Visible light responsive mesoporous graphene-Eu2O3/TiO2 nanocomposites for the efficient photocatalytic degradation of 4-chlorophenol

Article abstract of DOI:10.1039/c5ra27541e

Mesoporous graphene-Eu₂O₃/TiO₂ nanocomposites with different weight percentages were synthesized by hydrothermal method using Pluronic P123 as the structure directing template. The synthesized materials were characterized by XRD, HRTEM, SEM-EDAX, N₂ adsorption-desorption studies, XPS, UV-vis DRS, FT-IR and photoluminescence spectroscopy. The photocatalytic activity of the catalysts for the degradation of 4-chlorophenol, a model pollutant was evaluated under visible light irradiation. 0.7 wt% GR-Eu₂O₃/TiO₂ nanocomposites showed higher efficiency under visible light among the synthesized materials due to high surface area with narrow band gap and effectively suppressed electron-hole recombination. UV-Vis DRS showed the shift in the light absorption band-edge position to visible region for GR-Eu₂O₃/TiO₂ nanocomposites. The well reduced photoluminescence intensity revealed the effective charge separation for GR-Eu₂O₃/TiO₂ nanocomposites than GR-TiO₂, Eu₂O₃-TiO₂ and bare meso TiO₂. Eu³⁺ ions were trapping and transferring the exited electrons to graphene layer and adsorbed oxygen efficiently, hence the co-doping of europium and graphene synergistically suppressed the electron-hole recombination. The suppressed electron-hole recombination enhanced ^.OH radicals formation and subsequently the photocatalytic degradation of 4-chlorophenol.

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Page 1 of 32
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Visible light responsive mesoporous graphene-Eu2O3/TiO2 nanocomposites for
the efficient photocatalytic degradation of 4-chlorophenol
Mallanaicker Myilsamy,a Mani Mahalakshmi,*a Nallasivam Subha a Ariyamuthu
Rajabhuvaneswari b and Velayutham Murugesanc
a Department of Chemistry, SSN College of Engineering, Kalavakkamꢀ603110, Tamil Nadu,
India.
b Department of Chemistry, Sri Sairam Institute of Technology, West Thambaram, Chennaiꢀ 44
Tamil Nadu, India.
c Department of Chemistry, Abdur Rahman University, Chennaiꢀ 48, Tamil Nadu, India.
*
To whom correspondence should be addressed, Eꢀmail: mahalakshmim@ssn.edu.in
Fax: +91 44 27469772
Tel: +91 44 27469700
1

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Abstract
Mesoporous grapheneꢀEu2O3/TiO2 nanocomposites with different weight percentages were
synthesized by hydrothermal method using Pluronic P123 as the structure directing template. The
synthesized materials were characterized by XRD, HRTEM, SEMꢀEDAX, N2 adsorptionꢀ
desorption studies, XPS, UVꢀvis DRS, FTꢀIR and photoluminescence spectroscopy. The
photocatalytic activity of the catalysts for the degradation of 4ꢀchlorophenol, a model pollutant
was evaluated under visible light irradiation. 0.7 wt% GRꢀEu2O3/TiO2 nanocomposites showed
higher efficiency under visible light among the synthesized materials due to high surface area
with narrow band gap and effectively suppressed electronꢀhole recombination. UVꢀVis DRS
showed the shift in the light absorption bandꢀedge position to visible region for GRꢀEu2O3/TiO2
nanocomposites. The well reduced photoluminescence intensity revealed the effective charge
separation for GRꢀEu2O3/TiO2 nanocomposites than GRꢀTiO2, Eu2O3ꢀTiO2 and bare meso TiO2.
Eu3+ ions were trapping and transferring the exited electrons to graphene layer and adsorbed
oxygen efficiently, hence the coꢀdoping of europium and graphene synergistically suppressed the
•
electronꢀhole recombination. The suppressed electronꢀhole recombination enhanced OH radicals
formation and subsequently the photocatalytic degradation of 4ꢀchlorophenol.
Keywords: Mesoporous, Eu/TiO2, Graphene, Photocatalysis, 4-Chlorophenol,
Charge carriers
2

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Introduction
Titanium dioxide (TiO2) is the most widely used photocatalyst in environmental
applications because of its green, clean, lowꢀcost, and sustainable innovation.1 However, the
quantum efficiency of TiO2 is low because of the fast recombination of photo induced electron
hole pairs, and its wide band gap energy (3.2 eV) limits its use as a light source in solar light (with
3% to 5% UV light).2,3 The limited optical absorption results in low activity of TiO2 as
photocatalysts. Therefore, a lot of approaches have been explored to extend the optical absorption
of TiO2 into the visible light region, and thus aim to enhance its photocatalytic activity. These
approaches include dyeꢀsensitizing, compositing of semiconductors with a narrow band gap, nonꢀ
metal (e.g. N, C, S) doping and transition metal doping, etc.4ꢀ6
Surfactant assisted solꢀgel and hydrothermal methods have been employed to prepare the
mesoporous TiO2 nanoparticles for enhancing the surface area and pore volume. Several
surfactants and Pluronicꢀtype copolymers such as P123 and F127 have been used as structural
directing agents for synthesis of mesoporous nanomaterials.7,8 In particular, highly porous
nanostructured materials with either large pore volumes or ordered pores have been fabricated.9 In
addition, many articles have reported that doping metal ions could improve the mesopore
structure, meanwhile increase the photocatalytic activity of TiO2.10ꢀ12 Rareꢀearthꢀmetal
compounds, especially their oxides, have become of increasing interest in recent years because of
their special photoluminescence and catalytic properties.13,14 Lanthanide ions are known for their
ability to form complexes with various Lewis bases (e.g., acids,amines, aldehydes, alcohols,
thiols, etc.) in the interaction of these functional groups with the fꢀorbitals of lanthanides.15 The
doping with Eu3+, Pr3+,and Yb3+ increased the adsorption capacity and also adsorption rate of TiO2
catalysts simultaneously in aqueous salicylic acid, tꢀcinnamic acid, and pꢀchlorophenoxyꢀacetic
acid solutions.16 It is expected that incorporation of lanthanide ions into a TiO2 matrix could
provide a means to concentrate the organic pollutant at the semiconductor surface and therefore
enhance the photoactivity of titania. Eu3+, Nd3+ and Ce4+ increased the photocatalytic activities of
TiO2 catalysts in aqueous azo dye (Xꢀ3B) under visible light irradiation.17 Rare earth metals
having incompletely occupied 4f and empty 5d orbitals often serve as catalyst and promote
catalysis. The europium element, might have an ability to improve the photocatalytic activity of
TiO2 because of its electrons trap effect. However, until now, there were only a few reports on
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Euꢀdoped mesoporous TiO2.18,19 A systematic study about the preparation, characterization and
photocatalytic properties of Euꢀdoped mesoporous TiO2 is needed.
