Magnetization of TiO2/Reduced Graphene Oxide Nano Photocatalyst

Yousef Nazari1, Shiva Salem

2†

1MSc. Student of Chemical Engineering, Urmia University of Technology, Iran

2Associate Professor of Chemical Engineering Faculty, Urmia University of Technology, Iran

Abstract. From the practical point of view, titanium dioxide (TiO2) has limited applications in wastewater

treatment because of low photocatalytic purification a long with difficult separation from aqueous system. In

this research study, the photocatalytic activity and recycling of TiO2, synthesized by sol-gel method, has been

improved by combined application of Fe3O4 and reduced graphene oxide (RGO). X-ray powder diffraction

(XRD), scanning electron microscopy (SEM), and magnetization measurement have been utilized to

characterize the sample. Cationic dye degradation of magnetic TiO2/RGO has been evaluated by UV

spectroscopy. The results indicate the enhanced photocatalytic activity of composite which is attributed to the

decrease in the migration of photo-generated charge carriers to the interlayer by reduced graphene oxide. The

superparamagnetism of composites provides a convenient route for separation of the catalyst from the

reaction mixture by an external magnet.

Keywords: Nano photocatalyst, Magnetization, Titanium dioxide, Reduced graphene oxide, Recovery

1. Introduction

In last decade, advanced oxidation processes have increasingly applied to decrease the environmental

toxicity. Among advanced oxidation processes the photocatalytic degradation of pollutants in the wastewater

has found great interest [1]. Photocatalyst generates powerful oxidizing hydroxyl radicals which completely

destroy the pollutants in wastewater [2].

Owing to its desirable properties such as non-toxicity, weaving performance, corrosion resistance and

superior photocatalytic activity, TiO2 is perhaps the most prospecting photocatalyst in a variety of different

semiconductors [3], [4]. TiO2 has a wide band gap i.e., 3.2 eV hence can be activated under UV irradiation.

However the ultraviolet radiation occupies approximately 5-6% in day light source, which limits the

applicability of TiO2 as photocatalyst. In addition to the photocatalytic efficiency of single phase anatase was

seriously limited due to the rapid recombination of photo-generated electrons (e−) and holes (h+) pairs [5]. In

order to enhance the activity, many efforts have been presented. Ion doping is one of the techniques to

overcome such limitation and have been extensively studied. Doping TiO2 with metals ions like Fe, Sb, Co,

etc., can extends the absorption spectrum to infrared radiation i.e., to visible light spectrum (400 nm -800

nm), where the day light source could be utilized effectively [6]-[10]. In order to enhance the the

photocatalytic activity, many efforts have also been presented for coupling anatase with other

semiconductors to form nanocomposite. The strategy of coupled nanocomposites has been proved to prevent

the rapid recombination of photo-generated electrons and holes pairs. The appropriate matching in the

photocatalytic systems drives the electrons from one particle to neighbors as a result, electrons and holes

separation occurs, steadily. Thus, the interaction between components in the nanocomposite plays an

important role in the photocatalytic activity [11].

Graphene which is a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D)

honeycomb lattice has recently drawn much attention in photocatalysis technology due to its superior

† Corresponding author, Tel.: + (98443198249); fax: + (984431980251).

E-mail address: (s.salem@che.uut.ac.ir).

International Proceedings of Chemical, Biological and Environmental Engineering, V0l. 102 (2017)

DOI: 10.7763/IPCBEE. 2017. V102. 9

50

electrical conductivity, high specific surface area, and chemical stability. The enhanced photocatalytic

activity of semiconductor/graphene composites has been reported in many applications such as hydrogen

production [12], bacteria degradation [13], and organic pollutant elimination [14]. TiO2 nanorod combining

reduced graphene oxide composite has been synthesized by the sol–gel method. The application of

chloroform in the two-phase hydrothermal system can effectively hinder the aggregation of products and

plays a crucial role in the enhancement of photocatalytic performance and adsorption capacity [15], [16].

From the practical point of view, very small particle size is not favourable since it is difficult to be

separated from treated effluent. On the other hand, many researchers immobilized TiO2 over different

supporters e.g., glass, ceramics, polymers, etc. [17]-[19].The immobilized TiO2 is easy to be separated but

suffers from very low surface areas and difficult distribution inside photoreactor which limits its activity.

