Synthesis of CuAPTPP-TDI-TiO2Conjugated Microspheres and its Photocatalytic Activity

Cho Peng,Bing-hu Yo?,Wen Zhng,Jin-fen Niu,Jie Zho

a.Department of Applied Chemistry,Xi’an University of Technology,Xi’an 710054,China

b.Department of Civil Engineering,University of Arkansas,Fayetteville 72701,USA

(Dated:Received on October 22,2013;Accepted on December 20,2013)

Synthesis of CuAPTPP-TDI-TiO2Conjugated Microspheres and its Photocatalytic Activity

Chao Penga,Bing-hua Yaoa?,Wen Zhangb,Jin-fen Niua,Jie Zhaoa

a.Department of Applied Chemistry,Xi’an University of Technology,Xi’an 710054,China

b.Department of Civil Engineering,University of Arkansas,Fayetteville 72701,USA

(Dated:Received on October 22,2013;Accepted on December 20,2013)

The metal complex 5-(4-aminophenyl)-10,15,20-triphenylporphyrin copper(CuAPTPP)was covalently linked on the surface of TiO2microspheres by using toluene disocyanate(TDI) as a bridging bond unit.The hydroxyl group(-OH)of TiO2microspheres surface and the amino group(-NH2)of CuAPTPP reacted respectively with the active-NCO groups of TDI to form a surface conjugated microsphere CuAPTPP-TDI-TiO2that was conf i rmed by FT-IR spectra.The CuAPTPP-TDI-TiO2microspheres were characterized with UV-visible, elemental analysis,XRD,SEM,and UV-Vis dif f use ref l ectance spectra.The ef f ect of amounts of linked TDI on the performance of photocatalytic microspheres was discussed,and the optimal molar ratio of TDI:TiO2was established.The photocatalytic activity of CuAPTPPTDI-TiO2was evaluated using the photocatalytic degradation of methylene blue(MB)under visible-light irradiation.The results showed that,TDI,as a bond unit,was used to form a steady chemical brigdging bond linking CuAPTPP and the surface of TiO2microspheres,and the prepared catalyst exhibited higher photocatalytic activity under visible-light irradiation for MB degradation.The degradation rate of 20 mg/L MB could reach 98.7%under Xelamp(150 W)irradiation in 120 min.The degradation of MB followed the f i rst-order reaction model under visible light irradiation,and the rate constant of 5.1×10-2min-1and the halflife of 11.3 min were achieved.And the new photocatalyst can be recycled for 4 times, remaining 90.0%MB degradation rate.

Conjugated microspheres,Surface modif i cation,Sensitization,Visible-light photocatalysis,Methylene blue

I.INTRODUCTION

Due to its excellent performance and environmentally friendly features,titanium dioxide(TiO2)has been regarded as the most promising environment-friendly photocatalytic material that can be utilized for treatment and degradation of organic pollutants in waste water[1,2].However,the drawbacks,including the wide band gap(3.2 eV),the low quantum efficiency and exhibition of excellent photocatalytic activity only when irradiated under UV light,greatly restricted its practical application.Numerous studies indicate surface modif i cation could signif i cantly improve photocatalytic activity of TiO2[3,4].Many methods have been proposed for modif i cation.In recent years,researchers have been focusing on the surface modif i cation of TiO2. TiO2was endowed with novel photo-absorption properties and physicochemical performance through surface modif i ed by active substances,such as organic dyes[5, 6],polymer[7-9],fullerene C60[10,11],graphene[12, 13],nanotube NT[14,15],metal[16,17],metal oxides[18-20].Metalloporphyrin and its derivatives are dyes with excellent performance,optical and thermal stability.As a sensitizing material,metalloporphyrin can ef f ectively broaden the photo response range of TiO2[21-23].Huang et al.reported that porphyrin and iron metalloporphyrin sensitized TiO2was used to degrade Rhodamine B under UV-light irradiation[24]. The results showed that the photocatalytic properties of TiO2sensitized by iron-porphyrin was signif i cantly improved,the degradation rate of Rhodamine B approached 86.3%in 30 min under the high-pressure mercury lamp irradiation.Lu et al.synthesized a series of tetraphenylporphyrin derivatives with dif f erent functional groups(-OH,-CO2C2H5,-COOH)which were utilized to sensitize TiO2.The photocatalytic performance of sensitized TiO2was improved[25].Chang et al.used nickel-porphyrin sensitized TiO2to degrade 2,4-dichlorophenol in waste water under visible-light irradiation,the maximum degradation rate was up to 81%in 240 min[22].Murphya et al.utilized 4-(4-carboxyphenyl)porphyrin to modify TiO2,which de-graded pharmaceutical famotidine under visible-light irradiation[6].The results showed that the obtained catalysts exhibited more optimized performance than Degussa P25.However,for the metalloporphyrin sensitized TiO2,the sensitizer existed on the surface of TiO2only through simple physical adsorption,instead of strong chemical bond[26,27].Therefore,in the process of photocatalytic degradation,sensitizer would be prone to shedding,which would a ff ect sensitization and reduce photocatalytic efficiency[28,29].