The carbonaceous materials are of tremendous interest due to their unique pore structure,
electronic properties and adsorption capacity. These materials include activated carbon, carbon
nanotubes, and graphene.20 Graphene, which is a monolayer of carbon atoms closely packed in a
honeycomb lattice, has attracted a great deal of scientific interest due to its unique structure and
outstanding mechanical, electrical, thermal and optical properties.21 Compared with CNTs,
graphene has many advantages including high surface area (a theoretical value of 2630 m2/g)22
and the honeycombꢀlike graphene has a very large πꢀconjugated system and a twoꢀdimensional
planar structure, and these improve the mobility of charge carriers (≈200000 cm2 V−1 s−1).23,24
Since the first report of the synthesis of graphene/TiO2 nanocomposites have been developed as
efficient composite systems for many applications.25 Some studies have been focused on
grapheneꢀbased materials in capacitors,26 biosensors,27 solar cells28 and liquid crystalline
displays.29 P25ꢀgraphene composite from P25 and graphite oxide in ethanol solution using a
hydrothermal method, where graphite oxide was reduced to GS during the hydrothermal process.
Thus, the combination of TiO2 and graphene is promising to simultaneously possess excellent
adsorptivity, transparency and conductivity which could facilitates the charge separation in the
photocatalysis process.30
In this study, synthesis of mesoporous GRꢀEu2O3/TiO2 with different weight percentage
using triblock copolymer (Pluronic P123) as the structure directing template in ethanolꢀwater
solvent mixture by hydrothermal method. Both europium and graphene dopants can shift the light
absorption edge to the visible light region and can enhance the photocatalytic activity by
efficiently separating charge carriers (electrons / holes). To the best of our knowledge, there has
been no report on mesoporous GRꢀEu2O3/TiO2 as a visible light photocatalyst. Hence both
europium and graphene were selected for this coꢀdoping work. The spectroscopic characterization
of the obtained photocatalysts, photocatalytic activities and the degradation pathway of
4ꢀchlorophenol under visible light irradiation were also discussed.
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Experimental section
Materials
All the reagents and chemicals were of analytical grade and used without further
purification. Titanium (IV) isopropoxide (Aldrich), europium (III) nitrate pentahydrate (Aldrich)
were used as the source materials for Ti and Eu respectively. Triblock copolymer, [poly (ethylene
glycol)ꢀblockꢀpoly (propylene glycol)ꢀblockpoly(ethylene glycol)] (Pluronic P123, molecular
weight = 5800, EO20PO70EO20; Aldrich) was used as the structure directing template. Graphite
powder (Aldrich), absolute ethanol (Merck) and triple distilled water were used for the
preparation of catalyst. 4ꢀchlorophenol (SRL, 98%) dye was used as the model pollutant for the
degradation study.
Synthesis of mesoporous Eu2O3/TiO2 nanocomposites
Mesoporous europium doped TiO2 materials were synthesized by hydrothermal method
using soft template mechanism with Pluronic P 123 as the structure directing agent. The gel
composition of the synthesis with absolute ethanol as coꢀsolute was 1 Ti(OiPr)4: 18 H2O. The
optimum ethanol/water ratio was maintained as 50:1 for this synthesis method. Triblock
copolymer Pluronic P 123 (1g) was added to absolute ethanol (30 mL). After stirring for 4 h, a
clear solution was obtained. Titanium (IV) isopropoxide (3 mL) was added to the above clear
solution and stirred for 2 h at room temperature. Then water (18 mL) was added to the mixture
and the stirring was continued for 24 h at room temperature. The pH of the mixture was
maintained at 3.0 using 0.2M H2SO4. Subsequently, a given amount of Eu(NO3)3.5H2O were
added to the above solution and stirred for 1 h. After this, the resulting suspension was put into
200 ml Teflon sealed autoclave and maintained at 180◦C for 6 h. Finally, the suspension was
washed with deionized water and centrifuged several times to get pH 7. Then samples were dried
at 100◦C in the vacuum oven for 6 h and calcined at 400◦C to obtain Euꢀdoped TiO2 nanopowder.
It was denoted as 0.5 wt % Eu doped TiO2 sample. The different wt% of Eu to TiO2 (0.7 and 0.9
wt% ) were also prepared by the same procedure.
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Synthesis of mesoporous GR- Eu2O3/TiO2 nanocomposites
Graphene oxide (GO) was synthesized from graphite powder by a modified Hummers
◦
method.31 In a typical synthesis, 2.0 g of graphite powder was put into cold (0 C) concentrated
H2SO4 (100 mL). Then, KMnO4 (8.0 g) was added gradually under stirring, and the temperature
◦
of the mixture was kept below 10 C by cooling. The reaction mixture was continued for 2 h
◦
below 10 ◦C. Successively, the mixture was stirred at 35 C for 1 h, and then diluted with 100 mL
of deionized (DI) water. After adding 100 mL of DI water, the mixture was stirred for 1 h, and
was then further diluted to approximately 300 mL with DI water. After that, 20 mL of 30% H2O2
was added to the mixture to terminate the reaction. The mixture released a large amount of
bubbles, and the color of the mixture changed into brilliant yellow. Finally, the mixture was
filtered and washed with 5% HCl aqueous solution (800 mL) to remove metal ions, followed by 1
L of DI water to remove the acid until pH reached to 6. For further purification, the asꢀobtained
graphene oxide was reꢀdispersed in DI water, and after sonication for 1 h, the suspension was
centrifuged and filtered. The resulting solid was dried at 60 ◦C for 24 h.
