Enhanced photocatalytic activity for degrading phenol in seawater by TiO2-based catalysts under weak light irradiation

Article abstract of DOI:10.1039/c7ra04732k

The seawater system is a typical salt water system. A catalyst should overcome the disturbance from the salt ions for an efficient photodegradation of organic pollutants in the seawater system. Commercial photocatalysts (P25) and La³⁺-doped SiO₂-TiO₂ prepared using adsorbed-layer nanoreactor synthesis (ALNS) were first used for photodegrading different initial concentrations of phenol in seawater under weak UV light irradiation. The weak adsorption capacities for phenol and the hydrophilic surfaces of the two photocatalysts could not overcome the disturbance of salt ions and thus showed low photocatalytic activities. Based on this, graphene oxide (GO) was used as a support to prepare TiO₂ and La³⁺-doped TiO₂ using ALNS. The solvothermal treatment with alcohol was used as a solvent for both TiO₂ crystallization and GO reduction. Results showed that TiO₂ nanoparticles with sizes <10 nm formed and distributed homogeneously on the reduced GO surface. The small size of the TiO₂ particles and the decreased oxygenated functional groups on the GO surface both caused high separation efficiency of the photogenerated charge carriers, thereby increasing the photodegradation performance. The strong phenol adsorption of the photocatalyst was efficient enough to overcome the interference of salt ions and enhance the photodegradation efficiency in seawater. The activities of the two Red-GO-TiO₂ catalysts were more than twice those of P25 and La³⁺-doped SiO₂-TiO₂. La³⁺ doping caused mixed crystals to form and increased the shallow trapping sites for charge carriers. Therefore, La³⁺ doping increases the photocatalytic activity of Red-GO-TiO₂.

Full text of DOI:10.1039/c7ra04732k

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Enhanced photocatalytic activity for degrading
phenol in seawater by TiO2-based catalysts under
weak light irradiation†
Cite this: RSC Adv., 2017, 7, 31921
*
Ting Wang, Zhi-yong Xu, Li-guang Wu,
Yi-chen Zhu and Jing Cai
Bing-rui Li, Mei-xi Chen, Shi-yi Xue,
The seawater system is a typical salt water system. A catalyst should overcome the disturbance from the salt
ions for an efficient photodegradation of organic pollutants in the seawater system. Commercial
photocatalysts (P25) and La3+-doped SiO2–TiO2 prepared using adsorbed-layer nanoreactor synthesis
(ALNS) were first used for photodegrading different initial concentrations of phenol in seawater under
weak UV light irradiation. The weak adsorption capacities for phenol and the hydrophilic surfaces of the
two photocatalysts could not overcome the disturbance of salt ions and thus showed low photocatalytic
activities. Based on this, graphene oxide (GO) was used as a support to prepare TiO2 and La3+-doped
TiO2 using ALNS. The solvothermal treatment with alcohol was used as a solvent for both TiO2
crystallization and GO reduction. Results showed that TiO2 nanoparticles with sizes <10 nm formed and
distributed homogeneously on the reduced GO surface. The small size of the TiO2 particles and the
decreased oxygenated functional groups on the GO surface both caused high separation efficiency of
the photogenerated charge carriers, thereby increasing the photodegradation performance. The strong
phenol adsorption of the photocatalyst was efficient enough to overcome the interference of salt ions
and enhance the photodegradation efficiency in seawater. The activities of the two Red–GO–TiO2
catalysts were more than twice those of P25 and La3+-doped SiO2–TiO2. La3+ doping caused mixed
crystals to form and increased the shallow trapping sites for charge carriers. Therefore, La3+ doping
increases the photocatalytic activity of Red–GO–TiO2.