TiO2 supported on magnetic powder provides a good solution because of its sufficient surface area along

with magnetic core which facilitates separation using a magnetic separator [20]-[22].

Although great efforts have been recently made to modify the photocatalytic activity and recycling of

TiO2 [23], [24] however there are few papers which focus on promotion of both performance

contemporaneously. Zhang et al. were reported a new type of multifunctional TiO2–graphene nanocomposite

hydrogel and magnetic graphene with a facile one-pot hydrothermal approach and explored its environmental

applications as a photocatalyst. During the hydrothermal reaction, the graphene nanosheets and TiO2

nanoparticles self-assembled into three-dimensional interconnected networks, in which the spherical

nanostructured TiO2 nanoparticles with uniform size were densely anchored on to the graphene nanosheets

[25].

In this research study, both photocatalytic activity and recycling of magnetic TiO2/RGO, synthesized by

sol-gel method, has been evaluated and methylene blue (MB) as a cationic dye has been degraded under

sunlight irradiation.

2. Experiment

Chemicals mentioned hereafter were of reagent grade and used as received. All was reported in Table 1.

Table 1: Raw materials specification

Material name Formula Molar mass Producer Purity

Tetrabutoxytitanium Ti(OCH2CH2CH2CH3)4 340.32

Merck

97

Ethanol C2H5OH 47.07 99

Hydrochloric acid HCl 36.46 37

Ammonia NH3 17.03 25

Iron(III) nitrate nonahydrate Fe (NO3)39H2O 403.95 99

Ethylene glycol HOCH₂CH₂OH 62.07 99

Nitric acid HNO3 63.01 65

Toluene C7H8 92.14 99

3-Aminopropyltrimethoxysilane C6H17NO3Si 179.29 Sigma

Aldrich 97

2.1. Synthesis of Reduced Graphene Oxide

Graphene oxide was prepared from natural graphite powder using a modified Hummer method [11].

Then, 0.2 g of graphene oxide was loaded into a 100 mL beaker, and 40 mL of blackberry juice was added,

resulting in an inhomogeneous dispersion. This dispersion was then stirred for 6 h. After stirring process, the

dispersion was sonicated for 120 minutes and the resulting suspension was filtered. The obtained powder

was washed for several times with deionized water. Finally sample was dried in oven for 24 h at 65⁰C.

2.2. Synthesis of Nano-TiO2

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To prepare pure TiO2 tetrabutoxytitanium was added into the ethanol and stirred at 37°C. The level of

pH was controlled at 7 by adding appropriate amounts of hydrochloric acid and ammonia solution. The

obtained product was dried at 80°C overnight before calcination. The TiO2 powder was achieved after

calcination at 400°C for 2 h.

2.3. Synthesis of Nano-Fe3O4

The Fe3O4 was synthesized by modified sol–gel method using Ethylene glycol and Iron (III) nitrate

nonahydrate which were added to a flask, incubated at 40°C and stirred for 2h. Thereafter, the suspension was

heated at 80°C to form a brown gel. The viscous gel was dried at 150°C for 20h and then milled and finally

calcined at 400°C for 2h.

2.4. Synthesis of Nano Fe3O4@TiO2

Synthesized Fe3O4 was mixed with distilled water and ultrasonicated for 30 min to form a uniform

solution (Solution I). Then tetrabutoxytitanium and ethanol were mixed and HNO3 was added under stirring

to reach a uniform solution (Solution II). Subsequently, solution II was added dropwise into solution I with

stirring until pH 6-7 then the mixture was heated to 100°C under stirring for 3 h to complete the reaction.

The sample was finally filtered, washed with water and dried.

2.5. Synthesis of Nano Magnetic TiO2/RGO

Fe3O4@TiO2 particles were firstly modified by surface grafting of aminopropyltrimethoxysilane (APS);

the mixture of Fe3O4@TiO2, APS and toluene was refluxed for 24 h under nitrogen atmosphere, and then

centrifugalized to get surface amino-functionalized Fe3O4@TiO2 particles. Secondly, the deionized aqueous

dispersion containing the surface amino-functionalized Fe3O4@TiO2 particles and RGO was ultrasonicated at

pH 5 for 2 h. The hybrid photocatalyst of Fe3O4@TiO2/RGO oxide was obtained by centrifugation.