In this work,the toluene disocyanate(TDI)was used as a“bridging bond”molecule,the reaction between the highly reactive group(-NCO)of TDI molecule and the surface hydroxyl group(-OH)of TiO2was used to fi rmly fi x TDI molecule on the surface of TiO2microspheres,another group(-NCO)of TDI molecule reacted with the amino group(-NH2)of CuAPTPP complex to obtain the stable conjugate microspheres(CuAPTPPTDI-TiO2).The structure and morphology of the conjugate microspheres were investigated.The photocatalytic activity of the CuAPTPP-TDI-TiO2was also evaluated by the degradation of methylene blue (MB),in comparison with pure TiO2,TDI-TiO2and CuAPTPP-TiO2.

II.EXPERIMENTS

A.Main materials

Pyrrole,benzaldehyde,4-nitrobenzaldehyde,cupric acetate,methylene blue,titanium(IV)sulfate(AR) were purchased from Sinopharm Chemical Reagent Co.,Ltd.,toluene diisocyanate from Xi’an Chemical Reagent Factory,chloroform,methanol,N,N-dimethylformamide,ethanol(AR)from Tianli Chemical Reagent Co.,Ltd),and propanoic acid(AR)from Tianjin Fuyu Fine Chemical Co.,Ltd.Ultrapure deionized water was used throughout the experiments.

B.Synthesis of CuAPTPP complex

6.50 mLofbenzaldehydeand2.85g4-nitrobenzaldehyde(molarratioofbenzaldehyde to 4-nitribenzaldehyde=3:1)were dissolved in the 150 mL of propionic acid in the three-neck f l ask equipped with ref l ux condenser.The three-neck f l ask with mixture was heated at 140?C.Subsequently 4.60 mL of pyrrole was dripped into the reaction solution slowly within 60 min.The mixture solution was ref l uxed continuously for 120 min.After cooling,the mixture solution was distilled at the 70?C under vacuum and a majority of propionic acid was removed.The remaining solution was transferred to 80 mL of methanol on the ice-bath until the purple crystal precipitated.The precipitated material was 5-(4-nitro)-10,15,20-triphenylporphyrin(NPTPP).The crude product was dissolved in a small amount of CHCl3and f i ltered to remove insoluble substance.The CHCl3solution was purif i ed by the chromatography on a silica gel column with chloroform,the f i rst color band was collected,concentrated and dried,and then the purif i cation of NPTPP was complete.