Received 27th April 2017
Accepted 15th June 2017
DOI: 10.1039/c7ra04732k
rsc.li/rsc-advances
pollutant emissions and numerous sailing ships.3,4 The marine
system is rst a typical salt water system (salt concentration is
Introduction
generally between 3–5%) that interferes with traditional treat-
ment technologies, such as microbial degradation, for organic
pollutants.5,6 Second, the organic pollutant concentration in
seawater is low; therefore, effective treatments using traditional
adsorption technology are costly and difficult.7 At present, few
studies have focused on effectively eliminating organic pollut-
ants from seawater. For decades, developing advanced oxida-
tion technologies has provided new ideas for removing organic
pollutants in the ocean.8 TiO2-based heterogeneous semi-
conductor photocatalysis is the most intensively studied tech-
nology for pollutant treatment among advanced oxidation
technologies, because of its low energy consumption and
pollution-free process.9–11 Especially, heterogeneous photo-
catalysis shows excellent potential in degrading highly toxic
organic pollutants with low concentration. In heterogeneous
photocatalysis, small TiO2 particles, especially the quantum-
sized TiO2 particles (less than 10 nm), show high activity.12–14
However, small nanoparticles easily agglomerate during prep-
aration and application to form large particles and reduce or
even halt the catalytic activity. To mediate particle agglomera-
tion, supported catalysts have been presented and prepared in
The rapid development of industry has initiated numerous
environmental pollution problems.1,2 For example, the ocean is
facing difficult organic pollution problems due to terrestrial
School of Environmental Science and Engineering, Zhejiang Gongshang University,
Hangzhou, 310012, China. E-mail: wulg64@hotmail.com
† Electronic supplementary information (ESI) available: Dependences of
conductivity of seawater on time aer photocatalysts addition, dependences of
conductivity of seawater on time aer photocatalysts addition, the standard
curve of phenol in seawater and water measured by spectrophotometer, the
blank degradation experiments of phenol illuminated under 4 W UV lamp,
degradation of phenol with different initial concentration by two catalysts in
seawater, degradation of phenol with different initial concentration by two
catalysts in water, digital photographs of the dispersion of La3+ doped
SiO2–TiO2 in seawater, TEM images of Red–GO samples, TEM images of two
GO–TiO2 before solvothermal treatment, XPS proles of Ti 2p in two
Red–GO–TiO2 catalysts, digital photographs of the dispersion of GO–TiO2 and
Red–GO–TiO2 in seawater, HRTEM images of GO–TiO2 before solvothermal
treatment, photocurrent response of different photocatalysts, photodegradation
of different phenol concentrations using GO–TiO2, degradation curves using
Red–GO–TiO2 and degradation curves using La3+ doping Red–GO–TiO2 are
shown in Fig. S1–S16. The mechanism scheme on photodegradation process for
phenol in seawater by Red–GO–TiO2 with or without doping is shown in
Scheme S1. See DOI: 10.1039/c7ra04732k
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numerous studies.15,16 Considering its enhanced light adsorp-
tion and separation of photogenerated carriers, SiO2 and new
carbon nanomaterials (carbon nanotubes and graphene) have
the high potential as support materials.17,18
However, high-power light sources, such as high-pressure
mercury that emit several hundred watts, were used in most
studies as excitation light sources.19,20 The intensities of these
Preparation of graphene oxide with hydrophilic groups
Graphene oxide (GO) nanosheets with hydrophilic groups were
synthesized by oxidizing graphite powder based on the
improved Hummers method.32 In previous studies, GO nano-
sheets optimized the morphology and distribution in polymer
matrices upon addition of 5.0 g graphite powder, 2.5 g NaNO3,
and 20 g KMnO4 to the suspension during GO preparation.
These optimized conditions were maintained in the present
study for synthesizing GO nanosheets. The specic surface area
high-power light sources signicantly exceed the strong natural
sunlight environment with a light intensity of 1 W cmꢀ2
.
In
21
marine environments, light intensity is usually less than 1 mW
cmꢀ2. Therefore, the strong-intensity light sources used in
present studies limit the practical application of heterogeneous
photocatalysis for eliminating organic pollutants from the
ocean.19,20 Fujishima rst studied photocatalysis using a low-
of GO nanosheets was 52 m2 gꢀ1
.
Preparation of TiO2-based multiphotocatalysts
The preparation of SiO2–TiO2 doped with La3+ by ALNS. The
intensity light system and found that short carbon-chain alco- preparation process referred to our previous work.25 Aer we
hols and bacteria could be photodegraded using TiO2 lms sintered TiO2–SiO2 doped with 0.5 wt% La3+ at 900 C for 5 h
ꢁ
under weak UV light irradiation.22–24 Moreover, the quantum and added 1.5 mL water and 2.15 g tetrabutyl titanate during
yield of photoreaction under weak UV light irradiation was catalyst preparation, it achieved peak photocatalytic activity.
visibly higher than that of the photodegradation excited by the Therefore, these conditions were used to synthesize the undo-
strong UV light.19 In our previous work,25 TiO2 with a quantum ped or doped TiO2 photocatalysts in this study.
