VIETNAM JOURNAL OE CHEMISTRY VOL. 51(5) 594-598 OCTOBER 2013
GRAPHENE TITANIUM DIOXIDE COMPOSITES FOR WASTE WATER TREATMENT
Part 1. SYNTHESIS AND CHARACTERIZATION
Hung Van Hoang'*, Dai Hon Tran^
'Department of Physical Chemistry - Hanoi National University of Education
^Himg Nhan High School - Thai Binh Province Received 25 June 2013 Abstract
Graphene titaniimi dioxide GR/T1O2 nanoconqjosites have been successfully synthesized via a facile hydrothermal reaction using graphene oxide and titanium tetrachloride as startmg materials in an ethanol water solvent. These nanocomposites prepared with different ratios of graphene oxide were characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM), Foiuier transform infrared (FT-IR) spectroscopy and Thermogravimetnc analysis (TGA). The characterization results reveal that titanium dioxide in aU composites exists in anatase form and con^osite composed of 20% graphene possesses the most imiform distribution of TiOi and thermal stable in comparison to other composites.
Keywords: Graphene, Ti02, hydrothermal method.
1. INTRODUCTION
Titanium dioxide (Ti02) has been expected to play more and more important role in solving many serious environmental and pollution challenges due to its good chemical stability, favourable photooxidation power, nontoxicity, and low price [1, 2]. However, there are two bottlenecks to hinder its practical applications. One is that the band gap of Ti02 is 3.2 eV, hence it can absorb only the ulfraviolet light (radiation with wavelength, X < 400 nm), which occupies about 4% of the sunlight. The other drawback comes from the low separation probability of photo-induced elecfron-hole pairs in photocatalysts. Up to now, vanous processes have been proposed via either doping or compound modification to narrow its band gap and enhance the photocatalytic activity in the range of the visible wavelength radiation [1-4],
Graphene, a single aromatic sheet of sp^ bonded carbon, is another allofrope of carbon besides fuUerenes and carbon nano-tube, which can be considered a two-dimensional single atomic layer of graphite. It has been intensively considered due to the unique physical, chemical and mechanical properties. It processes high conductivity at room temperature [5], high specific surface area up to
2630 m g", complex band structure with conduction and valence bands overlapping for a multi-layer graphene and so on [6]. Among the applications of graphene, integrating graphene with other inorganic materials to fabricate composites or hybrids is the focus. Particularly, the composite of Ti02 and graphene has been considered as a potential photocatalyst m the freatment of polluted air and water [7, 8]. GTaphene-Ti02 composites have been successfully fabricated by various ways m recent years. Liang et al. reported that graphene-Ti02 nanocrystals hybrid has been prepared by directly growing Ti02 nanocrystals on graphene oxide sheets. The direct growth of the nanocrystals on graphene oxide sheets was achieved by a two-step method, in which Ti02 was first coated on graphene oxide sheets by hydrolysis and crystallized into anatase nanocrystals by hydrothermal freatment in the second step [9]. Kamat et al. reported that graphene-Ti02 composite is obtained via UV illuminated suspension of graphene oxide -TiOi under noble atmosphere condition, inhibited the UV light as reducer [10]. Liu et al. prepared the self- assembly of Ti02 with graphene composites in the stabilization of graphene in aqueous solution by assistance of anionic sulfate surfactant [11].
In this paper, we report a facile hydrothemul
VJC, Vol. 51(5), 2013
process for preparation of graphene-Ti02 composites with different ratios of graphene to titanium(IV) oxide using precursors of graphene oxide and titamum(IV) chloride, and the characterization of the obtamed materials using X-ray diffraction (XRD), scanning elecfron microscope (SEM), thermogravimetric (TG) and differential thermogravimetric (DTG) analysis.
2. EXPERIMENTAL PART 2.1. Chemicals
TiCU was purchased from Merck and used as received. The graphite (GR) flake has been purchased from Sigma Aldnch. Distilled water (18 Mii) was used, and all other chemicals were analytical grade reagents and used as received.
