王 程 施惠生 李 艷 郭曉潞

(1同濟大學先進土木工程材料教育部重點實驗室，上海 201804)(2石家莊經(jīng)濟學院材料科學與工程研究所，石家莊 050031)

靜電自組裝制備納米TiO2/SiO2光催化材料

王 程1,2施惠生＊,1李 艷2郭曉潞1

(1同濟大學先進土木工程材料教育部重點實驗室，上海 201804)(2石家莊經(jīng)濟學院材料科學與工程研究所，石家莊 050031)

采用靜電自組裝方法制備了納米TiO2/SiO2光催化材料。采用巰丙基三甲氧基硅烷偶聯(lián)劑對SiO2進行干法改性，采用雙氧水/冰醋酸將偶聯(lián)劑巰基基團氧化為磺酸基基團。在正負電荷的吸引下，帶負電荷的SiO2與帶正電荷的鈦聚合陽離子自發(fā)地組裝在一起，經(jīng)一定溫度熱處理得到納米TiO2/SiO2光催化材料。采用XRD、FTIR、PL、UV-Vis DRS、SEM和ICP等對材料進行了分析和表征。采用甲基橙溶液評價材料的光催化性能。結果表明：SiO2促使銳鈦礦的形成，抑制銳鈦礦向金紅石的轉變，減小TiO2的晶粒尺寸，使得TiO2光吸收波長發(fā)生藍移。TiO2與SiO2通過Si-O-Ti鍵發(fā)生結合。采用靜電自組裝方法制備的材料中TiO2的含量高于傳統(tǒng)方法，導致材料的光催化性能有所提高。

靜電自組裝；TiO2；SiO2；光催化材料；甲基橙

Nano-sized TiO2as one of the most promising photocatalysts has attracted much attention owing to its photocatalytic degradation of organic pollutants,photocatalytic dissociation ofwater,solarenergyconversion,and disinfection[1-4].However,it is difficult and expensive to recycle TiO2nanopowders in the practical utilization due to the formation of milky dispersion after mixing the nanopowders into the wastewater.

TiO2/SiO2oxides were found to be a good candidate material to overcome the above problem.Further more,it was demonstrated that SiO2could improve the thermal stability,decrease TiO2particle size and thus improve the UV photocatalytic activity of TiO2[5-8].So far,the TiO2/SiO2photocatalysts have been prepared by several approaches including impregnation[9],precipitation[10]and sol-gel techniques[5-8].However,during the preparation procedures mentioned above,the surface of SiO2is positively charged in acid aqueous environments,thus resulting in the mutual exclusion of SiO2and titanium polycation.This phenomenon may further lead to a low composite efficiency of SiO2with TiO2and furtherreduce the photocatalytic activity ofthe photocatalysts.Recently,researchers have adopted different methods to solve the above problems,for example,using liquid-phase deposition (LPD)method from a (NH4)2TiF6aqueous solution upon addition of boric acid (H3BO3)[11],using hydroxypropyl cellulose(HPC)as a surface esterification agent to modify the substrate[12].Decher et al.[13]reported in 1991 a novel method called electrostatic self-assembly method(ESAM).According to the ESAM,the opposite electric charge elements self-assemble with each other by electrostatic interaction.Related to that,Shin et al.[14]found that TiO2films on silicon prepared by ESAM showed densely packed anatase crystallites.Chen et al.[15]used ESAM to prepare Iron (Ⅲ)-doped TiO2/SiO2particles and the results showed that TiO2nanopowders were well distributed on the surface of SiO2.

In this paper,ESAM method was used to prepare nano TiO2/SiO2photocatalysts.The aim of this work was to investigate the photocatalytic property ofthe prepared photocatalysts and the interrelationship of SiO2with TiO2.Based on the above goals,XRD,FTIR,PL,UV-Vis DRS,ICP and SEM were employed to characterize the microstructure and morphology of the photocatalysts.The photocatalytic activtiy of nano TiO2/SiO2photocatalysts for the degradation of methyl orange was also evaluated.

1 Experimental

1.1 Photocatalysts preparation

Silane coupling agent of (OCH3)3Si(CH2)3SH(Wuhan University Silicone New Material Co.,Ltd.,China)was used to modify the SiO2powders(Zhoushan Mingri Nano-material Ltd.,China,average grain size of 10 nm).In a typical preparation,2wt%silane (it was diluted by the same volume of ethanol)was mixed with SiO2powders and then the mixture was stirred at 120℃for 30 min.The modified SiO2powders were oxidized by 30%H2O2/HOAc(hydrogen peroxide/glacial acetic acid)at 50℃ for 2 h.

