Ultrasonics Sonochemistry 17 (2010) 649–653

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Ultrasonics Sonochemistry

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Ultrasonic synthesis and photocatalytic characterizationof H3PW12O40/TiO2 (anatase)

Jia Lee a,*, Xiaoli Dong a,1, Xuewei Dong b

a School of Chemical and Material Engineering, Dalian Polytechnic University, Dalian 116034, PR Chinab School of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian 116028, PR China

a r t i c l e i n f o

Article history:Received 15 September 2009Received in revised form 9 December 2009Accepted 22 January 2010Available online 28 January 2010

Keywords:UltrasonicAnataseHeteropolyacidSolar photocatalytic degradation

1350-4177/$ - see front matter Crown Copyright � 2doi:10.1016/j.ultsonch.2010.01.009

* Corresponding author. Tel.: + 86 15042432456; faE-mail addresses: bigfacecat22@hotmail.com (Jia

Dong).1 Tel.: +86 41186323508; fax: +86 41186323736.

a b s t r a c t

A novel H3PW12O40/TiO2 (anatase) composite photocatalyst was prepared by a high-intensity ultrasonicmethod using a lower temperature (80 �C) and was characterized by XRD and FT-IR. Its photocatalyticactivity, using solar light, was evaluated through the degradation of organic dye methylene blue (MB)in aqueous. When MB solution (50 mg/l, 200 ml) containing H3PW12O40/TiO2 (anatase) (0.4 g) wasdegraded by solar irradiation after 90 min, the removal of concentration and TOC of MB reached 95%and 73%, respectively. The photocatalyst activity of H3PW12O40/TiO2 (anatase) was much higher thanTiO2 which was prepared in the same way. H3PW12O40/TiO2 remained efficient after five repeatedexperiments.

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1. Introduction

Photocatalytic processes have been receiving much attention,particularly for their complete destruction or mineralization ofthe toxic and non-biodegradable compounds to carbon dioxideand inorganic constituents [1].

In recent years, semiconductor metal oxides (mainly TiO2 in theanatase phase) and heteropolyacid (HPAs) have drawn much atten-tion as the green photocatalysts for environmental protection andremediation [2–9]. For TiO2 photocatalyst, electrons and holes arethe key factors to impact photocatalytic activity, however, becauseof the fast speed of combine again of electron and hole reduce thephotocatalytic activity of TiO2 [10–12]. For HPAs photocatalyst, ex-cited state formation of O ? M charge transfer is the main reasonfor catalytic oxidation. As the effective electron capture agents,HPAs receives electrons which produced by light excite electronicof TiO2, and then extends the time of electrons and holes combina-tion, at the same time maintains the stability of HPAs’s structure[13–15]. So it is an effective way to improve the photocatalyticproperties of the new complex photocatalyst of TiO2 and HPAs.General preparation methods are sol–gel and impregnation [16–19]. But these two general methods have some defects: activation

010 Published by Elsevier B.V. All

x: + 86 41186322228.Lee), dongxl@dlpu.edu.cn (X.

needs high-temperature calcinations, the time for impregnation istoo long.

Research of ultrasonic technology on synthesis of new mate-rials, chemical reaction and wastewater treatment was very ac-tive. Ultrasonic cavitation proves to be a very effectivetechnique in the field of specific materials preparation. The effectof ultrasonic cavitation makes high-temperature locally, highpressure, strong shock waves and micro-jet. Those affects pro-vide a new special physical and chemical environment for chem-ical reaction which is difficult to carry out under generalconditions [20–23].

In this work, we used the ultrasonic method to prepare a newcomplex photocatalyst with H3PW12O40 and TiO2. H3PW12O40/TiO2 compound has anatase phase which is directly synthesizedwithout high-temperature calcination at a lower temperature(80 �C), with high-intensity ultrasonic irradiation. The compoundhas the following characteristic: (a) the combination of the originalH3PW12O40 and the anatase TiO2 basement produced a supportedeffect which increased photocatalytic activity; (b) it can initiatethe photochemical reactions under solar irradiation. Solar photo-catalytic activity of the H3PW12O40/TiO2 was measured by thedecomposition of organic dyes. H3PW12O40/TiO2 exhibited higherphotocatalytic activity than TiO2 which prepared in the samemethod for decomposition of dyes under solar irradiation. Mean-while, the composite catalyst is easy to separate for recycling afterthe reaction, and the efficiency of catalyst remains almost the sameafter five times of activities.

