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Journal of Physics and Chemistry of Solids 69 (2008) 1980–1984

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Low-temperature preparation of anatase titania-coated magnetite

Jingjing Xua,b, Yanhui Aoa,b, Degang Fua,b,�, Chunwei Yuana,b

aSchool of Chemistry and Chemical Engineering, Southeast University. Nanjing 210096, ChinabState Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, China

Received 21 August 2007; received in revised form 9 February 2008; accepted 11 February 2008

Abstract

A composite photocatalyst with an anatase titania shell and a magnetite core was prepared in a novel way at low temperature (75 1C at

most) by coating photoactive titanium dioxide onto a magnetic Fe3O4 core. The photocatalytic activity of the prepared photocatalyst

was evaluated by the degradation of model contaminated water of phenol and compared to single-phase titania (either Degussa P25 or

prepared titania without magnetic Fe3O4). The results showed that the photoactivity was slightly depressed. Then, a remarkable

improvement in photoactivity was achieved by modifying the photocatalyst with a SiO2 layer between the Fe3O4 core and TiO2 shell. The

repetitive using of the modified photocatalyst was also investigated, and experimental results illustrated that the photocatalytic-degraded

ratio of phenol was still higher than 80% after six cycles.

r 2008 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures; B. Sol–gel growth

1. Introduction

In recent years, there has been an extensive interest in theuse of semiconductors as photocatalysts to degrade organiccontaminations [1–4]. As a popular photocatalyst, titaniahas been widely used because of its various merits, such asoptical and electronic properties, low cost, high photo-catalytic activity, chemical stability and non-toxicity [5].However, its practical application seems limited for severalreasons, among which one is the separation problembecause of the small particle size of the photocatalyst,another is the low photon utilization efficiency. To solvethese problems, the modification of these photocatalystshas also been attempted by depositing them onto magneticparticles [6–12]. But the preparation of such type photo-catalysts needed heat treatment, which was very detri-mental to both photoactivity and magnetic properties ofthe photocatalysts.

front matter r 2008 Elsevier Ltd. All rights reserved.

s.2008.02.015

ng author at: School of Chemistry and Chemical

utheast University. Nanjing 210096, China.

36250; fax: +86 25 83793091.

ss: andyao@seu.edu.cn (D. Fu).

In this paper, we report a new method to prepare anatasetitania-coated Fe3O4 (TF) at low temperature. First of all,we synthesized anatase TiO2 sols under mild conditions(i.e. 75 1C and ambient pressure) by hydrolysis of titaniumnbutoxide in abundant acidic aqueous solution. Then, thesols were deposited onto the magnetite which prepared byco-precipitation of iron(a) and iron(b) in the presence ofammonium hydroxide. For enhancement of photoactivityof the prepared photocatalyst, we modified the photo-catalyst with a SiO2 layer between the Fe3O4 core and TiO2

shell (TSF). Then, phenol was chosen as the modelpollutant to determine the photocatalytic activity of thedifferent photocatalysts.

2. Experimental

2.1. Synthesis procedure

Anatase TiO2 was prepared by a sol–gel method at lowtemperature using Ti(OBu)4 as precursor. The detaildescription was as following: Ti(OBu)4 diluted with PrOHwas added drop-wise into water under vigorous stirring,whose acidity was adjusted with HNO3 to 2.5. The molarratios of PrOH to Ti(OBu)4 was 1.42, and the effect of

ARTICLE IN PRESSJ. Xu et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1980–1984 1981

water/Ti(OBu)4 molar ratio on the crystallinity of thetitania was investigated. Then, the solution was keptunder reflux condition (around 75 1C) for 24 h. Finally,pure TiO2 sol was obtained. The sol was used to becoated onto the seed particles of magnetite synthesizedby chemical co-precipitation method. In a typical coatingprocedure, magnetite particles were dispersed in titaniasols in an ultrasonic bath for 1 h. Then, it was driedinto powders in a rotatory evaporator under vacuum at75 1C. The modified photocatalyst was prepared by thesame way except for that SiO2 deposited onto magnetiteparticles before the deposition of titania. In a typicalsynthesis of SiO2 deposited magnetite particles, the startingsolution was prepared by mixing 2ml of ethanol and18ml of water, which was adjusted to pH=2 with HCl,and then adding 1ml of TEOS into the solutionwith vigorous stirring. The TEOS was hydrolyzed bystirring for 30min. A total of 0.3 g magnetite particleswere added into the solution and stirred vigorously to coatsilica onto the surface. The composition of the preparedsamples were determined to be (Fe3O4/TiO2=33:67)and (Fe3O4/SiO2/TiO2=32.1:1.9:66) in TF and TSF,respectively.

Fig. 1. XRD patterns of the titania particles prepared with different molar

ratios of water to Ti(OBu)4: (a) 151, (b) 76, (c) 38.