GRꢀEu2O3/TiO2 nanocomposite was obtained via a hydrothermal method based on
Zhang’s work with a slight modification.32 Briefly, 0.0115g of GO was dissolved in a solution of
DI H2O (80 mL) and ethanol (40 mL) by ultrasonic treatment for 3 h, and then 200 mg of (TiO2
or Eu/TiO2) was added to the obtained GO solution and stirred for another 2 h to get a
homogeneous suspension. The suspension was then placed in a 300 mL Teflonꢀlined autoclave,
and maintained at 120 ◦C for 5 h to simultaneously achieve the reduction of GO and the
deposition of TiO2. Finally, the resulting composite was recovered by filtration, rinsed by DI
water several times, and dried at 120 ◦C for 6 h. It was denoted as 0.5 wt% GRꢀEu2O3/TiO2. In the
similar procedure, 0.7 wt% GRꢀEu2O3/TiO2 and 0.9 wt% GRꢀEu2O3/TiO2 composites were
synthesized when 0.0161 g and 0.0207 g starting material GO were used respectively.
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Catalysts characterization
The Xꢀray diffraction (XRD) patterns of mesoporous TiO2 were recorded on Bruker D2
Phaser desktop Xꢀray diffractometer using CuKα radiation (λ= 1.542 Å) as the Xꢀray source. The
average crystallite size (D) was determined from the broadening of the diffraction peak using the
Scherrer formula D = Kλ / β cosθ where D is the average particle size in (nm), K is the Scherrer
constant (0.89) λ is the wavelength of Xꢀray source, β is the full width at halfꢀmaximum and θ is
the Bragg’s angle. Transmission electron microscopic (TEM) images were taken using a electron
microscope (TECHNAI G2 305ꢀtwin D 905) operated at an accelerating voltage of 200 keV.
Scanning electron microscopic (SEM) pictures were recorded using a scanning electron
microscope with energy dispersive Xꢀray spectrometer (SEMꢀEDAX, Hitachi S 3400 N). The N2
adsorptionꢀdesorption isotherms were measured using Micromeritics ASAP 2020 sorption
analyzer. Prior to analysis, the samples were degassed for 3 h at 300 oC under vacuum (10−5 mbar)
in the degas port of the adsorption analyzer. The specific surface area was calculated by Brunauerꢀ
EmmettꢀTeller (BET) method and the pore size distribution was calculated using adsorption branch
of Barrettꢀ JoynerꢀHalenda (BJH) method. Xꢀray photoelectron spectroscopy (XPS) was recorded
with XPS KꢀAlpha surface analyzer using AlKα as the radiation source operated at 200 W
hemispherical electron energy analyzer. Diffuse reflectance spectra of the synthesized materials
were collected in the scan range 200–800 nm using UV–visible spectrophotometer (Shimadzu
model 2450) equipped with an integrating sphere and BaSO4 was used as the reference. The FTꢀIR
spectra were recorded on PerkinElmer FTꢀIR spectrometer using KBr pellet technique. The pellet
was scanned at 4 cm−1 resolution in the range of 4000ꢀ400 cm−1. Photoluminescence (PL) spectra
were recorded on PerkinElmer LSꢀ45 model fluorescence spectrometer using Xenon lamp as the
excitation source with 10 nm slit width and photomultiplier tube as the detector.
Photocatalytic degradation studies and analysis
The visibleꢀlight photocatalytic efficiency of the synthesized catalysts were investigated
by studying the degradation of 4ꢀ chlorophenol (4ꢀCP) aqueous solution. Photocatalytic
degradation was performed in aqueous medium in a slurry batch visibleꢀlight photoreactor. A
cylindrical photochemical reactor of 30 cm × 2 cm (height × diameter) with a water circulation
arrangement to maintain the temperature in the range 25–30 oC, was used in all the experiments. A
300 W tungsten Xenon lamp built into a housing with polished anodized aluminum reflectors was
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used as the visibleꢀlight source and placed 6 cm away from the reactor tube. These lamps emit
predominantly in the visible region at a wavelength above 410 nm with an intensity of 17
mW/cm2. The light intensity was measured using LXꢀ101 digital lux meter.
4ꢀchlorophenol stock solution containing 1000 mgL−1 was prepared in triple distilled water and
diluted to the required concentration. In a typical procedure, 100 mg of the catalyst was added to
100 mL 4ꢀchlorophenol solution of 30 mg L−1 in a glass tube and oxygen was bubbled into the
solution throughout the experiment. The resultant slurry was stirred for 30 min to attain
equilibrium. It was then irradiated with visibleꢀlight of wavelength above 410 nm with continuous
purging of air free from CO2. An aliquot of 5 mL of samples were withdrawn at the specific time
intervals of 30 minutes and analyzed after centrifugation followed by filtration with 0.2 ꢁm
membrane to remove the catalyst particles. The photocatalytic activities were evaluated by
detecting 4ꢀchlorophenol at the wavelength of 280 nm using UVꢀ Visible spectrophotometer
(Elico SL 218 double beam). The extent of mineralization of 4ꢀchlorophenol was followed by the
decrease in TOC value with irradiation time using a total organic carbon analyzer (Analytikjena
multi N/C 2100). The degradation products, obtained during the photocatalytic degradation were
analyzed using GC–MS (Jeol GCMS GCꢀMate II). The GC–MS conditions were as follows: DPꢀ
◦
1 (100% dimethylpolysiloxane) column, helium as carrier gas, initial temperature 80 C,
◦
◦
increment of 10 C/min, injection port temperature 220 ◦C and detector temperature 250 C.
Adsorption study
The extent of adsorption of 4ꢀchlorophenol on the various synthesized catalysts were
studied by mixing 100 ml of aqueous solution of 4ꢀchlorophenol (30 mg/L) with 0.1 g of the
catalyst under dark condition. The solution was kept in stirring to have uniform concentration
throughout the samples. All the experiments were carried out under identical conditions. Aliquots
were withdrawn at specific time intervals of 10 minutes and the change in 4ꢀchlorophenol
concentration was measured using UVꢀVisible spectrophotometer. The extent of adsorption was
determined from the decrease in 4ꢀchlorophenol concentration as Ct/C0. Where C0 is the initial
concentration of 4ꢀchlorophenol and Ct is the concentration of 4ꢀchlorophenol at time t.