2.6. Photocatalytic Degradation Performance

The MB degradation was evaluated for studding the photocatalytic performance of magnetic TiO2/RGO

under sunlight irradiation. First, the MB solution was prepared by mixing dye with deionized water (3.0 mg l−1) then the photocatalysts (12.0 mg) were added to prepared solution (25 ml). Prior to sunlight degradation,

the suspensions were stirred for 30 min in dark space for adsorption–desorption equilibrium. The bright blue

color of the solutions was gradually vanished, indicating the degradation of cationic dye. The variation of

MB concentration with time, 5–60 min, was monitored spectrophotometrically (UV–vis spectrophotometer,

Agilent, USA) at a wavelength range of 400–700 nm.

The dye degradation is presented as a conversion, 1−(C/C0), where C0 and C are the concentration of MB

at dark condition after adsorption–desorption equilibrium and the concentration of dye at different irradiation

times, respectively.

2.7. Characterization

The crystalline structures were characterized by X-ray diffraction (XRD) analysis on a Brucker X-ray

diffractometer (Model X/Pert pro, Philips, Netherland) at 40 kV and 30 mA with Cu-Kα radiation. The

samples were scanned at a rate of 0.02°s−1 in the 2θ range of 10–70°. The morphology of particles was

observed by scanning electron microscopy (SEM, Leo 1430 VP-Germany) on gold coated surfaces. The

magnetic measurements of the Fe3O4 and as-prepared Fe3O4@TiO2/RGO composite were recorded on a

superconducting quantum interference device magnetometer at 300 K.

2.8. Results and Discussion Fig.1 indicates the X-ray diffraction (XRD) patterns of TiO2 and magnetic TiO2/RGO composite. The

main diffraction peaks of (101), (004), (200), (211), and (204) reflections are observed in TiO2 sample,

which can be indexed as anatase (JCPDS card no. 02-0406), corresponding to 25.3, 37.8, 48.0, 55.1, and

62.7°. Compared with that data of JCPDS no. 01-11111 the (220), (311), (222), (400) and (422) peaks of

Fe3O4 can be found at XRD pattern of magnetic TiO2/reduced graphene oxide, suggesting that a Fe3O4/TiO2

composite are successfully fabricated. The diffraction signal at 27.5° due to the rutile phase (110) is not

52

observed in TiO2 powder. The fact that no peaks of other phases or impurities are observed in the XRD

patterns, implying the high purity of the TiO2 crystalline phase synthesized in this work.

The apparent crystallite size, D, of TiO2 and Fe3O4 were determined using the Scherrer formula [7] and

reported at Table 2:

D = (0.9λ)/(βcosθ) (1)

Where β is the breadth of observed diffraction peak at its half-intensity, θ is the Bragg angle, and λ is the

radiation wavelength, 1.5406 nm.

As seen in Table 2, the average crystal size of TiO2 sharply decreased from 16.9 to 4.7 by using reduced

graphene oxide and Fe3O4. This phenomenon could be ascribed to the heat distribution and prevention of

crystal growth resulted by well dispersion of reduced graphene oxide sheets in Fe3O4@TiO2 composite.

Fig. 1: XRD patterns of TiO2 and magnetic TiO2/reduced graphene oxide composite

Table 2: Crystallite size of anatase and Fe3O4 formed on magnetic TiO2/RGO composite

Sample Anatase crystallite size Fe3O4 crystallite size

Pure TiO2 16.9 -

TiO2/RGO composites 4.7 32.2

SEM analysis was used to elucidate the morphology and structural features of TiO2 and magnetic

TiO2/RGO nanocomposite (Fig. 2a and b). The graphene oxide sheets represent fully exfoliated and flaked

structure (Fig. 2a). SEM images of magnetic TiO2/RGO show that spherical Fe3O4@TiO2 nano-particles

were attached on the surface of reduced graphene sheets (Fig. 2b). Graphene layers with high surface area

distribute the heat and prevent the aggregation of Fe3O4@TiO2 nano-particles. Satisfactory distribution of

reduced graphene sheets led to less aggregation, great surface area, and uniform pores distribution.