size of less than 7 nm and prepared using adsorbed-layer
The preparation of Red–GO–TiO2 by ALNS. GO–TiO2 was rst
nanoreactor synthesis (ALNS) showed higher activity in photo- prepared using ALNS, as with our SiO2–TiO2 preparation in our
degrading methyl orange under weak light irradiation previous work,25 except that we used GO with hydrophilic
compared with P25 commercial photocatalyst. Given the groups as a support.
formation of few photogenerated charge carriers excited by
0.5 g of GO with hydrophilic groups, 1.5 mL of water and 200
weak light irradiation, the structure–activity relationship of mL of absolute alcohol were placed in a triask, while stirring at
catalysts was more sensitive than that under strong light 30 ꢁC. A water-rich adsorption layer formed gradually on GO
irradiation.
surface, due to the selective adsorption capacity for water. Aer
One challenging aspect still remains regarding the use of adsorption equilibrium (24 h), tetrabutyl titanate (2.25 g) dis-
heterogeneous photocatalysis in seawater systems. Numerous solved in alcohol (50 mL) was added. And tetrabutyl titanate
salt ions oen decrease photocatalytic activity or cause photo- could diffuse into the adsorption layer and the hydrolysis
catalyst inactivation.26–29 Based on studies and our previous reaction takes place.25 Aer 5 h of reaction, the product was
work,30,31 the photodegradation for phenol by P25 and La3+
doped SiO2–TiO2 via ALNS in seawater under weak UV light times.
-
obtained by centrifugation–washing cycles conducted three
irradiation was rst accomplished; aerward, TiO2 particles
The as-prepared TiO2–GO catalysts were dispersed in 150 mL
and La3+-doped TiO2 particles with graphene oxide (GO) as absolute alcohol and placed in a Teon-covered stainless steel
a support were prepared using ALNS. A solvothermal treatment autoclave (volume: 200 mL). The autoclave was then placed in
with alcohol as the reducing agent was used for both TiO2 a furnace for solvothermal treatment (temperature: 170 ꢁC,
crystallization and GO reduction to enhance the photocatalytic time: 24 h). Aer several centrifugation–washing cycles, the
activity for phenol in seawater.
photocatalysts were retrieved and designated as Red–GO–TiO2.
The preparation of Red–GO–TiO2 doped with La3+ prepara-
tion by ALNS. The preparation process was similar to that of
SiO2–TiO2 doped with La3+
.
25
Experimental
Materials
0.5 g of GO with hydrophilic groups, 1.5 mL of water and 200
mL of absolute alcohol were placed in a triask, while stirring at
Graphite powder (G, 8000 mesh) was purchased from the 30 ꢁC. A water-rich adsorption layer formed gradually on GO
Reagent Chemical Manufacturing (Shanghai, China). H2SO4, surface, due to the selective adsorption capacity for water. Aer
KMnO4, NaNO3, and lanthanum nitrate La(NO3)3 were obtained adsorption equilibrium (24 h), tetrabutyl titanate (2.25 g) and
from Shanghai Reagent Factory (Shanghai, China). Analytical lanthanum nitrate (0.77 mg) dissolved in alcohol (50 mL) was
grade tetrabutyl titanate was received from the Reagent Chem- added. Both tetrabutyl titanate and La3+ ions could diffuse into
ical Manufacturer (Shanghai, China) and was used without the adsorption layer and the hydrolysis reaction takes place.25
further purication. Analytical grade ethanol was purchased Aer 5 h of reaction, the product was obtained by centrifuga-
from Reagent Chemical Manufacture of Shanghai and was tion–washing cycles conducted three times.
˚
distilled and stored over 4 A molecular sieves before use.
The process of solvothermal treatment for this catalyst was
Nonporous hydrophilic colloidal silica (SiO2) A-200 (12 nm, 200 similar as that of Red–GO–TiO2. Aer the solvothermal treat-
m2 gꢀ1) and P25 (gas nanometer TiO2, 21 nm, 50 m2 gꢀ1) were ment, the photocatalysts were retrieved and designated as La3+
-
obtained from Degussa, Germany.
doped Red–GO–TiO2.
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conrm the results from the 4-aminoantipyrine spectrophoto-
metric method,35,36 we determined the total organic carbon
(TOC) in the aqueous solution without catalysts aer 5 h of
photodegradation. And it was found that the removal rate of
phenol by TOC was approximated by the results of spectro-
photometry, which meant phenol in seawater transfer to form
CO2 and H2O aer 5 h of photodegradation.