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Results and discussion
Catalysts characterization
The crystal structures of the synthesized bare and GRꢀEu2O3 coꢀdoped TiO2 samples were
analyzed using XRD. As shown in Fig.1a, all the patterns showed characteristic diffraction peaks at
2θ = 24.80, 37.78, 48.10, 53.89, 55.12, 62.86 and 68.85○ which could be perfectly indexed to the
(101), (004), (200), (105), (211), (204) and (116) crystal faces of anatase phase of TiO2 (JCPDS No.
21ꢀ1272). The average crystallite size of the synthesized materials calculated using the Scherrer’s
equation from the (101) crystal plane of anatase phases were 14.9, 12.3, 14.3, 10.8, 9.9 and 8.5 nm
for bare meso TiO2, 0.7 wt% Eu2O3ꢀTiO2, GRꢀTiO2, 0.5, 0.7 and 0.9 wt% GRꢀEu2O3/TiO2
respectively. Since Eu2O3 crystals on TiO2 lattice surface suppreseed the crystallite growth of TiO2
nanoparticles, the average crystallite size was found to decrease with an increase of Eu loading. The
ionic radius of Eu3+ is 0.95 Å which is much larger than that of Ti4+ (0.68 Å ), hence Eu3+ could not
replace Ti4+ in TiO2 lattice.33 It can be seen that the diffraction peak of anatase TiO2 was not shifted
to higher or lower angle after europium doping. The diffraction peaks of Eu2O3 were not found
since the dopant concentration was very low. Fig. 1b shows the enlarged diffraction peak (002) for
graphene at 25.07○ and anatase (101) peak at 24.80○ for both bare meso TiO2 and 0.7 wt% GRꢀTiO2
samples. Since the (002) diffraction peak of graphene was exactly overlaping with the (101)
diffraction peak of TiO2, a small shoulder peak could be observed in the composite sample which
revealed the presence of graphene.34,35
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Fig. 1 XRD patterns of (a) meso TiO2, 0.7 wt% Eu2O3ꢀTiO2, 0.7 wt% GRꢀTiO2, 0.5, 0.7 and 0.9
wt% GRꢀ Eu2O3/TiO2 samples. (b) enlarged (002) peak for graphene and anatase (1 0 1) peak for
both bare meso TiO2 and 0.7 wt% GRꢀTiO2 samples.
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The HRTEM images clearly demonstrated the morphology and structure of the
synthesized samples. The HRTEM images of mesoporous TiO2 and 0.7 wt% GRꢀEu2O3/TiO2
catalysts were recorded and shown in Fig. 2. The graphene layer, the spherical nanostructure of
TiO2 nanoparticles and the particle size distributions in the range of 9ꢀ15 nm were seen in the
TEM images. These values are consistent with the typical crystallite sizes calculated from XRD
results. The well resolved lattice fringes were observed in the magnified images of meso TiO2 and
0.7 wt% GRꢀEu2O3/TiO2 catalysts shown in Fig. 2b and d which revealed the crystalline nature of
the samples. In Fig. 2d, the lattice fringes with 0.354 and 0.314 nm spacings are assigned to the
tetragonal anatase phase of TiO2 (101) and cubic form of Eu2O3 (222) planes respectively. This
indicates that europium nanoparticles are on the TiO2 surface, not incorporating into the lattice of
TiO2. The graphene layers could serve as a supporting material for in situ growth of anatase TiO2
nanocrystals and also suppress the particles aggregation.36 This kind of interfacial interaction
would facilitate the transfer of photoinduced electrons from TiO2 to graphene, which improve the
charge transfer efficiency.
Fig. 2 HRTEM images of (a) meso TiO2, (b) magnified image of meso TiO2 (c) HRTEM image
of 0.7 wt% GRꢀEu2O3/TiO2 (d) magnified image of 0.7 wt% GRꢀEu2O3/TiO2.
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SEM images generally give information about the shape, size and EDAX tells about the
composition of the synthesized catalysts. The SEM images of meso TiO2 and 0.7 wt% GRꢀ
Eu2O3/TiO2 are shown in Fig. 3a and b. The large aggregates of spherical shapes with different
particle sizes are seen in the SEM images. The composition of the 0.7 wt%
GRꢀEu2O3/TiO2 sample is determined by energy dispersive Xꢀray spectroscopy (EDAX)
experiments. Fig. 3c showed the obvious signals for C, O, Ti and Eu. The EDAX analysis results
showed the atomic percentages of C, O, Ti and Eu are 4.34, 70.75, 24.85 and 0.07% respectively.
Fig. 3 SEM images of (a) meso TiO2, (b) 0.7 wt% GRꢀ Eu2O3/TiO2 and (c) EDAX spectrum of
0.7 wt% GRꢀ Eu2O3/TiO2 samples.
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The textural properties of meso TiO2, 0.7 wt% Eu2O3ꢀTiO2, 0.7 wt% GRꢀTiO2, 0.5, 0.7 nd
0.9 wt% GRꢀEu2O3/TiO2 were obtained from N2 adsorptionꢀ desorption analyses are shown in
Fig. 4A . All the obtained isotherms belong to type IV with H2 hysteresis loop at higher relative
pressure, which confirmed the characteristics of mesopores. Type H2 hysteresis loop indicates
that pore structure is made up of interconnected networks of different size and shape. Table 1
summarizes the crystallite size, specific surface area, pore volume and pore diameter of various
samples. The average pore sizes were obtained from the maximum of pore size distribution curves
for all the synthesized samples. The specific surface area of the photocatalysts are varied in the
region of 91 to 122 m2/g. Among all the prepared photocatalysts, BET surface area of
0.7 wt%
GRꢀEu2O3/TiO2 catalyst was found to be maximum of 122 m2/g. A greater specific surface area of
photocatalysts can supply more surface active sites and make charge carriers transport easier.