2.2. Photocatalytic studies

The photoreactor was a glass beaker (500ml capacity)covered by silver paper, and irradiation was provided byultraviolet lamp (UV) (20W, 365 nm). In an ordinaryphotocatalytic test performed at room temperature, 0.6 g ofphotocatalyst was added under stirring into 400ml ofphenol whose concentration was 50mg l�1 and maintainedin the dark for 1 h to reach complete adsorption anddesorption equilibrium before it was illuminated by the UVlamp. Then, samples of the suspension (5ml) were removedat regular intervals of 1 h for analysis.

20

(c)

(b)

(a)

Inte

nsity

/a.u

.

magnetite

anatase

2 theta/degree

30 40 50 60 70 80

Fig. 2. XRD patterns of (a) Fe3O4, (b) TF, (c) TSF.

2.3. Equipments and measurements

The crystalline structure of the prepared samples weredetermined by X-ray diffractometer (XD-3A, ShimadazuCorporation, Japan) using graphite monochromatic copperradiation (Cu–Ka) at 40 kV, 30mA over the 2y range20–801. The morphologies were characterized with atransmission electron microscopy (TEM, JEM2000EX).The magnetic measurements were carried out with avibrating sample magnetometer (VSM, PARR, Model4500). The HPLC system was Agilent 1100 with tunableabsorbance detector adjusted at 270 nm for the detection ofphenol. A reverse-phase column (length, 250mm; internaldiameter, 4.6mm) Aglient Eclipse XDB-C18 was used. Themobile phase was composed of acetonitrile and deionizeddoubly distilled water. The v/v ratio CH3CN/H2O was20/80 and the flow rate was 1mlmin�1. The compositionof the prepared samples was analyzed by ICP-AES(Perkin-Elmer, ELAN9000).

3. Results and discussion

3.1. XRD studies

The influence of molar ratios of water to Ti(OBu)4 onthe crystallinity of titania was investigated and the resultsare shown in Fig. 1. It can be seen that the titania preparedwith molar ratios 151 of water to Ti(OBu)4 shows thehighest crystallinity. XRD patterns of prepared magneticsamples are presented in Fig. 2. Fig. 2(a) shows XRDpattern of the Fe3O4, presenting the characteristic peaks ofcubic-spinel structure. It can also be seen from Fig. 2(b)and (c) that the Fe3O4 maintain cubic-spinel structure. Thisilluminates that the magnetic properties of Fe3O4 arebasically invariable, which is in agreement with VSM

ARTICLE IN PRESSJ. Xu et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1980–19841982

results showed in Fig. 4. The TiO2 coating layer has ananatase structure for the titania-coated magnetite and themodified samples, determined from XRD pattern inFig. 2(c), where the peaks signed by macula are thecharacteristic peaks of anatase structural TiO2. In theXRD plot, there is no signal for SiO2 that can be ascribedto the small amount of SiO2. Our preparation of anatasetitania avoids heat treatment of the composite sampleswhich was found to have a strong influence on thephotoactivity of the prepared samples.

3.2. TEM studies

The direct coating of titania onto the surface of themagnetite particles resulted in the formation of a core—shell type structure, in which most magnetite particles areas the core and the titania as the shell. These particles areshown in Fig. 3(a), from which we can see the mono-dispersity of the particles are very good. The titania coatingwith a thickness of 20–30 nm is believed to havepredominantly occurred through a hetero-coagulationmechanism between the precipitated titanium dioxide,

Fig. 3. SEM micrograph of (a) TF and (b) TSF.

and the seed particles due to a difference in their surfacecharges [7]. The magnetite particles prepared by chemicalco-precipitation method are paramagnetic, which can beseen from the value of remanent magnetization listed inTable 1. So the magnetite particles, which were coated bytitania layer, would not undergo aggregation. A TEMmicrograph of the modified photocatalyst (TSF) is shownin Fig. 3(b). It can be seen from the micrograph that thenanoparticles are aggregated into clusters since the SiO2

deposition.

3.3. Magnetic properties

The magnetic properties of the TF, TSF and Fe3O4 corewere measured by VSM, as shown in Fig. 4. From VSMexperiments, the magnetic parameters such as saturationmagnetization Ms, coercivity Hc and remanent magnetiza-tion Mr are given in Table 1. The decrease of saturationmagnetization Ms in the order of Fe3O4, TF, and TSF isconsistent with their Fe3O4 content in unit weight sample.The low values of Hc and Mr indicate that the preparedsamples exhibit paramagnetic behaviors at room tempera-ture [13]. The paramagnetic behaviors of the prepared TFand TSF make the photocatalyst, which can be separatedmore easily by a magnet or an applied magnetic field. In themeantime, the very low remanent magnetization largelyreduced the aggregation of the catalyst after it wasseparated by applied magnetic field from original reactionsolution, so the photocatalysts can be easily re-dispersed ina solution for re-using.

3.4. Photocatalytic activities

The photoactivity of the prepared photocatalyst wasassessed by applying it to degrade model contaminatedwater of phenol aqueous solution whose initial concentra-tion was 50mg l�1. In the absence of photocatalyst, there is

-4000

-10

-8

-6

-4

-2

0

2

4

6

8

10

(b)

(a)

M/e

mu.

g-1

H/Oe

-2000 20000 4000

Fig. 4. Magnetization vs. applied magnetic field for the different samples:

(a) TF and (b) TSF.