2.2. Preparation of Graphene Oxide (GO) GO was synthesized from GR by a modified Hummer's method [12]. In a 1000 mL beaker, 4.0 g GR and 2,0 g NaNOj were mixed with 100 mL concenh^ted H2SO4 (98%). The mixture was stirred for 3.0 hours at ambient temperature. During stimng, 5.0 g KMn04 was slowly added to the suspension and the temperature was maintained at ambient temperature. After addition of KMn04, the reaction mixture was then stirred at 30°C for 7.0 hours. Then, 250 mL dilute H2SO4 (5%) was slowly added to the mixture with vigorous stirring. The diluted suspension was again stirred at 90^0 for 4.0 hours. Finally, 50 mL 30% H2O2 was added. The whole reaction mixture was washed by centrifuging with 5% H2SO4 followed by distilled water for 6 to 8 times and filtered to obtain gray GO sheets.
2.3. Hydrothermal preparation of graphene TvOi Suspension of GO was prepared by mixing GO with ethanol under condition of vigorous stirring.
Then the ethanol solution of T1CI4 was slowly added to GO suspension followed by stirring for another 1.0 hour. The mixture was finally fransferred to 200 mL autoclave maintained at 120°C for 24.0 hours.
The resulting composite was obtained by filtering tiie precipitated portion following by washing with distilled H2O and drying at 120"C for 24.0 hours.
The amount of TiCU solution added to GO suspension was varied in order to produce composite samples with different GO/Ti02 weight ratios of 5:95, 10:90, 20:80, 40:60, 60:40 and 80:20, respectively. For convenience, composite samples are denoted as GXTi02 (X is percentage of TiO: in
Hoang Van Hung, et sample corresponding to the values of 5, 10, 20, 40, 60 and 80).
2.4. Characterization
Scanning elecfron microscope (SEM) was performed by a Hitachi S-4800 field emission scanning elecfron microscope at 5 kV. X-ray diffraction (XRD) patterns of the samples were measured on a Bruker-D5005 powder X-ray diffractometer using copper Ka-radiation with X =
1.5406 A. Thermogravimetric analysis (TGA) was carried out on a Shimadzu DTG-60H instrument at a heating rate of lOVmin under air flow.
3, RESULTS AND DISCUSSION 3.1. X-ray DiiTraction
Figure 2 shows the XRD patterns of graphene/
Ti02 composites with different contents of Ti02 ranging from 5 to 80%. XRD patterns exhibits diffraction peaks at 20 ^ 25.3, 37.0, 48.0, 54.0, 55.0 and 62.7" which were all characteristic peaks of anatase phase [13]. It was agreed that rutile form of Ti02 which can only be formed under highly acidic conditions therefore, in our case rutile form was hardly formed [14]. Peaks in patterns are all relatively broad. This result suggests that the Ti02 particles formed on the graphene sheets with small particle size. In comparison to two other forms of Ti02 (rutile and brookite), anatase form has higher forbidden bandwidth and larger specific surface area which would be beneficial to the photocatalytic processes.
The XRD patterns of the graphene/ri02 composites were the same as the XRD pattern of pure Ti02 reported in [15], illusfrating that the existence of graphene did not influence the particles of pure Ti02 fabricated on it [16].
3.2. Morphology Analysis
hi order to get more information of materials, the resultant graphene/Ti02 composites were characterized using scanning elecfronic microscope (SEM). The results are shown in figure 1. From images, one can see that Ti02 particles are spread on surface of graphene sheets. There is a difference m distiibution of Ti02 at different samples where contents of graphene in samples are varied. The particle density of Ti02 on surface of graphene sheets increases as Ti02 content in sample increase.
The most uniform distribution can be seen in Figure Id in which sample is composed of 20% wt. of TiO:.