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650 Jia Lee et al. / Ultrasonics Sonochemistry 17 (2010) 649–653

2. Experiments

2.1. Preparation and characterization of photocatalyst

A three-necked flask, filled with distilled water (40 ml), was putin an ultrasonic generator (40 kHz), programmed temperature(30–80 �C), phosphotungstic acid (H3PW12O40) (0.03–0.07 mM)was dissolved with the above distilled water. Then, tetrabutyl tita-nate (Ti(OC4H9)4) (3 ml) mixed with absolute ethyl alcohol(C2H5OH) (6 ml) was added drop by drop. The mixture was contin-ually stirred for 3 h under ultrasonic radiation (0.2 W/cm3). Afterthis reaction, the production was separately washed three timeswith water and absolute ethyl alcohol, and then dried at thermo-stat water bath in 80 �C. The product H3PW12O40/TiO2 is whitepowder. The preparation of TiO2 was via the same method withoutH3PW12O40.

The crystal phase of the obtained materials was examined bylarge-angle powder X-ray diffraction (XRD) measurements re-corded on a Japan Rigaku D/max3B X-ray diffract meter usingCu Ka radiation. The FT-IR spectra were detected in the range4000–400 cm�1 by PE Spectrum One-B FT-IR spectrometer.

2.2. Measurement of photocatalytic activity

The solar catalytic activity of H3PW12O40/TiO2 catalytic wastested with methylene blue (MB) as the target dye. Photodegrada-tion of MB was carried out at atmospheric pressure using air as oxi-dant in the photoreactor. Usually, the photocatalytic procedurewas carried out as follows: catalyst (0.4 g) was suspended in afresh aqueous dye solution (C0 = 50 mg/l, 200 ml). At given inter-vals of solar irradiation (April 2009–June 2009, 09:00–15:00), asample suspension was taken and centrifugalized. The upper clearsolution was analyzed by UV–vis spectroscopy (PE Lambda 35 UV–vis spectrophotometer) and totally organic carbon (TOC) concen-trations (Japan Shimadzu TOC-VCPH).

3. Results and discussion

3.1. Characterization of H3PW12O40/TiO2 photocatalyst

As shown in Fig. 1, H3PW12O40/TiO2 prepared via ultrasonicmethod in 30 �C exhibits weak diffraction peaks, which becamestrong in 70 �C, and much clearer and intensity reached highest

Fig. 1. X-ray diffraction (XRD) patterns for H3PW12O40/TiO2 prepared via ultrasonicmethod in different preparation temperature (30, 50, 70, 80 �C, respectively).

in 80 �C. H3PW12O40/TiO2 (80 �C) had anatase structure with itscharacteristic diffraction peaks of 2h values locating at 25.4(101), 37.8 (004), 47.8 (200), 54.6 (105), 54.96 (221), 63.3 (211)and 69.68 (116), respectively [24,25]. In thermodynamics, crystalsystem must meet the requirements of energy, crystal particlescould be aligned and produced to an orderly structure. In the pro-cess of ultrasonic, as a result of the extremely fast ‘‘hotspot” cool-ing (>1010 K/s), instantaneous temperature could not make thecrystal particles align in an orderly structure. Therefore, the reac-tion temperature is required to reach a certain temperature beforethey can be crystallized [26]. H3PW12O40/TiO2 prepared by ultra-sonic method transformed into anatase substance in 70 �C andsteady-state in 80 �C.

Fig. 2B and C shows the XRD patterns of TiO2 and H3PW12O40/TiO2 prepared via the ultrasonic method using a lower temperature(80 �C) which possess uniform anatase structure. However, con-ventionally, H3PW12O40/TiO2 prepared via sol–gel method wasamorphous (Fig. 2D). The precipitate prepared by sol–gel methodis amorphous in nature, requiring further heat treatment at a high-er temperature to induce crystallization. The thermal process re-sults in particle agglomeration and grain growth, and hence mayinduce unexpected phase transformation [27,28]. At this consider-ably lower temperature (80 �C), the decomposition of the phospho-tungstic acid and the collapse of the pores can be avoided (theKeggin unit begins to decompose at 350 �C [29]). The temperaturefor calcinations of amorphous TiO2 to its anatase structure is at500 �C [30,31], while it needs only 80 �C via the ultrasonic method(Fig. 2C). Because the Ti(OC4H9)4 has a low vapor tension, chemicaleffect of ultrasound occurred on the border area of cavitation bub-ble and aqueous solution, and forms lots of ‘‘hotspots” in the reti-form gel. Those ‘‘hotspots” provide a special environment forsufficient hydrolysis and crystallization of gel [32–34].