The NPTPP and copper acetate(molar ratio of NPTPP to copper acetate=1:1)were dissolved in DMF. The mixture solution were heated at 150?C under stirring and ref l uxed continuously for 180 min in the oil bath.After cooling,the mixture solution was distilled at 85?C under vacuum and the DMF was removed.The resulting sepia sticky substance was transferred into an oven and dried at 80?C for 12 h.The obtained substance was dissolved in chloroform and f i ltered to discard the insoluble substance.The chloroform extraction solution was washed by deionized water for 2-3 times.The extraction liquid was dried by MgSO4for 12 h and then f i ltered to remove MgSO4.The extraction solution was distilled at 60?C in vacuum.The 5-(4-nitro)-10,15,20-triphenylporphyrin cupper(CuNPTPP) was generated.A certain amount of SnCl2concentrated hydrochloric acid solution(0.620 g of SnCl2·H2O was dissolved in 10 mL of concentrated hydrochloric acid) reacted with CuNPTPP at 78?C for 90 min.After cooling,the mixture solution was extracted by chloroform.The extraction solution was washed by deionized water for 1-2 times and dried by MgSO4for 12 h. Then the MgSO4was removed by f i ltration.The extraction solution was distilled at 60?C in vacuum,concentrated and dried.The 5-(4-aminophenyl)-10,15,20-triphenylporphyrin cupper(CuAPTPP)was generated. The synthesis of CuAPTPP is shown in Fig.1.

C.Synthesis of TDI-modif i ed TiO2

TiO2was dispersed into acetone by ultrasonic dispersion in a three-neck f l ask to form a suspension.The suspension was stirred slowly on heat,while TDI(molar ratio of TiO2to TDI=1:0.5)was dripped into the f l ask. And then the mixture solution was ref l uxed 30 min at 50?C.The mixture solution was f i ltered and washed with acetone 3-4 times,and let dry naturally.Then the TDI cross-linked TiO2(TDI-modif i ed TiO2)was obtained.The synthesis of TDI-modif i ed TiO2is shown in Fig.2.

D.Preparation of CuAPTPP-TDI-TiO2conjugate microspheres

20 mg of CuAPTPP was dissolved in 100 mL of acetone.An appropriate amount of TDI-TiO2was added to the CuAPTPP solution with ultrasonic dispersion for 30 min.The mixture solution was transferred into a 250 mL three-necked f l ask with stirring at 50?C and ref l uxed for 120 min in oil bath.The mixture solution was distilled in vacuum to remove acetone,then the TDI-modif i ed TiO2was sensitized by using CuAPTPPto obtain the conjugate microspheres(CuAPTPP-TDITiO2).Figure 3 shows the synthetic pathway of the CuAPTPP-TDI-TiO2conjugate microspheres.

FIG.1 Synthesis of CuAPTPP complex.

FIG.2 Synthesis of TDI-modif i ed TiO2.

E.Preparation of CuAPTPP-TiO2

20 mg of CuAPTPP was dissolved in 100 mL of acetone.An appropriate amount of TiO2was added to the CuAPTPP solution with ultrasonic dispersion for 30 min.The mixture solution was transferred into a 250 mL three-necked f l ask with stirring at 50?C and ref l uxed for 120 min in oil bath.The mixture solution was distilled in vacuum to remove acetone, then the TiO2sensitized by CuAPTPP was obtained (CuAPTPP-TiO2).

F.Characterization of conjugate microspheres

The FT-IR spectra were obtained using a Shimadzu FT-IR 8900(Japan)with the reference of KBr.The morphology of the conjugate microspheres was observed by JSM-6700F f i eld emission scanning electron microscope(Japan).The crystalline phase analysis of sample was characterized by Shimadzu XRD-7000S X-ray dif f ractometer(Japan)at tube current of 30 mA,tube voltage of 40 kV,and scanning speed of 10?/min.Elemental analysis(N,C,H)was performed by Vario EL cube elemental analyzer instrument(Germany).The metalloporphyrin and its derivates were characterized by UV-2102 PC UV-visible spectrophotometer(China). The UV-Vis dif f use ref l ectance spectra(DRS)of obtained photocatalytic microspheres were characterized by TU-1901 double-beam UV-Vis dif f use ref l ectance spectrophotometer(China)with BaSO4as reference.

G.Evaluation of visible-light catalytic activity

Self-made photocatalytic reaction device was used for the evaluation of photocatalytic activity of samples. The reactor includes a light source(Xe lamp,150 W), sample tube(100 mL quartz tube:length 22.0 cm,diameter 2.0 cm,from the light source 10 cm),a cold trap, a snorkel and other accessories.0.05 g of photocatalysts and 50 mL of MB solution(20 mg/L)were added into the sample tube.The air tube was inserted into the bottom of the sample tube,maintaining a controlledair f l ow at 3 L/min to achieve the suspended catalyst in the degradation solution.Adsorption was performed in dark for 30 min,and then sampling once every 15 min with pipette.The absorbance of the supernatant from high-speed centrifugation was measured at 665 nm.According to the relationship between the absorbance and MB concentration,the degradation rate was calculated using the equation

FIG.3 Synthesis of CuAPTPP-TDI-TiO2conjugate microspheres.