The modified and oxidized SiO2powders were mixed up with decuple distilled water and heated up to 70℃.Subsequently,TiCl4solution (the theoretical proportion of TiO2in the photocatalyst was 30wt%)was added drop wise into the above mortar and the pH value of the dispersion was adjusted to about 2.0.The system was vigorously stirred at 70 ℃ for 4 h.After aging for 12 h,the product was filtrated and washed repeatedly three times with distilled water,and then dried at 80℃for 2 h.The above samples were finally calcined at 200～700℃ for 2 h in a muffle furnace to obtain the TiO2/SiO2photocatalysts.

Unmodified SiO2powders were used to prepare TiO2/SiO2photocatalyst(referring to as conventional method here after)in order to compare with the photocatalyst prepared by ESAM.Pure TiO2powders were also prepared by the conventional method too.

1.2 Characterization

The structure of samples was characterized by XRD (D/max 2550VB3+/PC,Rigaku International Corporation,Japan)under the following conditions:graphite monochromatic copper radiation (Cu Kα,λ=0.154 18 nm);40 kV as accelerating voltage;40 mA as emission current;and the 2θ range of 3°～70°at a scan rate of 4°·min-1.The sizes of TiO2were estimated using the Scherrer equation.FTIR spectra, for the determination of Si-O-Ti bond of the photocatalysts,were recorded by a spectrometer (Nexus,Thermo Nicolet Corporation,America)using KBr pellets at room temperature in the region of 400～4 000 cm-1.The photoluminescence (PL)spectra of the samples were recorded with RF-540 spectrofluorophotometer(RF-540,Shimadzu Instruments,Japan),using the 280 nm excitation line of a Xe lamp.UV-Vis absorption spectra(UV-Vis DRS)of the samples were obtained for the drypressed disk samples using a UV-Visible spectrophotometer with an integrating sphere(UV-3010,HITA-CHI,Japan).The morphology of the photocatalyst was examined by field-emission scanning electron microscope (FESEM)(Quanta 200 FEG,FEI Company,America)with an accelerating voltage of 10 kV.The content of TiO2in the photocatalyst was examined by ICP(Optima 4300DV,Perkin Elmer Ltd.,America).

1.3 Photocatalytic activity tests

The photocatalytic activity of the sample was evaluated by the degradation of methyl orange(MO)in an aqueous solution.The photocatalytic reactor system consisted of a 40 W UV lamp centered at 253.7 nm and a magnetic stirrer for obtaining dispersion.In a typical test for MO degradation,the photocatalyst(1 g)was suspended in a 100 mL MO solution (10 mg·L-1)and then treated under UV light for different times.The concentration of the MO solution was monitored at regular time intervals by measuring the maximum absorbance of MO (464 nm),using 722S spectrometer(722S,Shanghai Precision&Scientific Instrument Co.,Ltd.,China).The decoloration rate of the MO solution was calculated according to P=(A0-At)/A0×100%,where P is the decoloration rate of the solution;A0is the absorbency of the original solution;Atis the absorbency of the MO solution after treating for t h.

2 Results and discussion

2.1 XRD analysis

Fig.1 shows the XRD patterns of nano TiO2/SiO2photocatalysts calcined at 200～700 ℃.The patterns reveal that all the samples are composed of anatase phase without any rutile phase even at 700℃.With the ascent in calcination temperature,the peak intensity of anatase phase increases significantly,indicating the decrease in crystallite oxygen vacancies and intrinsic defects of anatase phase.Simultaneously,the width of the(101)peak becomes narrower,suggesting the growth of anatase crystallites.The crystallite sizes of TiO2calculated by the Scherrer equation are about 2.5 nm,3.8,4.7,5.1,8.6 and 9.3 nm,respec-tively,as the calcination temperature increases from 200℃to 700℃.