Only the anatase phase was present in the XRD of H3PW12O40/TiO2 and no separate phosphotungstic-related phase was observed.This finding indicated that H3PW12O40 (Fig. 2A) was either in theoctahedral interstitial sites or the substitutional positions of TiO2

[35].For H3PW12O40, the Keggin characteristic absorption peaks can

be seen clearly at 1080.29, 982.63, 888.40, 804.55 cm�1 belongingto Vas(P–O), Vas(W–Od), Vas(W–Ob–W), Vas(W–Oc–W), respectively(Fig. 3) [36,37]. For H3PW12O40/TiO2 (anatase), 1065.38 and 959.53belong to Vas(P–O) and Vas(W–Od), Vas(W–Ob–W) and Vas(W–Oc–W) are covered with Ti–O stretching vibration peak, which indi-

Fig. 2. XRD for original H3PW12O40 (A), anatase TiO2 (B) and H3PW12O40/TiO2

(anatase) (C) prepared by ultrasonic method in 80 �C, H3PW12O40/TiO2 (amorphous)(D) prepared by sol–gel method without high-temperature calcination.

Fig. 3. FT-IR spectra for original H3PW12O40, TiO2 and H3PW12O40/TiO2 (anatase).Fig. 5. UV–vis spectral of MB (50 mg/l, 200 ml) changes by H3PW12O40/TiO2 (0.4 g)under solar irradiation.

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cates that the Keggin unit has not been destroyed. Vas(P–O) andVas(W–Od) of H3PW12O40/TiO2 (anatase) blue-shifted slightly andformed a small peak. The blue-shift of this band has been attrib-uted to a distorted tetrahedral coordination environment or theexistence of some titanium species in an octahedral coordinationenvironment [38]. Namely, the introduction of H3PW12O40 inTiO2 framework results in the changes of coordination environ-ment of TiO2 crystalline. H3PW12O40 joins the framework of TiO2,and bonding up with it. This FT-IR spectra result is consistent withthe conclusion obtained from the XRD patterns.

3.2. Studies of solar photocatalytic degradation of MB withH3PW12O40/TiO2

We studied the degradation of MB as a model reaction to inves-tigate the conditions for preparation and activity of H3PW12O40/TiO2. MB is a heteropolyaromatic dye, and the structural formulaof MB is illustrated in Fig. 4. Due to the great absorption coefficientof MB, the absorbance of the high concentration of MB solutioncannot be measured by UV–vis spectrometer, so the MB(C0 = 50 mg/l) solution was diluted 10 times. Then the maximalabsorption wavelength of MB at 664 nm, and the UV–vis spectralwas obtained as shown in Fig. 5. The standard curve of absorptionand concentration was indicated in Fig. 4.

Fig. 4. The standard curve (concentration and absorbance) of MB. Inset shows thestructural formula of MB.

The present photocatalytic tests were operated in an aqueoussolution containing molecule oxygen from air dissolved in thesolution. The changes in the absorbance of MB solution recordedfollowing UV–vis irradiation are shown in Fig. 5. The chromophoricgroup of MB was reduced rapidly at the first 15 min, and decoloredsteadily in 30 min, while there was still persistent group in UV wasreduced slowly. The removal of the solution raised equably be-tween 60 and 90 min, and MB was completely removed after300 min.

Fig. 6 shows the photocatalytic activity of H3PW12O40/TiO2 in-creased as the preparation temperature increased, and the bestclearly effect of degradation MB solution with H3PW12O40/TiO2

which was prepared at 80 �C. The removal of TOC of MB reached79% after 120 min solar irradiation (illustration in Fig. 6). As theFig. 1 shows, H3PW12O40/TiO2 (80 �C) has an obvious anatase phasecrystal structure which has strong catalytic capacity.

Effect of degradation MB by H3PW12O40/TiO2 loading differentoriginal H3PW12O40 is in Fig. 7. The loading has a best value. Whenthe loading was small, active site increases with the growth ofloading, the life of carriers becomes longer, then the photocatalytic

Fig. 6. UV–vis spectral and TOC of MB (50 mg/l, 200 ml) changes by H3PW12O40/TiO2 which prepared in different preparation temperature (30, 50, 70, 80 �C,respectively) under 120 min solar irradiation.

Fig. 7. Effect of degradation of MB (50 mg/l, 200 ml) by H3PW12O40/TiO2 whichprepared in different original H3PW12O40 (0.03, 0.04, 0.05, 0.06, 0.07 mM, respec-tively) under 90 min solar irradiation.

Fig. 9. The change of concentration and TOC (illustration) of MB solution (50 mg/l,200 ml) with H3PW12O40/TiO2 (0.4 g, ultrasound, 80 �C, 0.05 mM originalH3PW12O40) composite and TiO2 (ultrasound, 80 �C) under 120 min solarirradiation.