Thus, graphene may play a role in enhancing the photocatalytic activity. The pore size distribution
was investigated by adsorption branch of the BarrettꢀJoynerꢀHalenda (BJH) method is shown in
Fig. 4B and inset. Particle size decreases with an increase of dopant concentration, hence the
surface area increased upto 0.7 wt% GRꢀEu2O3/TiO2. At higher dopant concentration though
particle size decreases, the well dispersed Eu2O3 and graphene may cover the surface sites of TiO2
and thus obstruct the pores of the TiO2, resulting in a decrease of the specific surface area.
Table 1 Physicoꢀchemical and textural properties of meso TiO2, 0.7 wt% Eu2O3ꢀTiO2, 0.7 wt%
GRꢀTiO2, 0.5, 0.7 and 0.9 wt% GRꢀEu2O3/TiO2
Catalysts
Crystallite
size (nm)
Bandgap
(eV)
Specific surface
area (m2/g)
Pore volume
(cm3 /g)
Pore diameter
(nm)
meso TiO2
14.9
11.3
14.3
10.8
9.9
3.16
3.06
3.10
3.01
2.90
2.80
91
0.45
0.47
0.46
0.53
0.56
0.54
13.4
13.6
14.8
10.8
15.4
13.6
0.7 wt% Eu2O3ꢀTiO2
0.7 wt% GRꢀTiO2
98
105
116
122
118
0.5 wt% GRꢀEu2O3/TiO2
0.7 wt% GRꢀEu2O3/TiO2
0.9 wt% GRꢀEu2O3/TiO2
8.5
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Fig. 4 (A) N2 adsorptionꢀdesorption isotherms of meso TiO2, 0.7 wt% Eu2O3ꢀTiO2, 0.7 wt% GRꢀ
TiO2, 0.5, 0.7 and 0.9 wt% GRꢀEu2O3/TiO2. (B) Pore size distribution curves of meso TiO2, 0.7
wt% Eu2O3ꢀTiO2, 0.7 wt% GRꢀTiO2 and inset figure shows (a) 0.5, (b) 0.7 and (c) 0.9 wt% GRꢀ
Eu2O3/TiO2.
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The chemical components of the synthesized GRꢀEu2O3/TiO2 samples were analyzed by
Xꢀray photoelectron spectroscopy The XPS survey spectrum of 0.7 wt% GRꢀEu2O3/TiO2
.
(Fig. 5a) catalyst indicates that the sample contains Ti, O, C, and Eu atoms. The C element could
be ascribed to the adventitious carbonꢀbased contaminant, and the binding energy for C 1s peak at
284.5 eV was used as the reference for calibration. The highꢀresolution XPS spectra with scanning
over the area corresponding to the binding energies for the Ti 2p region around 470 eV were
analyzed in Fig. 5b. For Ti 2p, the two peaks observed at 459.5 and 465.2 eV were respectively
assigned to the Ti 2p3/2 and Ti 2p1/2 spinꢀorbital splitting photoelectrons in the Ti4+ chemical
states.37 Fig. 5c shows the O 1s core level spectrum of 0.7 wt% GRꢀEu2O3/TiO2. The shoulder
peak at 529.5 eV can be ascribed to the lattice oxygen in TiꢀO bond on the TiO2 surface. The peak
at 532.1 eV was associated to the surface adsorbed hydroxyl groups38 and residual oxygenꢀ
containing groups in reduced graphene oxides.39 The XPS spectrum for the Eu 3d is shown in Fig.
5d. The characteristic peaks at 1121.1 and 1164.9 eV, which are assigned to Eu 3d5/2 and Eu 3d3/2
respectively. The spin orbital has 43 eV splitting of the Eu 3d doublets, which is a character of
40
Eu3+ anion in the form of Eu2O3.
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Fig. 5 XPS spectrum of 0.7 wt% GRꢀEu2O3/TiO2 nanocomposite. (a) survey spectrum
(b) Ti 2p spectrum (c) O 1s spectrum and (d) Eu 3d spectrum.
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The diffuse reflectance UVꢀVis absorption spectra of meso TiO2 and doped TiO2 are
shown in Fig. 6. The onset of absorption for meso TiO2, 0.7 wt% Eu2O3ꢀTiO2, 0.7 wt% GRꢀTiO2,
0.5, 0.7 and 0.9 wt% GRꢀEu2O3/TiO2 were observed at 392, 404, 400, 411, 420 and 428 nm
respectively. The band gap energy values calculated using KubelkaꢀMunk equation Eg = 1240 / λ
(where λ is the wavelength in nm) based on the onset of absorbance were found to be 3.16, 3.06,
3.10, 3.01, 2.90 and 2.80 eV for meso TiO2, 0.7 wt% Eu2O3ꢀTiO2, GRꢀTiO2, 0.5, 0.7 and 0.9 wt%
GRꢀEu2O3/TiO2 respectively. The optical absorption onset showed a redꢀshift with the increasing
graphene and Eu dopant concentration. Europium ion doping will produce lattice deformation and
form vacancy, which will probably result the impurity states in the band gap of TiO2. The
existence of impurity states can narrow the band gap and improve the absorption.41 The
hybridization of O2p and C2p atomic orbits to form a new valence band above the valence band
of TiO2 because of their similar energy levels reduced the band gap energy on the graphene
loading.42 Graphene incorporation in the composite samples modified the optical properties of
TiO2 similar to TiO2ꢀCNT and Cꢀdoped TiO2.43 Thus, the band gap energy was reduced in the
graphene and europium doped TiO2. Hence, the graphene and europium coꢀdoping improved the
photocatalytic activity of TiO2 under visible light.
Fig. 6 UVꢀ Vis diffuse reflectance absorption spectra of meso TiO2, 0.7 wt% Eu2O3ꢀTiO2,
0.7 wt% GRꢀTiO2, 0.5, 0.7 and 0.9 wt% GRꢀEu2O3/TiO2.