FIG.4(a)UV-Vis and(b)Q-band adsorption spectra of samples.

where A0is the initial absorbance of MB solution,Atis the absorbance of MB solution at dif f erent time,η is used to evaluate the photocatalytic activity of synthetic samples.

III.RESULTS AND DISCUSSION

A.UV-Vis analysis

FIG.5 FT-IR spectra of TPP and CuTPP derivatives.

Figure 4(a)shows the UV-Vis absorption spectra of the TPP,CuTPP,CuNPTPP,and CuAPTPP samples. The characteristic Soret band of TPP was captured at 419 nm.There are four weak peaks of TPP between 500-700 nm:515.7 nm(λ1),550.6 nm(λ2),590.5 nm (λ3),and 647.3 nm(λ4),which are the characteristic peaks of Q-band absorption of TPP[30].Figure 4(b) shows the Q-band absorption spectra of samples.In comparison with the absorption peaks of TPP,absorption peaks of CuTPP,CuNPTPP and CuAPTPP differed greatly.The peak position of Soret band essentially remained unchanged.Only one absorption peak (λ1)was present in the Q-band,and the other three absorption peaks disappeared.The λ1absorption peaks of metalloporphyrin spectrum exhibited red shift from 515.7 nm to 539.0 nm.

B.FT-IR analysis

Figure 5 shows the FT-IR spectra of TPP,CuTPP, CuNPTPP,and CuAPTPP.From the FT-IR spectrum of TPP,the peak at 3307 cm-1was a result of stretching vibration of the two N-H bonds at the center of porphyrin ring.Formation of metal ligand leads to disappearance of N-H vibration absorption peak[30].Compared with TPP,CuTPP,CuNPTPP,and CuAPTPP spectra have peaks at 1000 cm-1,due to bond stretching/bending vibration between Cu2+and porphyrin[32, 33].The FT-IR spectrum of CuNPTPP shows the peak at 1344 cm-1,which was due to the N-O bond stretching vibrations.After amination,the peak of N-O bond weaken signif i cantly at the FT-IR spectrum of CuAPTPP,and the peak of N-H bond stretching vibrations was observed clearly at 3510 cm-1.The results showed that-NO2has been transformed to-NH2in theamination reaction.

FIG.6 FT-IR spectra of samples.(a)TDI,(b)pure TiO2, (c)TDI-TiO2,(d)CuAPTPP-TiO2,(e)CuAPTPP-TDITiO2.

To reveal the interactions between modif i ed TiO2nanoparticles and sensitizer,the FT-IR spectra of TDI,pure TiO2,TDI-TiO2,CuAPTPP-TiO2,and CuAPTPP-TDI-TiO2samples are exhibited in Fig.6. In the FT-IR spectrum of TDI,the characteristic absorption peak at 2268 cm-1was due to isocyanate (-NCO)of TDI[34].The stronger peak of TiO2at 600 cm-1was a typical Ti-O-Ti absorption vibration. The peak of TDI-TiO2spectrum exhibited strong absorption at 2268 cm-1,it corresponds to the isocyanate (-NCO)of TDI,indicating there was still residual isocyanate on the surface of TiO2.The residual isocyanate group(-NCO)could react with amino group(-NH2)of CuAPTPP to conf i rm the CuAPTPP on the surface of TiO2.Simultaneously,the new peaks were observed at 1648 and 1228 cm-1,which could be due to the asymmetric stretching vibration and the symmetric stretching vibration of-NHCOOTi[34].The FT-IR spectrum of CuAPTPP-TDI-TiO2showed that the peak at 2268 cm-1weakened compared to the characteristic absorption peaks of-NCO for TDI-TiO2.It must be noted that the reaction between some amounts of-NCO on the surface of TDI-TiO2and-NH2of CuAPTPP led to the weakened peak at 2268 cm-1.The newly formed absorption peak at 1685 cm-1corresponded to the formation of-NHCONH-,indicating that the CuAPTPP was f i xed f i rmly on the surface of TiO2via TDI linking.