Fig.2 shows the XRD patterns of pure TiO2calcined at 200～700 ℃.The obtained TiO2are anatase and plenty of brookite,anatase,anatase and little rutile,anatase and rutile,anatase and plenty of rutile,rutile,respectively,as the calcined temperature increases from 200℃ to 700℃.The crystallite size of TiO2calcined from 200℃to 700℃calculated by the Scherrer equation are about 4.5,5.2,8.2,11.7,16.5 and 25.6 nm,respectively.Contrasting with the crystal forms and crystallite size of TiO2in the TiO2/SiO2photocatalysts,it can be suggested that the SiO2facilitates the formation of anatase,restrains the transformation of anatase to rutile and decreases the crystallite size of TiO2.

2.2 PL analysis

Fig.3 shows the PL spectra of TiO2/SiO2calcined at 200,300 and 400 ℃,respectively.A 280 nm He-Cd laser was used as the excitation source for the PL measurements.It can be found that all of the samples can exhibit obvious PL signal centered at approximately 468 nm and the PL intensity increases with the increase of calcination temperature.These PL signals result from the surface oxygen vacancies and defects of TiO2[16].Generally speaking,a lower PL intensity indicates a lower recombination rate of electron-hole pairs, higher separation efficiency and higher photocatalytic activity[17].These results suggest that TiO2/SiO2calcined at 200℃may have the higher photocatalytic activity than that of the other photocatalysts.

2.3 FTIR analysis

Fig.4 shows the FTIR spectra of SiO2and TiO2/SiO2in the range of 400～4 000 cm-1.As shown in the spectrum of SiO2,the board peak around 3430 cm-1and 1 630 cm-1can be assigned to the O-H stretching and flexural vibration of adsorbed water,respectively.The peak at 1088 cm-1corresponds to the Si-O-Si stretching vibration[18].Compared with the spectrum of SiO2,the spectrum of TiO2/SiO2photocatalyst calcined at 200℃obviously changes.The peak of Si-O-Si stretching vibration shift to 1 094 cm-1.The peak at 960 cm-1corresponds to the Si-O-Ti bond which suggests that the TiO2combines with SiO2by the formation of Si-O-Ti chemical bond[8,18].

2.4 UV-Vis DRS anaylsis

Fig.5 shows the UV-Vis DRS spectra of 200 ℃calcined pure TiO2and TiO2/SiO2.The absorption edge of TiO2/SiO2shifts to 395 nm in comparison with that of pure TiO2at about 414 nm.It shows that SiO2causes the absorption edge of TiO2to shift to higher energy region.

2.5 Morphology analysis

The SEM images of TiO2/SiO2calcined at 200℃are displayed in Fig.6.It can be seen that the samples are composed of well-defined micro-size particles which can be relatively easy to separate from the aqueous solution.

2.6 Photocatalytic activity

Fig.7a shows the degradation curves of MO solution catalyzed by the TiO2/SiO2photocatalysts calcined at 200～700 ℃.It can be observed that all of the decoloration rates of MO solution are relatively low due to the weak UV light intensity employed in this experiment.With the increase in illumination time,the decoloration rateofMO solution increases.The decoloration rate of MO solution catalyzed by the sample calcined at 200 ℃ reaches to 70.96%after illumination for 7 h which is slightly higher than that of 300℃.On the other hand,the photocatalytic activities of the samples calcined from 400℃to 700℃are obviously lower than that of 200℃.This can be attributed to the smallercrystallite size,lower recombination rate of electron-hole pairs and higher photocatalytic activity of TiO2contained in the sample calcined at 200℃.

The photocatalytic activity of photocatalyst prepared by ESAM compared with that of conventional method was further evaluated,and the results are shown in Fig.7b.The decoloration rate of MO solution catalyzed by photocatalyst prepared by conventional method is 39.71%after illumination for 7 h,which is significantly lower than that of the former.

The content of the TiO2in the two photocatalysts were examined by ICP.The results show that the content of TiO2in the sample prepared by ESAM and conventional method is 43.7%and 29.9%,respectively.This result may be the main reason for the higher photocatalytic activity of sample prepared by ESAM method.The content of TiO2in the sample prepared by ESAM method was higher than the theoretical value due to the unique formation mechanisms of TiO2/SiO2photocatalysts.