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The reduction of graphene oxide was also confirmed using FTꢀIR measurements. The FTꢀ
IR spectra of pure meso TiO2, graphene oxide, GRꢀTiO2 and 0.7 wt% GRꢀEu2O3/TiO2 are shown
in Fig. 7. All the spectra, the intense broad band observed in the region around 3200–3500 cm−1
was due to OꢀH stretching vibration of surface adsorbed water. The corresponding bending
vibration occurred close to 1625 cm−1. The characteristic peaks of graphene oxide at 1134, 1558,
1705 and 3432 cm−1 were assigned to oxygenꢀcontaining functional groups i.e., CꢀO (alkoxy)
stretching, OꢀH deformation, C=O stretching and O−H stretching respectively.44 In the spectrum
of GR/TiO2 sample, these characteristic absorption peaks especially the CꢀO stretching decreased
dramatically which revealed the significant reduction of graphene oxide. Hence the broad
absorption band between 500ꢀ800 is attributed to the TiꢀOꢀTi stretching vibrations in TiO2 based
composites. The absence of CH2 and CꢀO stretching and bending vibrations in the spectra of bare
meso TiO2 and 0.7 wt% GRꢀEu2O3/TiO2 calcined samples revealed the complete removal of
alcohol and polymer template in the samples.
Fig. 7 FTꢀIR spectra of meso TiO2, graphene oxide, 0.7 wt% GRꢀTiO2 and 0.7 wt% GRꢀEu2O3/TiO2.
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Photoluminescence (PL) emission spectra are commonly used to investigate the efficiency
of the trapping, transfer and separation of charge carriers, within an activated semiconductor
photocatalyst. PL spectra of and pure meso TiO2 and doped composite samples are recorded in the
wavelength range 320ꢀ600 nm are shown in Fig. 8. The emission intensity of 0.7 wt% GRꢀ
Eu2O3/TiO2 was much lower than meso TiO2, 0.7 wt% Eu2O3ꢀTiO2 and GRꢀTiO2, indicating the
recombination of electron and hole was effectively prohibited. It was reported that Eu doping has
the ability to trap the electrons, which can effectively reduce the photogenerated electronꢀhole
pairs.40 The exited electrons are trapped by Eu3+ and transported to the graphene layer. Further,
the graphene layers with a twoꢀdimensional πꢀconjugated structure could serve as an effective
electronꢀaccepting material to transport on the surface thereby the direct recombination of
electronꢀhole pairs in TiO2 was inhibited.
Fig. 8 Photoluminescence spectra of meso TiO2, 0.7 wt% Eu2O3ꢀTiO2, 0.7 wt% GRꢀTiO2 and
0.7 wt% GRꢀEu2O3/TiO2.
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Evaluation of photocatalytic activity of graphene and europium oxide co- doped TiO2
nanoparticles
The photocatalytic activities of different synthesized catalysts were evaluated. The
adsorption profile of the bare meso TiO2, 0.7 wt% Eu2O3ꢀTiO2, 0.7 wt% GRꢀTiO2 and 0.7 wt%
GRꢀEu2O3/TiO2 catalysts are shown in Fig. 9. All the catalysts showed linear increase in
adsorption of 4ꢀchlorophenol for the initial 60 min with different adsorption rate and attained
equilibrium after 60 min. The adsorption of 4ꢀchlorophenol on 0.7 wt% GRꢀEu2O3/TiO2 was
higher than on the other catalysts due to the enhanced surface area with sufficiently large pore
volume and pore diameter. The 0.7 wt% GRꢀEu2O3/TiO2 composite sample has the superior
adsorption capability for 4ꢀchlorophenol than other samples due to giant πꢀ conjugation system
and twoꢀdimensional planar structure of graphene with enhanced surface area. Moreover, the
enhanced adsorption of 4ꢀchlorophenol was driven by the πꢀ π stacking between 4ꢀchlorophenol
and aromatic regions of graphene.45
Fig. 9 Adsorption profile of 4ꢀchlorophenol on various catalysts. Reaction conditions:
4ꢀchlorophenol concentration ꢀ 30 ppm; catalyst concentration ꢀ 0.1 g; in dark under stirring
conditions; in the absence of air flow.
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The photocatalytic degradation rates of 4ꢀchlorophenol using Degussa Pꢀ25, meso TiO2,
0.7 wt% Eu2O3ꢀTiO2, GRꢀTiO2, 0.5, 0.7 and 0.9 wt% GRꢀEu2O3/TiO2 catalysts were evaluated
under visible light irradiation at above 410 nm are shown in Fig. 10. The degradation efficiency of
Degussa Pꢀ25 was lower than bare meso TiO2 due to the enhanced surface area and meso porosity.
The rate was also found to be lower on bare mesoporous TiO2 than europium doped samples. This
is due to the fact that the band gap of TiO2 is wide and visible light cannot be utilized effectively.
On europium doping, a new impurity energy level was formed below the conduction band of TiO2
due to the shift in the light absorption edge to the visible region. Hence europium doped samples
exhibited higher photocatalytic activity under the visible region with enhanced surface area. The
electron trap effect of europium due to the alterable valence states is also playing (Eu3+ and Eu2+)
an important role in inhibiting the electronꢀhole recombination.46 The rare earth elements have the
ability of oxygen storage, hence europium facilitates more adsorbed oxygen on the surface of the
catalyst. The trapped electrons were subsequently transferred to the adsorbed oxygen to form
superoxide radicals by the following process.47
Eu3+ + eꢀ
Eu2+ + O2
Eu2+
ꢀ
O2˙ + Eu3+
Neverthless, GRꢀEu2O3/TiO2 showed higher photocatalytic activity than Eu2O3ꢀTiO2,
since graphene is a competitive candidate as an electron acceptor material due to its
πꢀconjugation structure. The excited electrons in impurity energy state could also be transferred to
graphene and further transferred to adsorbed oxygen to form superoxide radicals.48 Thus, the
simple recombination between exited electrons and holes were greatly inhibited, the redox
probability increased and hence the photocatalytic activity was enhanced for the GRꢀEu2O3/TiO2
composite material. Among all the synthesized catalysts 0.7 wt% GRꢀEu2O3/TiO2 showed the
highest photocatalytic activity. Further increase in the dopant concentration beyond the 0.7 wt%,
the photocatalytic degradation efficiency was decreased due to the lower specific surface area.
