conrm the presence of Ti3+ defects and further prove the
in the XRD proles that amorphous TiO2 transforms into the successful doping of Gd ion, XPS spectra were measured to
crystalline anatase phase during the hydrothermal process of determine the chemical states of the 3% GTR sample. Fig. 5(a)
combining with GO, resulting in intense and clearly visible shows the full-scale XPS prole of the 3% GTR sample, which
peaks belonging to anatase TiO2. Thus, for these composites, indicates the chemical binding energies of approximately
the temperature of 180 ꢁC, which was used for the hydrothermal 459.6 eV, 530.3 eV, 284.8 eV, and 144.2 eV correspond to Ti 2p 3/
reduction of GO, was found to be sufficient to transform 2, O 1s, C 1s, and Gd 4d, respectively. As shown in Fig. 5(b), the
amorphous TiO2 into the anatase crystalline form.
C 1s core-level spectrum of the 3% GTR composite can be
Fig. 4 illustrates the N2 adsorption–desorption isotherms devolved into four peaks at 284.7, 285.4, 286.3, and 289.0 eV,
and the corresponding pore size distribution curves of pure which are closely assigned to C]C, C–C, C–O (hydroxyl or
TiO2 and the GTR composites containing different amounts of epoxy), and C–O–Ti, respectively. The presence of Ti–O–C
Gd. Following the IUPAC classication, all the adsorption– structure disclosed that some of the Ti atoms were substituted
with the C atoms in the TiO2 lattice during the preparation
process. The O 1s spectra are displayed in Fig. 5(c), which show
three peaks corresponding to the Ti–O–Ti bond (530.6 eV), Ti–
O–H bond (531.3 eV), and C–OH species (532.7 eV), respectively.
Fig. 5(d) shows the spectra of Ti 2p, wherein two main peaks are
observed at 459.6 eV (Ti 2p 3/2) and 465.3 eV (Ti 2p 1/2). These
binding energy values are in agreement with those of TiO2.
Moreover, the difference between these two binding energies
represents the standard binding energy of Ti element. The Ti 2p
3/2 peak shied slightly toward higher zone of binding energy;
this shi may be explained by a variation in the chemical state
and coordination structure of Ti 2p 3/2 owing to the interactions
between rGO and TiO2. In Fig. 5(e), the peak centered at
144.2 eV could be owing to the presence of Gd 4d 5/2. It was
reported that the binding energy of Gd 4d 5/2 in Gd2O3 is
142.1 eV.41,42 Therefore, our experimental data shows a higher
binding energy than that of Gd2O3, which indicates that Gd
exists in the form of Gd3+, and the chemical environment of
Fig. 3 XRD profiles of GO, pure TiO2 and GTR composites with
different.
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Table 1 Parameters obtained from nitrogen desorption isotherm experiments by BET and BJH methods and average anatase crystallite size of
the composites
Pore diameter,
DBJH (nm)
Anatase crystallite
size, D (nm)
Sample
Pore volume, VTOTAL (cm3 gꢂ1
)
Surface area, SBET (m2 gꢂ1
)
Pure TiO2
GO@TiO2
2% GTR
3% GTR
4% GTR
7.22
4.61
3.82
3.84
4.32
0.25
0.31
0.34
0.35
0.37
49.0
119.6
122.7
125.6
112.7
11.3
8.6
8.9
8.1
7.2
reduced during the hydrothermal preparation.44,45 The above
results indicate that the reduction of GO takes place in the TiO2
and rGO phases via bridging oxygen in both TiO2@rGO and the
3% GTR composite.