TABLE I The elements analysis of the TPP and CuTPP derivatives.

C.Elemental analysis

The elemental analysis results of synthesized TPP and porphyrin derivatives are shown in Table I.The experimental element contents of NPTPP,APTPP and metalloporphyrin derivatives(CuNPTPP,CuAPTPP) matched the theoretical value.

The experimental results show that,the synthesized NPTPP,APTPP,and metalloporphyrin derivatives were nitro-monosubstitued and amino-mono substituted derivatives.

D.XRD analysis

Figure 7 shows XRD patterns of pure TiO2,TDITiO2,CuAPTPP-TiO2,andCuAPTPP-TDI-TiO2samples.It is clear that the four kinds of photocatalysts were anatase phase structure.Various distinct characteristic dif f raction peaks corresponded to dif f erent crystalline surface.The synthesized pure TiO2has an anatase crystalline phase[101],25.28?corresponds to[101]plane,37.80?corresponds to[004]plane,48.04?corresponds to[200]plane,53.89?and 55.60?correspond to[105]and[211]planes,62.68?corresponds to [204]plane,76.01?corresponds to[301]plane,and 83.14?corresponds to[312]plane.After surface modif i cation and sensitization,the characteristic dif f raction peaks of CuAPTPP-TDI-TiO2in XRD patterns do not exhibit relocation or any change in the peak shapes.It was apparent that modif i cation and sensitization only occurred on the surface of TiO2and there were no signif i cant ef f ect on the crystalline phase of catalyst.CuAPTPP-TDI-TiO2crystalline anatase phase still dominated[36].

E.SEM analysis

FIG.7 XRD patterns of samples.(a)Pure TiO2,(b)TDITiO2,(c)CuAPTPP-TiO2,(d)CuAPTPP-TDI-TiO2.

FIG.8 SEM image of CuAPTPP-TDI-TiO2and TiO2(inset)sample.

As shown in Fig.8,TiO2(inset)appears smooth surface and uniform size,and the diameter average size is about 40 nm.In comparison,the microstructures of prepared CuAPTPP-TDI-TiO2appear loose and irregular spheres with rough surface.The average size of these microspheres diameter estimated from the SEM image is about 4-10μm.The loose structure of microspheres surface was due to the organic shell (CuAPTPP-TDI),which was generated from the reaction between CuAPTPP and TDI molecules on the surface of TiO2microspheres.The morphology of image implied that all the TiO2microspheres were coated by the CuAPTPP-TDI to form a organic shell.The organic shell was a compatible substrate to contact MB molecule,benef i cial to enhance adsorption of MB and play a part of sensitive efficiency.

F.UV-Vis DRS analysis

Figure 9 shows the UV-Vis DRS of modi fi ed TiO2with di ff erent amounts of TDI.It shows that the modifi ed TiO2exhibited visible absorption at 400-800 nm. The capability of visible absorption of modi fi ed TiO2strengthened with the increase of the modi fi ed amounts of TDI.The absorption edges of TDI0.01-0.1-TiO2(0.01-0.1 are the molar ratio of di ff erent amounts modi fi ed TDI:TiO2)moved signi fi cantly with a red shift. The TDI-TiO2showed strong UV absorption from 200 nm to 400 nm in the ultraviolet light range.This phenomenon is attributed to the TDI UV absorption property.It indicates that TDI,as sensitizer,could evidently improve the photo response activity of TiO2.When the molar ratio of TDI:TiO2is 0.5:1,the absorption intensity reaches maximum.The degradation rate of MB on TDI1-TiO2decrease signif i cantly.The agglomerate phenomenon of TDI1-TiO2in the MB solution is attributed to the modif i cation of TiO2with TDI,which lead to the photoabsorption decrease.The agglomerate phenomenon may be due to the cladding of TiO2with TDI and the formation of a small organic molecule shell, which results in the worsened dispersibility of TDI1-TiO2.