The higher concentration of surrounding europium ions influences the activity significantly,
because they will act as recombination centers for photogenerated electrons and holes.18 In
addition, the excessive graphene in composite increases the opportunity for the collision of
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electrons and holes and hence promotes the recombination of the photogenerated electronꢀhole
pairs, thus decreasing the photocatalytic activity.49
Fig. 10 The rate of degradation of 4ꢀchlorophenol using Degussa Pꢀ25, meso TiO2, 0.7 wt%
Eu2O3ꢀTiO2, 0.7 wt% GRꢀTiO2, 0.5, 0.7 and 0.9 wt% GRꢀEu2O3/TiO2 catalysts.
The higher photocatalytic activity of 0.7 wt% GRꢀEu2O3/TiO2 catalyst was also confirmed
by total organic carbon (TOC) analysis. The TOC analysis was performed and the plot of TOC
versus irradiation time is shown in Fig. 11. Among the various synthesized catalysts, the
photocatalytic mineralization of 4ꢀchlorophenol was also found to be high with 0.7 wt% GRꢀ
Eu2O3/TiO2 catalyst.
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Fig. 11 TOC profile of 4ꢀchlorophenol with irradiation time on various catalysts. Reaction
conditions; 4ꢀ chlorophenol concentration – 30 ppm; catalyst concentration ꢀ 0.1 g; visible light
source – tungsten halogen vapour lamp ( λ > 410 nm); in the presence of oxygen flow.
Photocatalytic mechanism
GRꢀEu2O3/TiO2 displayed the highest photocatalytic activity towards 4ꢀchlorophenol
under visible light, which could be ascribed to the synergistic effect of graphene and europium
coꢀdoping. The proposed mechanism for the photocatalytic degradation of 4ꢀchlorophenol on
GRꢀEu2O3/TiO2 catalyst is shown in Fig. 12. Upon irradiation, the electrons excited from the new
valence band of C2p above the valence band of TiO2 due to the graphene loading to the impurity
energy level formed below the conduction band of TiO2 due to europium doping. Since the band
gap was narrowed, more photons were absorbed in the visible region and utilized to generate
exited electrons and holes. The surface electrons can be trapped by the Eu3+ ion to form a
reduction species Eu2+ ions.50 The standard reduction potential of Eu3+/Eu2+ is more negative
.ꢀ
(ꢀ0.43 eV) than the standard reduction potential of O2/O2 (ꢀ0.33 eV). Hence, the Eu2+ could be
easily oxidized back to Eu3+ by the transferring the electrons to the surface adsorbed oxygen.51
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Additionally, lanthanide ions have special electronic structure of 4fx5dy which would be able to
form the labile oxygen vacancies with the relatively high mobility of bulk oxygen species,52 hence
the availability of oxygen on the surface of the catalyst was enhanced by europium loading.
The excited electrons in impurity energy state could also be transferred to graphene in the
nanocomposite via percolation mechanism.20 Graphene could act as electronic conductive
channels and transfer the electrons to the adsorbed oxygen. The excited electrons could be either
directly transferred to graphene or Eu2+ ions coud deꢀtrap the electrons to graphene.53 Therefore,
rapid transport of photogenerated electrons could be achieved and an effective charge separation
is subsequently accomplished. As a result, the electron hole recombination rate was suppressed
.
ꢀ
and more superoxide anion radicals (O2
)
were formed. The produced holes migrated to the
ꢀ
˙
catalyst surface and formed OH radicals with surface adsorbed OH ions. The well reduced
photoluminescence intensity revealed that a better charge separation for GRꢀEu2O3/TiO2
nanocomposites than other catalysts. These superior properties contributed by both europium and
graphene loading greatly inhibited the electronꢀhole recombination, which results in more charge
carriers to form reactive species and promotes the degradation of organic compounds. Thus, coꢀ
doping of europium and graphene synergistically enhanced the photocatalytic activity due to the
high surface area, large shift in the light absorption edge to the visible region and great
suppression in the electronꢀhole recombination.
CO2 + H2O
Eu2+
O2
e-
.
OH
- •
O2
CB
CB
Eu3+
e-
e-
e-
Eu3+
O2
e-
e-
e-
- •
Impurity
energy
level
Visible
light
O2
CO2 + H2O
e-
TiO2
e-
e-
-
e
.
C 2p
e-
VB
OH
e-
e-
h+
e-
h+
h+
VB
OH -
e-
Fig. 12 The proposed mechanism for the photocatalytic degradation of 4ꢀchlorophenol on
GRꢀEu2O3/TiO2.
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Photodegradation products
The photodegradation products formed at 60 min and 150 min irradiations were analyzed
using GC–MS technique. Table 2 summarizes the degradation products and their molecular
weights after 60 min and 150 min irradiation. The intermediate products hydroquinone (1),
benzoquinone (2), 4ꢀchlorocatechol (3), hydroxyhydroquinone (4) were identified at 60 min
irradiated sample. The hydroxybenzoquinone (5) and muconic acid (6) were identified at 150 min
irradiated sample. Based on the intermediate products found and earlier reports on the
products,54ꢀ56 the plausible degradation pathway for 4ꢀchlorophenol was proposed as shown in
Scheme 1. In 4ꢀchlorophenol, the attack of hydroxyl radical to the para position leads to the
formation of hydroquinone (1). It undergoes oxidation and reduction to form benzoquinone (2).
The degradation product 4ꢀchlorocatechol (3) is obtained by the addition of a hydroxyl radical to
the ortho position of the hydroxyl group of 4ꢀchlorophenol. The 4ꢀchlorocatechol (3) is converted
into hydroxyhydroquinone (4) by oxidation with another hydroxyl radical followed by Cl•
abstraction. The hydroxybenzoquinone (5) is formed either by the oxidation followed by reduction
of hydroxyhydroquinone (4) and the hydroxyl radical attack of benzoquinone (2). An attack of
hydroxyl radicals on hydroxybenzoquinone (5) and benzoquinone (2) lead to the ring opening to
form muconic acid (6). The aliphatic compound, muconic acid (6) is further oxidized into the
smaller molecular weight aliphatic compounds and finally mineralized into CO2 and H2O.