Fig. S4 (ESI†) provides the details on the blank experiments
under weak UV light irradiation with an intensity of 12 mW
cmꢀ2. The light intensity of the four-watt UV lamp was weak for
degrading phenol.
Characterization
The morphologies of TiO2–GO catalysts were characterized using
transmission electron microscopy (TEM, JEM-1230, Jeol Co., Ltd.).
The Autosorb 1-C sorption system (Quantachrome, USA) was used
to determine the specic surface area. The structure of TiO2–GO
catalysts was determined using Fourier transform infrared (FTIR)
spectroscopy (Nexus-670, Nicolet Co.). Moreover, the catalysts
were analyzed with X-ray diffraction (XRD) using a D/max-rA XRD
˚
instrument (XD-98) with Cu Ka radiation (1.5406 A). The photo-
luminescence (PL) spectra were recorded on a uorospectropho-
tometer (PL: RAMANLOG 6, USA) at room temperature using
a 250 nm xenon lamp as the irradiation source. The surface
properties of catalysts were investigated using X-ray photoelectron
spectroscopy (XPS) with Al Ka X-ray (hm ¼ 1486.6 eV) radiation
operated at 150 W (XPS: Thermo ESCALAB 250, USA).
Results and discussion
Comparison of photodegradation in seawater and in water
excited by weak UV light
Photocatalysis experiments
The photodegradation of different phenol concentrations in
Our experiment used simulated seawater composed of 2.5% seawater using P25 and SiO2–TiO2 doped with La3+ was rst
NaCl, 1.1% MgCl2, 0.40% Na2SO4, and 0.16% CaCl2 in distilled investigated. The photodegradation experiments in water using
water without CO2.33,34 Conductivity measurements were used to the same catalysts were done simultaneously for comparison.
evaluate the effects of photocatalyst addition on seawater salt The photodegradation curves of phenol in seawater and water
concentration. As shown in Fig. S1 and S2 in the ESI,† the are listed in Fig. 1 and 2, respectively.
solution conductivity did not change aer photocatalyst addi-
The curves in Fig. 1 and 2 shows that all catalysts reach
tion. It is suggested that the adsorption amount of salt ions by adsorption equilibrium for phenol both in seawater and water
these photocatalysts in seawater was too small to be measured aer 30 min. Two catalysts had poor adsorption for phenol in
by the conductivity measurements.
seawater or water, due to their hydrophilic surfaces. Under weak
The photodegradation of phenol in seawater was studied to UV light irradiation, the degradation for phenol by the two
evaluate the performance of different catalysts irradiated under catalysts in seawater and water almost followed a rst-order
weak ultraviolet (UV) black-light lamps (250 nm wavelength, 4 reaction kinetics, which agreed with previous results.37,38
W).25 Prior to irradiation, the suspension (800 mL of phenol Under same conditions, SiO2–TiO2 doped with La3+ had higher
solution with different initial concentration, 1.0 g of different photocatalytic activity than P25 under weak UV light irradiation,
photocatalysts) was stirred in the dark for 60 min to establish which corresponded with our previous results.25 According to
a adsorption/desorption equilibrium. At 30 min intervals, the heterogeneous photocatalysis studies, the amount of catalyst
sampled suspension was centrifuged, and the upper clear surface active center was limited. At a high initial concentra-
solution was extracted. UV-vis spectrophotometer (Beijing, tion, a large amount of phenol molecules could not efficiently
China) was used to measure the changes in the phenol be photodegraded by catalysts. The catalyst activity was reduced
concentration (510 nm wavelength) in the solution using the 4- and the degradation process was deviated from a rst-order
aminoantipyrine spectrophotometric method (Fig. S3, ESI†). To reaction kinetics.
Fig. 1 Photodegradation of different phenol concentrations in seawater using two catalysts. The initial phenol concentrations (mg Lꢀ1): (a) 5; (b)
10; (c) 15; (d) 20; (e) 25.
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Fig. 2 Photodegradation of different phenol concentrations in water using two catalysts. The initial phenol concentrations (mg Lꢀ1): (a) 5; (b) 10;
(c) 15; (d) 20; (e) 25.
decreased obviously, whereas in water, the activities of the two
catalysts were reduced at a high initial phenol concentration
($20 mg Lꢀ1).