VJC Vol. 51(5), 2013 Graphene titanium dioxide composites...
In samples with more than 20% wt. of Ti02, tt can uniform distiibution of Ti02 particles on graphene be seen the accumulation of T1O2 particles and non- sheets.
Figure 1: SEM images of GXT1O2 composites: (a) Ti02, (b) G5T1O2, (c) GIOT1O2, (d) G20TiO2, (e) G4OT1O2, (f) G60TiO2 and (g) G8OT1O2
3.3. Thermo-Gravimetric Analysis (TGA) The composition and structure of the graphene/Ti02 composites were further studied by
thermogravimetric analysis. As shown m Figure 3, all the materials show a small amoimt of mass loss at tiie temperature less than 200''C (9; 5; 4; 6; 8.5, 6 and 8 for pure TiO:, G5Ti02, G10TiO2, G20TiOi,
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G40TiO2, G6OT1O2, GSOTiOj, respectively) due to the de-intercalation of H2O from graphene or evaporation in the case of pure T1O2 [17]. For GXTi02 samples, the oxidation of graphene occurs vigorously at the temperature ranging ftom 550°C to 700''C. Most of the graphene mass presented in sample is lost in this range of temperature. However, most of weight loss occurs at under 200''C for pure T1O2 sample.
Hoang Van Hung, el graphene sheets. The uniform distnbution m the G20TiO2 composite makes it more stable in comparison to other GXTi02 composites.
2e/degree
Figure 2: X-ray diffraction patterns of graphene/IiOj composites: (a) G80TiO2, (b) G60TiO2, (c) G4OT1O2, (d) G20TiO2, (e) GlOTiO,
and (f) G5Ti02 Table 1
Samp
o TiOi GSTiO;
GlOTiOi G20TiOj G40TiO2 G60TiOj GSOTiO;
TGAdataofGXTiOi
Weight loss % at temperature (°C) 200
9.0 5.0 4.0 6.0 8.5 6.0 8.0
400 14.0 12.0 12.0 12.0 14.5 U.O 14.0
500 15.5 17.5 18 15 20 14.5 15.5
omposites
Major degr* temperatur
"1
60 600 558 640 636 534 600
Weight ret (%) at 70
O 3 a 85.5 4.0 84.0 66.5 50.5 54 5 76.5 It is interesting that, weight loss of graphene in sample GIOTiOj, m the range from 550°C to 700°C, takes place slower than that of in other GXTiOi samples with less steeper slope. The mass reduction of sample lasts until 790°C, while weight loss of other GXTiOi samples maintams less than 750°C.
The temperature stability of composite may depend on the distribution of TiOi particles on surface of
1 0 0 .
aa.
geo.
Weight loss
0 -
^ " - ^ ^ ^ ^
^ * > ! . f - _
^ ^ - -
V • v\_^.
\v =
V tTemparature (°C)
Figure 3: Thermogravimetnc analysis of of GXT1O2 composites: (a) Ti02, (b) G5Ti02, (c) GIOT1O2, (d)
G20TiO2, (e) G40TiO2, (f) G6OT1O2 and (g) GSOTiOj
4. CONCLUSION
In this paper, we present the preparation and characterization of graphene titanium dioxide nanocomposites from graphene oxide and titanium tefrachlonde using a facile hydrothermal reaction.
In all prepared composites, titanium dioxide exists in composites with characteristic peaks in XRD patterns of anatase form. The results from SEM and TGA show that composite composed of 20%
graphene possesses the most uniform disfribution of T1O2 and thermal stable in comparison to other composites. Therefore, it will be used for the next our study in treating waste water containing some stable orgamc compounds.
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Corresponding author: H o a n g V a n H u n g
D e p a r t m e n t of Physical C h e m i s t r y Faculty of C h e m i s t r y - H a n o i National University of Education
E m a i l : h u n g h v @ h n u e . e d u . v n .