Fig. 7 displays the diffuse reectance absorption spectra of
pure TiO2 and the as-prepared GTR samples. It is seen that the
GTR samples exhibit a red shi in the photoabsorption edge
with increasing content of Gd along with a strong optical
absorption in the visible-light region range.46 The curves in
Fig. 7(a) displays the DRS spectra of TiO2, GO@TiO2, 1% GTR,
2% GTR, 3% GTR, and 4% GTR powders from bottom to top,
respectively. It can be observed that the introduction of rGO and
Gd ions in the GTR with gray color extends its absorption to the
visible-light region and enhances visible-light absorption
ability. A plot of the transformed Kubelka–Munk function as
a function of energy of light is shown in Fig. 7(b), from which
the band gaps of GO@TiO2, 1% GTR, 2% GTR, 3% GTR, and 4%
GTR are roughly estimated as 2.82, 2.51, 2.41, 2.38, and 2.24 eV,
respectively. This reveals that a qualitative observation of a red
shi of the photoabsorption edge of the GTR composites as
compared to pure TiO2.
3.2 Photocatalytic activity and reusability
Fig. 4 (a) N2 adsorption–desorption isotherms and (b) pore size
distribution curves of pure TiO2 and GTR composites with different
amounts of Gd.
The photocatalytic activities of the as-synthesized composites
were evaluated by the degradation of RhB under visible-light
illumination. The visible light was turned on to initiate the
photocatalytic reaction for 180 min aer the adsorption–
desorption equilibrium was reached within 60 min of stirring in
a dark environment. The photocatalytic activities of different
samples were determined and the experimental results are
shown in Fig. 8.
The as-synthesized GTR composites are well dispersed in an
aqueous continuous phase for the removal of the RhB dye
during the pre-adsorption process because of the hydrophilic
property of rGO. Aer stirring for 60 min, the adsorption
equilibrium had been almost reached. The adsorption
percentages of RhB for the synthesized GTR photcatalysts and
pure TiO2 were around 61% and 6%, respectively. As shown in
Fig. 8, the photoactivity of the GTR composite was higher than
that of pure TiO2. The removal efficiencies of the GTR samples
with Gd doping ratios of 1, 2, 3, and 4% were determined as
88.3, 86.9, 91.5, and 85.9%, respectively, aer visible-light
irradiation for 180 min. Aer photodegradation process, the
blank experiment result showed that the dye on its own for the
full duration of the experiment were stability.
Gd3+ in the 3% GTR sample changed, possibly because of the
formation of Gd–O–Ti linkages.43
The FTIR spectra of GO, TiO2@rGO, 3% GTR composite, and
TiO2 samples are presented in Fig. 6. The bands in the spectra of
GO, TiO2@rGO, and 3% GTR composites appearing at 1120,
1570, and 1610 cmꢂ1 arose from the stretching vibrations of
oxygen-containing functional groups, such as C–O–H/C–O–C,
C]C, and carbonyl C]O. The broad band located at around
3420 cmꢂ1 originates from the OH stretching vibrations. It can
be seen that the spectrum of GO exhibits many strong absorp-
tion peaks corresponding to various oxygen-containing func-
tional groups. The spectra of TiO2@rGO, 3% GTR composite,
and TiO2 samples show strong and wide peaks in the range of
450 to 1000 cmꢂ1, which correspond to the bending vibrations
of Ti–O–Ti bonds. The bands of oxygen-containing groups
clearly observed in the GO spectrum changed in the spectra of
3% GTR composites, which indicates that GO was successfully
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Fig. 5 XPS survey spectra (a); C 1s (b), O 1s (c), Ti 2p (d), and Gd 4d (e) XPS spectra of 3% GTR composite.
We adjusted the Gd content in the GTR composite to
determine the inuence of Gd on the photodegradation
performance. The 3% GTR composite afforded a 3.5 times
higher apparent rate constant for the photodegradation of RhB
than that of pure TiO2. It is well known that GO has been
studied to be a competitive acceptor material owing to its two-
dimensional p-conjugated structure.47 Such structures can
improve the conductivity and transport of charge carriers of the
GTR composite.48 For this reason, photo-induced electrons
could rapidly transfer from the conduction band of TiO2 to rGO
through interfacial interactions. It can suppress the recombi-
nation rate of electron–hole (eꢂ–h+) pairs in the composite
system, generating more charge carriers to react with the RhB
Fig. 7 (a) UV-vis diffuse reflectance spectra of the synthesized pho-
tocatalysts in different doping ratios. (b) Plot of transformed Kubelka–
Munk function versus the energy of light for GTR composites with
different Gd contents.