Based on the comparison of the results in seawater and in
water, the catalyst must meet two requirements to efficiently
degrade organic matter in seawater systems. Firstly, the TiO2
photocatalysts should have the necessary microstructures, such
as small particle size, high dispersibility, and appropriate
crystal form, in the photodegradation process in water. On the
other hand, the previous results25 indicated that weakening the
surface hydrophilicity of the catalyst and increasing the
adsorption performance of the catalyst for target organics in
seawater are also necessary. Based on current and previous
results,26,27 we rst employed solvothermal treatment using
ethanol as a reduction solvent to change the surface properties
Fig. 3 Dependence of initial phenol concentration on the photo- of the SiO2–TiO2 catalyst. Fig. S7 (ESI†) shows the digital photos
degradation rate constants using two catalysts in seawater. The initial
of the SiO2–TiO2 catalyst before and aer the solvothermal
treatment in seawater. The surface hydrophilicity of the SiO2–
phenol concentrations (mg Lꢀ1): (a) 5; (b) 10; (c) 15; (d) 20; (e) 25.
TiO2 catalyst aer the solvothermal treatment was signicantly
Comparing all photodegradation curves in Fig. 1 and 2, the
changes in photodegradation in seawater and water vary as the
phenol concentration increases. Under the same condition, the
same catalyst had lower activity in seawater than in water. In
seawater, the salt ions were much more than the phenol
molecules. Combining the results in studies,39–41 large amount
of salt ions would move around the catalyst surface, due to the
hydrophilic surface of P25 or SiO2–TiO2 doped with La3+. This
inhibited the phenol diffusion from the bulk to the catalyst
surface and decreased the catalyst activity center.
For further comparison, the rate constants in seawater and
water were obtained by plotting ln(C0/C) to irradiation time t (as
shown in Fig. S5 and S6, ESI†) and are listed in Fig. 3 and 4. The
changes in the two gures indicate that the activities of two
catalysts in seawater were lower than those in water under the
same conditions, due to the presence of so many salt ions. In
seawater, the catalyst activity was more sensitive to the initial
concentration of phenol than that in water. At the initial
degradation rate constants using two catalysts in water. The initial
concentration of 15 mg Lꢀ1, the activities of the two catalysts phenol concentrations (mg Lꢀ1): (a) 5; (b) 10; (c) 15; (d) 20; (e) 25.
Fig. 4 Dependence of initial phenol concentration on the photo-
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adsorption layer can be used as a nanoreactor for preparing
TiO2 nanoparticles.
For a clear comparison, GO sample was denoted as Red–GO
aer the solvothermal treatment, and its TEM image is listed in
Fig. S8 (ESI†). As shown in the gure, TEM morphologies of
both GO and Red–GO nanosheets showed a yarn-like sheet
structure, indicating that the solvothermal treatment did not
change the nanosheet morphology. The specic surface area of
Red–GO nanosheets was 55 m2 gꢀ1, similar to that of the GO
nanosheets, which conrmed that the nanosheet morphology
was unchanged by the solvothermal treatment.
Fig. 7 displays the TEM images of Red–GO–TiO2 and La3+-
doped Red–GO–TiO2, whereas Fig. S9 (ESI†) presents those of
GO–TiO2 and La3+-doped GO–TiO2 before solvothermal treat-
ment for comparison. Manifested as the black points on the
nanosheets, the TiO2 particle morphology can be observed both
in Fig. 7 and S9 (ESI†).
Fig. 5 TEM image of GO nanosheets as a support.
weakened. However, the SiO2–TiO2 catalyst aer the sol-
The ALNS method employed a water-rich adsorption layer
vothermal treatment was unable to disperse uniformly in water, with several nanometer thick as a nanoreactor on the GO
therefore, photodegradation could not be done. Combining our surface; therefore, TiO2 nanoparticles with size of less than
former results25,30 and those of others,26,27 we rst used GO with 10 nm were formed and were distributed homogeneously on the
a hydrophilic surface as a support in preparing small TiO2 GO surface, as shown in Fig. S9 (ESI†). La3+ doping did not
particles using ALNS. The solvothermal treatment was used to change the TiO2 morphology on the GO surface, corresponding
reduce the oxidation groups on the catalyst surface, aiming to with previous ndings.25 Moreover, TiO2 small particles that
decrease the surface hydrophilicity and increase the adsorption formed in the adsorption layer interacted tightly with the GO
of the catalyst for organics in seawater. The TEM image of the surface, and this could effectively inhibit the aggregation of
GO nanosheets is shown in Fig. 5.
both small TiO2 particles and GO nanosheets during the sol-
vothermal treatment. Therefore, no difference was observed in
the catalyst morphology before and aer the solvothermal
Preparation of different Red–GO–TiO2 by coupling ALNS and
solvothermal treatment
treatment. The specic surface areas of Red–GO–TiO2 and La3+
-
doped Red–GO–TiO2 were 51 and 50 m2 gꢀ1, respectively, which
were close to that of GO.