652 Jia Lee et al. / Ultrasonics Sonochemistry 17 (2010) 649–653

activity of composite catalysts was increased. When the loadingwas greater than the best loading, doped ion became the recombi-nation centers of electron and hole, and excessive doped of TiO2

decreases space charge layer thickness of particles surface, andreducing the amount of absorption of incident light, reducing pho-tocatalytic activity [39]. When adding 0.05 mM originalH3PW12O40, H3PW12O40/TiO2 has the best photocatalytic activity.

We noted in Fig. 8 that the removal of concentration and TOC ofMB only decreased by 29% and 10%, respectively, in dark by TiO2,and the adsorption/desorption reached equilibrium after 30 min.When by H3PW12O40/TiO2, the removal of concentration and TOCreached between 44% and 23%, respectively, in 30 min in dark,and became higher by 53% and 37%, respectively, and reachedadsorption/desorption equilibrium in 90 min, which suggestingthat strong adsorption reaction occurred. This high photocatalyticactivity was also attributed to the bimodal porous structure ofthe H3PW12O40/TiO2, which provided enhanced mass transportfor molecules into and out of the pore structure [40]. With a largesurface area, the composite catalyst exhibited a binary function for

Fig. 8. The change of concentration (illustration) and TOC of MB solution(50 mg/l, 200 ml) with H3PW12O40/TiO2 (0.4 g, ultrasound, 80 �C, 0.05 mM originalH3PW12O40) composite and TiO2 (ultrasound, 80 �C) in dark in 120 min.

degradation of MB solution through both photocatalysis andadsorption, and adsorption played important roles to decomposethe MB molecules.

Fig. 9 shows that on stirring the suspension of aqueous MB solu-tion with catalyst under solar irradiation, the chroma removal ofMB was similar in the presence of H3PW12O40/TiO2 and TiO2. Butthe removal of TOC of MB differed a lot, H3PW12O40/TiO2 reached79% while TiO2 only reached 40% under 120 min solar irradiation(illustration in Fig. 9). These data illustrate that the solar photocat-alytic activity of H3PW12O40/TiO2 complex is higher than that ofthe TiO2 catalyst, and the degradation of MB solution mainly orig-inated from the supported effect yielded by the combination ofH3PW12O40 and TiO2. The supported effect was that in the systemof anatase TiO2 crystalline particles coupled by homogeneouslydispersed Keggin units, and the interfacial electron transfer tookplace from the TiO2 conduction to H3PW12O40 after solar irradia-tion. Such an effective electron transfer could inhibit the fast elec-tron–hole recombination on TiO2, and the trapped holes had

Fig. 10. Cycling runs in the photodegradation of MB in the presence ofH3PW12O40/TiO2 (anatase) under solar irradiation.

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sufficient time to react with H2O to generate OH� radicals. The OH�

radicals photo-oxidized MB molecule, resulted in the decoloriza-tion of the MB solution [41,42]. Pure anatase TiO2 had no the sup-ported effect, therefore the interfacial electron transfer took placein TiO2 itself, resulting in the fast electron–hole recombination,as a result, the photocatalytic efficiency of pure anatase TiO2

decreased.After the reaction, the suspension was centrifuged, and the

H3PW12O40/TiO2 catalyst was separated. The loss amount of masspercent was 0.1%, which confirmed the less solubility of the com-posite catalyst during the reaction process. After the treatment,the catalyst was reused in the photocatalytic experiment. The cat-alytic activity of H3PW12O40/TiO2 in the degradation of MB solutionwas maintained efficient after five repeated experiments (Fig. 10).

4. Conclusions

H3PW12O40/TiO2 (anatase) photocatalyst was prepared by aultrasonic method of through Ti(OC4H9)4 hydrolysis at a lowertemperature (80 �C), anatase phase is directly prepared withouthigh-temperature calcination. The primary Keggin structure ofH3PW12O40 remained intact after formation of the composite cata-lyst, and chemical interactions between H3PW12O40 and TiO2 viacovalent bonding existed in the composite. The obtained compos-ite catalyst exhibited greater photocatalytic activity in solar irradi-ation for the deterioration of organic dye which has complexstructure. Meanwhile, it is easy to separate the catalysts for recy-cling uses after the reaction is finished. The recycled catalystscould keep great and stable photocatalytic activity. The processof preparation and catalysis of H3PW12O40/TiO2 was environmen-tally friendly. The studies suggest that the H3PW12O40/TiO2 hasgreat potential for practical solar energy photodegradationapplications.

Acknowledgements

The authors would like to acknowledge the financial support ofthe Specialized Research Fund for the Creative Team Item by theEducation Office of Liaoning Province in China (2007T005) andthe fund by Dalian of Liaoning Province in China (2006E21SF084).

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