Fig. 2: SEM images of TiO2 (a) and magnetic TiO2/reduced graphene composites (b)

a b

53

Fig. 3: Magnetization curves of Fe3O4 (a) and magnetic TiO2/RGO nano-composite (b)

Fig. 4: UV spectra of MB solution (3 mg L

-1 ) degraded in the presence of TiO2 (a) and magnetic TiO2/RGO

nanocomposite (b) over 0-60 min.

For recovery and reuse of TiO2 nanocatalysts, Fe3O4@TiO2 composite that possess superparamagnetic

behavior at room temperature are preferred. In order to evaluate the magnetic response of the Fe3O4 and

Fe3O4@TiO2/RGO composite to an external field, the magnetization of samples were measured (Fig. 3a and

b

a

a

b

54

b). The magnetization curve of the Fe3O4@TiO2/RGO (Fig. 3b) composite shows that the prepared sample is

superparamagnetic at room temperature. The saturation magnetization (Ms) of Fe3O4 and Fe3O4@TiO2/RGO

were determined to be 89.2 and 4.98 emu g-1

, respectively, which indicate that the saturation magnetization

value drastically decrease in composite powder in comparison to pure Fe3O4 (Fig. 3a).

Fig. 4a and b indicate the typical UV spectra of MB aqueous solutions in the presence of TiO2 and

TiO2/RGO nanocomposite under the sunlight irradiation over 0-60 min. The treated samples show a certain

absorption in the visible light region, ranging from 400 to 700 nm. The intensity of major absorption peaks at

663 nm, corresponding to the MB molecules, decreases with application of nanophotocatalysts. The

decoloration of solution in the presence of magnetic TiO2/RGO nanocomposite (Fig. 4b) is considerably

higher than that of pure TiO2 (Fig. 4a). The dye removal efficiency of TiO2/RGO nanocomposite after 60 min

treatment is around 90%.

3. Conclusion

In summary, a magnetic TiO2/RGO nanocomposite has been successfully fabricated by the sol–gel

technique and the photocatalytic activity of resultant nanophotcatalyst has been investigated in detail. The

anatase formation was of a key importance in ensuring the synthesis of Fe3O4@TiO2/RGO photocatalyst, in

which the Fe3O4@TiO2 nanoparticles were homogeneously dispersed on sheets. The assessment of

photocatalytic results show that the coupling of anatase with graphene oxide ensures the efficient separation

of photogenerated electron–hole pairs, which is prerequisite for the enhanced photocatalytic performance in

comparison to pure TiO2. Meanwhile, separation and recovery of photocatalyst has been easily performed by

using magnetic field. The degradation of MB reaches 90% within 60 min, which exceeds that of pure anatase

under the sunlight irradiation.

4. References

[1] M. Liqun, et al. Synthesis of nanocrystalline TiO2 with high photo activity and large specific surface area by sol–

gel method. Materials research bulletin. 2005, 40.2: 201-208.

[2] A. Fujishima, et al. Titanium dioxide photocatalysis. Journal of Photochemistry and Photobiology C:

Photochemistry Reviews. 2000, 1.1: 1-21.

[3] Y. Yang, et al. Preparation of continuous TiO2 fibers by sol–gel method and its photocatalytic degradation on

formaldehyde. Applied surface science. 2012, 258.8: 3469-3474.

[4] N. Wiwat, et al. Structural characterization and morphology of electrospun TiO2 nanofibers. Materials Science and

Engineering: B. 2006, 131.1: 147-155.

[5] R. Xinshan, et al. Preparation, characterization and photocatalytic application of TiO2–graphene photocatalyst

under visible light irradiation. Ceramics International. 2015, 41.2: 2502-2511.

[6] W. Yanqin, et al. The photoelectrochemistry of transition metal-ion-doped TiO2 nanocrystalline electrodes and

higher solar cell conversion efficiency based on Zn2+

-doped TiO2 electrode. Journal of Materials Science. 1999,

34.12: 2773-2779.