From Fig. 5, the highly transparent nanosheets with yarn-like
sheets are characteristic of the GO morphology, which indi-
cate a monolayer or a few layers of GO. These ndings are
consistent with previous work.32 Fig. 6 lists the adsorption
curves of water by GO and the adsorption equilibrium aer 4 h
(240 min). Assuming that the adsorption layer is entirely
composed of water, the adsorption layer thickness of GO is
approximately 1 nm, thereby proving that the water-rich
adsorption layer will form on the GO surface. Thus, this
Fig. 8 displays the XPS results for different catalysts and
suggests TiO2 formation in all photocatalysts by ALNS. XPS
analysis was used to identify the chemical state of Ti in the Red–
GO–TiO2 and La3+-doped Red–GO–TiO2 catalysts, as shown in
Fig. S10 (ESI†). The Ti 2p levels of all catalysts depicted two
peaks at approximately 464.6 and 458.3 eV, which were denoted
as Ti 2p1/2 and Ti 2p3/2, respectively.42 Data suggested that Ti
existed as Ti4+ in the Red–GO–TiO2 and La3+-doped Red–GO–
TiO2 catalysts.
The solvothermal treatment had little inuence on the
catalyst morphology. However, the FTIR spectra of different
photocatalysts (Fig. 9) indicated that the surface properties of
the catalysts changed signicantly aer the solvothermal
treatment. As shown in Fig. 9, GO–TiO2 had a similar charac-
teristic FTIR spectra with GO. The absorption bands corre-
sponded to the C–O stretching at 1050 cmꢀ1, C–O–C stretching
at 1250 cmꢀ1, C–OH stretching at 1400 cmꢀ1, C–C stretching at
1620 cmꢀ1, and C]O carbonyl stretching at 1720 cmꢀ1
. The
43
resonance peak at 3383 cmꢀ1 was assigned to the absorbed
hydroxyl groups in catalysts.44 When TiO2 formed on the GO
surface through ALNS, the TiO2–GO catalysts exhibited weak
absorption peaks of C–O, C–OH, C]O, and absorbed hydroxyl
groups, thereby indicating that the oxygenated functional
groups on GO surface slightly decreased during the formation
of TiO2 particles.
Fig. 6 Dependences of water adsorbed by GO on adsorption time.
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Fig. 7 TEM images of different Red–GO catalysts (a) Red–GO–TiO2; (b) La3+-doped Red–GO–TiO2.
spectra of Red–GO–TiO2 and La3+-doped Red–GO–TiO2 showed
some remaining hydroxyl groups on the two catalyst surfaces,
which would mainly be distributed on the TiO2 surface. This
could help in the efficient distribution of Red–GO–TiO2 and
La3+-doped Red–GO–TiO2 in seawater (as shown in Fig. S11†).
HRTEM results (Fig. 10) show that TiO2 crystallization was
simultaneous with the GO surface reduction during the sol-
vothermal treatment. The HRTEM image of GO–TiO2 is listed in
Fig. S12 (ESI†) for comparison.
In the HRTEM image of GO–TiO2 and the images in Fig. 10,
the TiO2 crystallization formed in both Red–GO–TiO2 and La3+-
doped Red–GO–TiO2 aer the solvothermal treatment. These
two catalysts had TiO2 nanocrystals with less than 5 nm in size
due to the small nanoreactors and Red–GO protection on TiO2
particles. Several distortions were observed in the lattice fringes
of the TiO2 nanocrystals in the two catalysts, especially in the
La3+-doped Red–GO–TiO2. Based on the results both in Fig. 7
and in our previous study,25 the tight interaction between TiO2
particles and the supports, as well as La3+ doping, caused the
lattice distortions of TiO2 during TiO2 crystallization.
Fig. 8 XPS profiles of GO and different photocatalysts.
The XRD results in Fig. 11 conrm the formation of TiO2
crystallization and the difference in TiO2 crystallization between
the two catalysts.
Fig. 11 shows that GO–TiO2 has a similar XRD pattern with
GO, and no TiO2 crystallization peaks are present in the XRD
pattern; therefore, the TiO2 in GO–TiO2 existed as amorphous
TiO2. Aer the solvothermal treatment, amorphous TiO2
transformed into anatase TiO2 crystals in Red–GO–TiO2.