ARTICLE IN PRESS

Table 1

Magnetic parameters of the prepared samples

Sample Ms (emu/g) Hc (Oe) Mr (emu/g)

Fe3O4 80.79 50.25 4.26

TF 9.86 51.11 1.84

TSF 2.17 55.87 0.43

-10

10

20

30

40

50

(d) (c)

(b)

(a)

conc

entra

tion

of p

heno

l/mg.

L-1

time/hour0 1 2 3 4 5

Fig. 5. Photoactivity of four different samples (a) TF, (b) pure titania,

(c) P25 and (d) TSF.

10

20

40

60

80

100

phen

ol/%

cycles

adsorbed degraded

2 3 4 5 6

Fig. 6. Repetitive use of the photocatalyst TSF.

J. Xu et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1980–1984 1983

only 3% of phenol was degraded in 5 h by photolysis.Fig. 5 shows the results of the photoactivity testing. Thisplot shows the changing of concentration of phenol inaqueous solution as the UV irradiation proceeded. Theseresults illustrate two main observations: firstly, the activityof the particles which prepared by direct deposition ofanatase titania onto the magnetite core was lower than thatof single-phase titania samples (either Degussa P25 ortitania prepared by the same way); secondly, a remarkableimprovement in photoactivity was achieved by modifyingthe photocatalyst with a SiO2 layer, through which directcontact between the titania and the magnetite phase can beavoided.

The lower photoactivity of magnetite/titania comparedto the neat titania prepared by the same method could beascribed to the electronic interactions between the twosemiconductors, which has been described by others[8,14–16]. Electronic interactions occur at the point ofcontact of the different phases (heterojunction), leading tothe transfer of charge carriers across this junction, whentwo or more semiconductors are in contact [8]. So thephotogenerated charge carriers in the excited titania can betransferred to Fe3O4 phase because of their lower lyingconduction band and upper lying valence band. Thenarrower band gap of Fe3O4 is also thought to lead to anincrease in the incidence of electron–hole recombinationand subsequently lower the photoactivity of magnetite/titania composite photocatalyst.

The photoactivity of the modified photocatalyst isindeed higher than that of single-phase titania samples(either Degussa P25 or prepared titania) that was differentfrom the result illustrated by Beydoun et al. [8] Theyshowed that the photoactivity of the magnetic titania wasstill lower than that of Deggussa P25 titania even after themodification by applying a SiO2 layer into the intermediatelayer of titania shell and Fe3O4 core. The calcinations ofthe samples at high temperature (450 1C) was the majorcontributing factor for the lower photoactivity of thecomposite photocatalyst prepared by Beydoun et al. Theadsorption kinetics are related to both the chemicalstructure of the organic being degraded and the surfaceproperties of the catalyst, the higher the adsorption activityfor titania, the higher the photooxidation efficiency of theorganic [17]. The calcinations at high temperature wouldlead to the changing surface properties of titania involvingdecrease of surface area and losses of species such ashydroxyl and adsorbed water which dominate the surfacechemistry and adsorption activity of titania [18], and theheat treatment would induce high-Fe-doped titania [19]which had been found to be very detrimental to thephotoactivity of anatase titania.

3.5. Reuse of the photocatalyst

The regeneration of TiO2 photocatalyst was one of keysteps to make heterogeneous photocatalysis technologyfor practical applications. The separation problem of thephotocatalyst has been solved without depressing theactivity of titania by modifying the magnetic photocatalystwith a layer SiO2 between the magnetite core and titaniashell. So, the photocatalyst can be reused easily withoutany mass loss. The results of adsorption ratio anddegradation ratio of phenol in repetitive use of thephotocatalyst are shown in Fig. 6. The sample was usedrepeatedly for six cycles. At the first cycle, most of phenolwas removed from the solution. The adsorption activity

ARTICLE IN PRESSJ. Xu et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1980–19841984

and photoactivity of the catalyst weakened slightly when itwas reused. It can be seen from the plot that the degradingratio was still higher than 80% after the photocatalyst wasused for six cycles.

4. Conclusions

We have illustrated a simple route for successfullyfabricating anatase titania-coated magnetite at low tem-perature through directly depositing anatase titania ontothe magnetite. The photocatalyst, thus, prepared wasapplied to degrade model-contaminated water of phenol.The results show that the photoactivity of the compositephotocatalyst depressed compared to that of single-phasetitania (either Degussa P25 or neat titania prepared by thesame way without depositing onto magnetite). Then,modified photocatalyst was prepared by depositing aSiO2 layer between the magnetite core and titania shell,whose photoactivity was higher than the single-phasetitania. The enhanced photoactivity can be attributed tothe preparing method without heat treatment.

The photocatalyst shows good magnetic properties andcan be separated easily by an external magnetic filed. So,the photocatalyst can be reused without any mass loss. Thedegrade ratio of phenol was still higher than 80% after thephotocatalyst was used for six times.

Acknowledgments

This work was supported by the National NaturalScience Foundation of China (No. 60121101). We are alsovery grateful to Dr. Liu Ji-wei in Southeast University forhis help in VSM experiments.

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