Fig. 6 FTIR spectra of some samples.
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Fig. 10 Schematic illustration of photodegradation of RhB by GTR
composites under irradiation by visible light region of the simulated
sunlight.
Fig. 8 Photocatalytic activities of pure TiO2 and GTR photocatalysts
with different doping ratios.
dye molecules.49,50 The superior performance of GTR, therefore,
can be largely assigned to the effective charge transport of the
photogenerated electrons in RhB and TiO2 to graphene and Gd
nanoparticles.
Fig. 9 shows the adsorption and photocatalytic performance
of the 3% GTR composite for a RhB (10 mg Lꢂ1) solution aer
eight repeated experiments, in which the composites were
reused aer washing with ethanol at least eight times in each
cycle until the degradation reaches stable. The results show that
aer eight cycles, the removal efficiency of RhB by GTR
decreases to 76.65%, suggesting that the obtained GTR
composite has good stability. This indicated that the GTR
composites could be applied as a suitable candidate for treating
with sewage water in environmental.
arrangement, which prevent the recombination of electron–
hole. The generated large electric eld promotes the light-
induced electron orientation of rGO to the interface between
Ga and TiO2, and the electron–hole recombination is effectively
prevented in this process. Thus, during the photocatalytic
reaction, holes and superoxide are the most important active
species, which is consistent with the results of the active
component trapping experiment.
Herein, both Gd nanoparticles and graphene are excellent
co-catalysts, and we propose that their presence increases the
total reaction sites of the system and synergistically contributes
to TiO2, thus signicantly improving the photodegradation of
RhB and hydrogen generation. The mechanism can be
described as eqn (1)–(7).
GTR + RhB / GTR–RhB
rGO/TiO2 + hn / rGO/TiO2 (h+ + eꢂ)
Gd + hn / Gd (h+ + eꢂ)
eꢂ + O2 / O2cꢂ
(1)
(2)
(3)
(4)
(5)
(6)
(7)
3.3 Mechanism of degradation
Fig. 10 presents a schematic diagram of the photocatalytic
degradation of RhB by the GTR photocatalyst. Based on the
characterization and photodegradation results, we can assume
that both electrons (eꢂ) and holes (h+) can interact with O2 and
H2O to form superoxide (O2cꢂ) and hydroxyl radicals ($OH).51
The Gd/TiO2 petaloid ambulacra attached to the graphene
sheets transmit their activated electrons to the rGO due to band
h+ + OHꢂ / $OH
O2cꢂ + H2O / $OH
$OH + RhB / CO2 + H2O
4. Conclusion
In conclusion, we prepared GTR composites via a simple and
efficient one-pot solvothermal method. The characterization
results (FTIR and XPS) showed that GO was successfully
reduced to rGO during the hydrothermal process and perfect
Ti–O–C bonds existed between TiO2 and rGO. The results
showed that all the as-prepared GTR composites performed
better in the photodegradation of RhB in simulated wastewater
than the pure TiO2 nanoparticles, which revealed that the
Fig. 9 Recycling of 3% GTR photocatalyst for RhB dye removal.
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There are no conicts to declare.
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Acknowledgements
This work was supported by the Research Project of Education
Ministry of Heilongjiang Province of China (135209223,
135209217, YSTSXK201843, 135309116). In addition, the
authors also would like to express their gratitude to Research
Project of the Ministry of Human Resources and Social Security
of Heilongjiang Province of China (2018) and Research Project
of the Ministry of Human Resources and Social Security of
China (2015) for nancial supports.
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