Moreover, a few rutile TiO2 crystals appeared in the La3+-doped
Red–GO–TiO2, although anatase TiO2 was the main crystal
form. This phenomenon was not found in our previous
works25,30,31 and might be due to the strong inhibition by SiO2 on
TiO2 crystallization than that of GO. In our previous work,25 SiO2
strongly inhibited the crystallization transformation of TiO2,
and the anastase TiO2 did not transfer to rutile TiO2 even with
sintering at 900 ꢁC. Therefore, a low level of La ion doping did
not change the crystallization transformation of TiO2. Due to
the weak effect of GO on the crystallization transformation of
TiO2, a low level of La ion doping could improve the trans-
formation of anatase TiO2 to rutile TiO2, which could cause the
lattice distortions in the TiO2 crystallization.25,45
Fig. 9 FTIR spectra of GO and different photocatalysts.
Aer solvothermal treatment, the oxygenated functional
groups on the surface of Red–GO–TiO2 and La3+-doped Red–
GO–TiO2 were reduced signicantly and the absorption peaks of
C]O and C–O–C almost disappeared. The GO surface was
reduced during the solvothermal treatment with alcohol as the
reducing agent, thereby conrming that alcohol reduced the
oxygenated functional groups on the catalyst surface. The FTIR
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Fig. 10 TEM and HRTEM images of different TiO2–RGO catalysts. (A) Red–GO–TiO2; (B) GO–TiO2 doped with La3+
.
were reduced by alcohol and Red–GO could transport photo-
generated electrons,47,48 thereby weakening the PL peaks. The
PL spectra also show that the photogenerated charge carrier was
transferred rapidly from the TiO2 particles to the Red–GO
nanosheets, since the small TiO2 particles tightly attached to
the Red–GO surface in the two Red–GO–TiO2 catalysts. This
could signicantly enhance the separation efficiency of photo-
generated charge carriers and weaken the PL peak intensity.
Moreover, a few lattice distortions in TiO2 nanocrystals could
work as shallow trapping sites for charge carriers. Especially,
the mixed crystal forms, including anatase and rutile TiO2,
could introduce numerous shallow trapping sites as charge
carriers into the catalysts. Therefore, the La3+-doped Red–GO–
TiO2 had the weakest PL peak among the PL spectra in Fig. 12.
The photocurrent response of different catalysts could conrm
further the results in PL test, as shown in Fig. S13 (ESI†). From the
gure, it can be found that GO–TiO2 has very weak photocurrent,
due to oxygenated functional groups on the GO surface and
amorphous TiO2. The value of Red–GO–TiO2 and La3+ doping
Red–GO–TiO2 were signicantly higher than that of GO–TiO2,
since the oxygenated functional groups were reduced by alcohol
and TiO2 crystallization formed. La3+ doping introduced a few
lattice distortions in TiO2 nanocrystals as shallow trapping sites,
so it had the strongest photocurrent response.
Fig. 11 XRD patterns of GO and different photocatalysts.
The changes in the surface property and TiO2 crystallization
would certainly affect the photocatalytic process. A low recom-
bination rate of photogenerated electron–hole pairs result in
high catalytic activity that could be attributed to the separation
efficiency of photogenerated charge carriers and small intensi-
ties of PL peaks.46
Fig. 12 shows the differences in the PL spectra of GO, GO–
TiO2, Red–GO–TiO2, and La3+-doped Red–GO–TiO2. Oxygenated
functional groups on the GO surface could work as deep trap-
ping sites for photogenerated charge carriers. Thus, GO and
GO–TiO2 showed similar PL peaks with strong intensities. Aer
the solvothermal treatment, the oxygenated functional groups
Performance of different Red–GO–TiO2 by coupling ALNS and
solvothermal treatment
The photodegradation experiments for phenol with different
initial concentration of GO–TiO2 were carried out. And the
degradation curves are listed in Fig. S14.† Both amorphous TiO2
and large amount of oxygenated functional groups on the GO
surface worked as deep trapping sites for photogenerated
charge carriers. Therefore, the GO–TiO2 showed very low pho-
tocatalytic activity for phenol in seawater, under irradiation of
weak UV light. In addition, the adsorption capacity of phenol by
GO was also weak due to the hydrophilicity of GO surface.