[7] N. R. Lakshmi, et al. Photocatalytic decolourization of basic green dye by pure and Fe, Co doped TiO2 under day

light illumination. Desalination. 2011, 269.1: 249-253.

[8] Ch. Jianhua, et al. Investigation of transition metal ion doping behaviors on TiO2 nanoparticles. Journal of

Nanoparticle Research. 2008, 10.1: 163-171.

[9] Ch. Xiao-qing, et al. Preparation and photocatalytic properties of Fe-doped TiO2 nanoparticles. Journal of Central

South University of Technology. 2004, 161-165.

[10] A. Rossano, et al. Preparation, characterisation, and photocatalytic behaviour of Co-TiO2 with visible light

response. International Journal of Photoenergy. 2008, 17: 1-9.

[11] Sh. Salem, et al. Facile decoration of TiO2 nanoparticles on graphene for solar degradation of organic dye. Solid

State Sciences, 2016, 61: 131-135.

[12] Z. Xiaoyan, et al. A green and facile synthesis of TiO2/graphene nanocomposites and their photocatalytic activity

for hydrogen evolution. International Journal of Hydrogen Energy. 2012, 37.1: 811-815.

55

[13] C. Baocheng, et al. High antibacterial activity of ultrafine TiO2/graphene sheets nanocomposites under visible light

irradiation. Materials Letters, 2013, 93: 349-352.

[14] Z. Donglin, et al. Enhanced photocatalytic degradation of methylene blue under visible irradiation on graphene@

TiO 2 dyade structure. Applied Catalysis B: Environmental. 2012, 111: 303-308.

[15] C. Baocheng, et al. High antibacterial activity of ultrafine TiO2/graphene sheets nanocomposites under visible light

irradiation. Materials Letters. 2013, 93: 349-352.

[16] D. Zhang, et al. Two-phase hydrothermal synthesis of TiO2–graphene hybrids with improved photocatalytic

activity. Journal of Alloys and Compounds. 2014, 572: 199-204.

[17] G. Allah, et al. Treatment of synthetic dyes wastewater utilizing a magnetically separable photocatalyst

(TiO2/SiO2/Fe3O4): Parametric and kinetic studies. Desalination. 2009, 244.1: 1-11.

[18] N. Negishi, et al. Preparation of the TiO2 thin film photocatalyst by the dip-coating process. Journal of Sol-Gel

Science and Technology. 1998, 13.1: 691-694.

[19] J. Krýsa, et al. Photocatalytic degradation rate of oxalic acid on a semiconductive layer of n-TiO2 particles in a

batch mode plate photoreactor Part II: Light intensity limit. Journal of applied electrochemistry. 1999, 29: 429-

435.

[20] Ph. Carlson, et al. Solvent deposition of titanium dioxide on acrylic for photocatalytic application. Industrial &

Engineering Chemistry Research. 2007, 46: 7970-7976.

[21] R. N. Nageswara , and Vibha Chaturvedi. Photoactivity of TiO2-coated pebbles. Industrial & engineering

chemistry research. 2007, 46: 4406-4414.

[22] K. Jung Mi, et al. Novel immobilization of titanium dioxide (TiO2) on the fluidizing carrier and its application to

the degradation of azo-dye. Journal of hazardous materials. 2006, 134: 230-236.

[23] Z. Zhang, et al. One-pot self-assembled three-dimensional TiO2-graphene hydrogel with improved adsorption

capacities and photocatalytic and electrochemical activities. ACS applied materials & interfaces.2013, 5: 2227-

2233.

[24] M. Rezaei and Sh. Salem. Optimal TiO2–graphene oxide nanocomposite for photocatalytic activity under sunlight

condition: synthesis, characterization, and kinetics. International Journal of Chemical Kinetics. 2016, 48: 573-583.

[25] Z. Zhang, et al. Multifunctional magnetic graphene hybrid architectures: one-pot synthesis and their applications

as organic pollutants adsorbents and supercapacitor electrodes. RSC Advances. 2015, 101: 83480-83485.

56