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Table 2 The intermediates identified in the photocatalytic degradation of 4ꢀ chlorophenol
Product Retention time MW (m/z)
Structure of
No
(minutes)
intermediates
OH
1
2.8
110
108
OH
hydroquinone
O
2
3
1.65
7.27
O
benzoquinone
OH
OH
144
Cl
4ꢀchlorocatechol
OH
OH
4
5
4.93
3.05
126
124
OH
hydroxyhydroquinone
O
OH
O
Hydroxybenzoquinone
O
HO
OH
6
4.45
142
O
muconic acid
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OH
O
OH
Cl
+2 h+/oxidation
.
ꢀ 2 H+
+ OH
.
+ OH
.
+2 H+
+2 eꢀ/reduction
ꢀ Cl
oxidation
COOH
O
OH
COOH
(2)
(1)
.
+ OH
.
(6)
oxidation
.
+ OH
ꢀ H
.
+ OH
O
OH
OH
+2h+/oxidation
ꢀ 2H +
Further oxidation to
aliphatic intermediates
OH
OH
.
OH
+ OH
.
+2H+
ꢀ Cl
+2eꢀ/reduction
Cl
O
OH
CO2 + H2O
(3)
(5)
(4)
Scheme 1 Plausible pathway for the photocatalytic degradation of 4ꢀ chlorophenol on 0.7 wt%
GRꢀEu2O3/TiO2
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Conclusion
A high photocatalytic degradation efficiency under visible light irradiation has been
achieved over the GRꢀEu2O3/TiO2 nanocomposite synthesized by a hydrothermal method.
Europium and graphene coꢀdoped nanocomposite enhanced the specific surface area, shifted the
light absorption to the visible region and improved the charge transfer efficiency. The optimal
weight percentage of graphene and europium was found to be 0.7 wt %. The results demonstrated
that the unique features of graphene and europium were excellent supporting materials for
semiconductor nanoparticles to separate the photogenerated electronꢀhole pairs. The electron trap
effect, high oxygen availability on surface of the catalyst due to europium and electron accepting
ability of graphene efficiently suppressed the electronꢀhole recombination,which was revealed by
PL study. It can be concluded that both europium and graphene synergistically enhanced the
photocatalytic activity of TiO2 nanoparticles for the degradation of 4ꢀchlorophenol under visible
light irradiation.
Acknowledgments
The authors gratefully acknowledge the Department of Science and Technology, New
Delhi (Ref. No.SR/FT/CS ꢀ14/2011) for the financial support under Fast Track Scheme for young
scientists.
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References
1. X. Chen and S. S. Mao, Chem. Rev., 2007, 107, 2891ꢀ2959.
2. Y. Liu, L. Chen, J. Hu, J. Li and R. Richards, J. Phys. Chem. C, 2010, 114, 1641ꢀ1645.
3. K. Li, X. Yang, Y. Guo, F. Ma, H. Li, L. Chen and Y. Guo, Appl. Catal. B: Environ., 2010,
99, 364 ꢀ375.
4. B. Hamal and K. J. Klabunde, J. Colloid Interface Sci., 2007, 311, 514 ꢀ522.
5. M. Li, S. Zhou, Y. Zhang, G. Chen and Z. Hong, Appl. Surf. Sci., 2008, 254, 3762ꢀ3766.
6. J. Wang, D. N. Tafen, J. P. Lewis, Z. Hong, A. Manivannan, M. Zhi, M. Li and N. Q. Wu,
J. Am. Chem. Soc., 2009, 131, 12227ꢀ12290.
7. S. D. Shen, Y. Deng, G. B. Zhu, D. S. Mao, Y. H. Wang, G. S. Wu, J. Li, X. Z. Liu, G. Z. Lu
and D. Y. Zhao, J. Mater. Sci., 2007, 42, 7057ꢀ7061.
8. M. M. Mohamed, W. A. Bayoumy, M. Khairy and M. A. Mousa, Micropor. Mesopor.
Mater., 2006, 97, 66 ꢀ77.
9. L. Chen, B. Yao, Y. Cao and K. Fan, J. Phys. Chem. C, 2007, 111, 11849ꢀ11853.
10. J. H. Pan and W. I. Lee, Chem. Mater., 2006, 18, 847ꢀ853.
11. S. Yuan, Q. Sheng, J. Zhang, H. Yamashita and D. He, Micropor. Mesopor. Mater., 2008,
110, 501ꢀ507.
12. X. S. Li, G. E. Fryxell, M. H. Engelhard and C. Wang, Chem. Commun., 2007, 10, 639ꢀ641.
13. M. J. Capitan, P. Malet, M. A. Centeno, A. MunozꢀPaez, I. Carrizosa and J. A. Odriozola,
J. Phys. Chem., 1993, 97, 9233ꢀ9240.
14. J. Lin and J. C. Yu, J. Photochem. Photobiol. A: Chem., 1998, 116, 63ꢀ67.
15. J. P. Shoffner, Anal. Chem., 1975, 47, 341ꢀ343.
16. K. T. Ranjit, I. Willner, S. H. Bossmann and A. M. Braun, Environ. Sci. Technol., 2001, 35,
1544
17. Y. Xie, C. Yuan, X. Li, Mater. Sci. Eng. B, 2005, 117, 325–333.
18. S. Yao, C. Sui and Z. Shi, J. Rare Earth., 2011, 29, 929ꢀ933.
19. P. HaroꢀGonza´ lez, M. Pedroni, F. Piccinelli, L. L. Martı´n, S. Polizzi, M. Giarola, G.
Mariotto, A. Speghini, M. Bettinelli and I. R. Martı´n, J. Lumin., 2011, 131, 2473ꢀ2477.
20. H. Zhang, X. Lv, Y. Li, Y. Wang and J. Li, ACS Nano, 2010, 4, 380ꢀ386.
21. A. Peigney, C. Laurent, E. Flahaut, R. R. Bacsa and Rousset, Carbon, 2001, 39, 507ꢀ514.
22. M. D. Stoller, S. Park, Y. Zhu, J. An and R. S. Ruoff, Nano Lett., 2008, 8, 3498ꢀ3502.
29