Fig. 13 and 14 display the photodegradation of different
phenol concentrations using Red–GO–TiO2 and La3+-doped
Red–GO–TiO2. From these curves, the two Red–GO–TiO2 cata-
lysts are shown to have strong adsorption capacity for phenol in
seawater aer the solvothermal treatment. We also list the C/C0
Fig. 12 PL spectra of GO and different photocatalysts with 250 nm
excitation wavelength.
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Fig. 13 Photodegradation of different phenol concentrations using
Red–GO–TiO2. The initial phenol concentration in seawater (mg Lꢀ1):
(a) 15; (b) 20; (c) 25.
Fig. 15 Degradation efficiency of different phenol concentrations
using different photocatalysts in seawater within 2 h (a) P25; (b) La3+
-
doped SiO2–TiO2; (c) Red–GO–TiO2; (d) La3+-doped Red–GO–TiO2.
seawater. The La3+-doped Red–GO–TiO2 showed higher activity
than Red–GO–TiO2 without doping. From the HRTEM and XRD
analyses in Fig. 8 and 9, La3+ doping caused mixed crystals to
form, including anatase and rutile TiO2, which could increase
the shallow trapping sites in the catalysts for charge carriers.
Thus, the photocatalytic activity of Red–GO–TiO2 increased
aer La3+ doping. However, the enhancement on the activity
was small, which was inconsistent with our previous studies.
And this results suggested that the strong adsorption capability
for phenol and the rapid transformation of photogenerated
electrons played the more important roles on the enhancement
on the photo activity in seawater than that of La3+ doping. This
might be due to the difference between the catalysts prepared
with GO as the carrier and the catalysts prepared with SiO2 as
the carrier. These require further studies.
Fig. 14 Photodegradation of different phenol concentrations using
La3+-doped Red–GO–TiO2. The initial phenol concentration in
seawater (mg Lꢀ1): (a) 15; (b) 20; (c) 25.
Conclusions
graphs where C0 is the concentration at the time that the light is Due to the weak phenol adsorption and hydrophilic surface, P25
switched on, as shown in Fig. S15 and S16.† As shown in all and La3+-doped SiO2–TiO2 could not overcome the disturbance
degradation curves, the photocatalytic activities of the two of salt ions using ALNS. Therefore, they showed low activity in
catalysts exceeded those of P25 and SiO2–TiO2 doped with La3+
,
photodegrading phenol in seawater under weak UV light irra-
especially at high initial concentrations ($20 mg Lꢀ1). The total diation, especially at high initial phenol concentrations. Using
phenol removal rates at 2 h using different photocatalysts are ALNS coupling with solvothermal treatment with alcohol as the
listed in Fig. 15.
As shown in Fig. 15, both Red–GO–TiO2 and La3+-doped Red– occurred simultaneously. The tight interaction between TiO2
GO–TiO2 had signicantly higher activities than P25 and La3+
particles and the GO surface effectively inhibited TiO2 aggre-
reducing agent, TiO2 crystallization and RGO formation
-
doped SiO2–TiO2 at high initial phenol concentrations. The gation during solvothermal treatment. Given the small size and
total removal rate of the two Red–GO–TiO2 catalysts within 2 h efficient distribution of TiO2 nanoparticles, the two Red–GO–
was more than twice those of P25 and La3+-doped SiO2–TiO2. TiO2 catalysts showed high separation efficiency for photo-
This is mainly caused by both strong adsorption capability for generated charge carriers. Moreover, the strong phenol
phenol in seawater and the rapid transformation of photo- adsorption on the two Red–GO–TiO2 catalysts was essential for
generated electrons from TiO2 to Red–GO, as shown in Scheme improving the photodegradation efficiency in seawater. The
S1 (ESI†).49
activities of the two Red–GO–TiO2 catalysts were more than
Comparing Red–GO–TiO2 with or without doping, the two twice those of P25 and La3+-doped SiO2–TiO2. Moreover, La3+
catalysts had similar adsorption capacities for phenol in doping caused mixed crystals to form, including anatase and
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rutile TiO2, which could increase the shallow trapping sites in 23 S. Kayano, K. Yoshihiko, H. Kazuhito and A. Fujishima,
the catalysts for charge carriers. Thus, La3+ doping increases the
photocatalytic activity of Red–GO–TiO2.
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Acknowledgements
Financial support from the National Science Foundation of China
Grants (Contract 20806071 and 51008277), the Natural Science
Foundation of Zhejiang Province (Contract LY14B060001 and
LY14E080001) and the Public Projects of Zhejiang Province
(Contract 2015C31025) are gratefully acknowledged.
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