During the treatmentofSiO2surfaceswith sulfhydryl silane coupling agent,the alkoxy groups hydrolyzein an aqueousenvironment,producing hydroxyl groups,one or more of which condense with the hydroxyl groups commonly found on SiO2surfaces as well as with one another.Subsequent drying leads to the formation of both covalent bond linkages with the surface and development of a cross-linked silane film[19].30%H2O2/HOAc solution was used to oxidize the sulfhydryl group of the silane to sulfonic acid group,which is proved in our early study[20].TiCl4hydrolysis products of titanium polycation can self-assemble with the electronegative SiO2particles to form microparticles by electrostatic attraction.The nano TiO2/SiO2photocatalysts can be obtained after the above samples calcining at a certain temperature.

In this preparation procedure of ESAM method,the content of electronegative SiO2nanoparticles is much more than titanium polycation which may result in the excess and accumulation of electronegative SiO2nanoparticles,and further,partly excessive and accumulated SiO2nanoparticles may probably lose during the preparation procedure.This may be the reason why the content of TiO2is higher than the theoretical value.A further study in this aspect is required.

3 Conclusions

Nano TiO2/SiO2photocatalysts were prepared by ESAM method.Photocatalysts calcined from 200 ℃ to 700℃are composed of anatase phase without any rutile phase.Contrasting with the crystal forms and crystallite sizes of pure TiO2,it can be suggested that the SiO2facilitates the formation of anatase,restrains the transformation of anatase to rutile and decreases the crystallite size of TiO2.TiO2combines with SiO2by formation of Si-O-Ti chemical bond.SiO2causes the absorption edge of TiO2to shift to 395 nm in comparison with that of pure TiO2at about 414 nm.The photocatalysts are composed of well-defined microsized particles which can be relatively easy to separate from the aqueous solution.The decoloration rate of MO solution catalyzed by the sample calcined at 200℃reaches to 70.96%after illumination for 7 h which is higher than that of 300～700 ℃.This may be attributed to the smaller crystallite size,lower recombination rate of electron-hole pairs and higher photocatalytic activity of TiO2contained in the photocatalyst calcined at 200℃.The decoloration rate of MO solution catalyzed by photocatalyst prepared by conventional method is 39.71%after illumination for 7 h,which is significantly lower than that of ESAM.The content of TiO2in the sample prepared by ESAM and conventional method was 43.7%and 29.9%,respec-tively.This result can be obviously attributed to the higher photocatalytic activity of the sample prepared by ESAM.

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Preparation of Nano TiO2/SiO2Photocatalysts by Electrostatic Self-assembly Method

WANG Cheng1,2SHI Hui-Sheng＊,1LI Yan2GUO Xiao-Lu1

(1Key Laboratory of Advanced Civil Engineering Materials(Tongji University),Ministry of Education,Shanghai 201804,China)
(2Institue of Material Science and Engineering,Shijiazhuang University of Economics,Shijiazhuang 050031,China)

The nano TiO2/SiO2photocatalysts were prepared by electrostatic self-assembly method (ESAM).Silane coupling agent of(OCH3)3Si(CH2)3SH was used to modify the surface of SiO2powders by dry process.Sulfhydryl group(-SH)of the silane was oxidized to easy ionized sulfonyl(-SO3H)by 30%H2O2/HOAc oxidant.The electronegative SiO2was assembled with titanium polycation by electrostatic attraction.The as-prepared compounds were calcined under certain temperature to obtain the nano TiO2/SiO2photocatalysts.The materials were characterized by XRD,FTIR,photoluminescence (PL),UV-Vis DRS,SEM and ICP.The photocatalytic activity was evaluated by the degradation of methyl orange in aqueous solution.The results show that SiO2facilitates the formation of anatase,restrains the transformation of anatase to rutile,decreases the crystallite size of TiO2and causes the absorption edge of TiO2to shift to higher energy region.TiO2combines with SiO2by the formation of Si-O-Ti chemical bond.Photocatalysts prepared by ESAM method exhibit higher photocatalytic activity than that of traditional method attributed to the higher content of TiO2in the samples.

electrostatic self-assembly;TiO2;SiO2;photocatalyst;methyl orange

O614.41+1；O613.72；TB34

A

1001-4861(2011)11-2239-06

2011-06-24。收修改稿日期：2011-07-15。

河北省自然科學基金(No.E2008000537)、河北省科學技術研究與發(fā)展指導計劃項目(No.07215156)、河北省自然科學基金－鋼鐵聯(lián)合基金(No.E2009000946)和同濟大學先進土木工程材料教育部重點實驗室青年基金(No.2010412)資助項目。

＊通訊聯(lián)系人。 E-mail：shs@#edu.cn