FIG.9 UV-Vis DRS of TiO2with dif f erent amounts of TDI surface modif i cation and pure TiO2.The inset is plots of(αhν)2vs.hν.(a)Pure TiO2,(b)TDI0.01-TiO2, (c)TDI0.05-TiO2,(d)TDI0.1-TiO2,(e)TDI0.5-TiO2,(f) TDI1-TiO2.

According to the relationship between semiconductor band gap and UV-Vis absorption coefficient:

where hν is the photo energy,α is the absorption coeffi cient,K is the semiconductor constant and Egis the band energy gap.The(αhν)2vs.hν curves of di ff erent samples are shown in inset of Fig.9.According to the value from tangent of curve intersects the axis abscissa,the band gap values were calculated.Figure 9 shows that the band gap of pure TiO2is 3.19 eV,the band gap of TiO2modi fi ed with TDI decreased in varying degree:Eg(TDI0.01-TiO2)=3.09 eV,Eg(TDI0.05-TiO2)=3.06 eV,Eg(TDI0.1-TiO2)=3.13 eV,Eg(TDI0.5-TiO2)=3.08 eV,Eg(TDI1-TiO2)=3.12 eV.The narrow of band energy gap indicated that the modi fi cation of TDI can extend the range of TiO2light absorption.The decrease of transition energy of photogenerated electrons is caused by the UV-absorption property of TDI.

Figure 10 is UV-Vis DRS spectra for TDI,TiO2, and a series of catalysts.It shows that the spectra of CuAPTPP-TiO2and CuAPTPP-TDI-TiO2exhibited strong characteristic absorption peaks of prophyrin at 420 nm,indicating that TDI-TiO2has been sensitized by CuAPTPP.The surface modif i ed TiO2with TDI showed more intensive characteristic absorption peaks of prophyrin at the same amount of sensitizer,indicating the surface of TDI-TiO2could be f i xed with more CuAPTPP via-NCO band.The results showed thatsensitization of CuAPTPP improved the photoresponse activity of TDI-TiO2greatly.

FIG.10 UV-Vis DRS of(a)pure TiO2,(b)TDI,(c)TDITiO2,(d)CuAPTPP-TiO2,and(e)CuAPTPP-TDI-TiO2.

G.Evaluation of photocatalytic activity

1.Degradation experiments of MB by CuAPTPP-TDI-TiO2

MB is a chromogenic agent of diltiazem benzene.In the process of photocatalytic degradation,the-(CH3)2within molecules accepted the photo-generated electron and then demethylation occurred.The degradation process was indicated by the reduction of characteristic absorption peak(λ=664 nm).After formation of phenyl thioridazine,the small organic were degraded and mineralized to inorganic molecules gradually[37].

As shown in Fig.11,the absorption peaks of MB solution are gradually reduced during the photocatalytic degradation by a series of samples under Xe lamp(150 W)irradiation.In Fig.12(a),the degradation rate of 10 mL MB solution by CuAPTPP-TDI-TiO2was up to 98.7%within 120 min.It was signif i cantly higher than the degradation rates of CuAPTPP-TiO2(Fig.11(b),65.1%),TDI-TiO2(Fig.11(c),62.0%)and pure TiO2(Fig.11(d),33.7%).It can be apparently seen that the absorbance of the characteristic peaks at 290 and 664 nm both decline continually and nearly disappear f i nally in Fig.11(a).It implies that MB not only decolorized but also mineralized under visible light irradiation.The unique and excellent photocatalytic activity of sample may be due to the role of CuAPTPPTDI,which has increased the hydrophobicity of TiO2and favored samples adsorption.Before bridging bond linked by CuAPTPP-TDI,only a small amount of MB is adsorbed on TiO2surface and then degraded,the hydrophobic attractions between CuAPTPP-TDI and MB are increased,leading to enhancement of the surface coverage of MB on CuAPTPP-TDI-TiO2powders. In addition,for the CuAPTPP-TDI-TiO2composite, the complex structure established the conjugated interaction between TiO2and CuAPTPP molecules,the sensitization efficiency was improved.As mentionedabove,CuAPTPP-TDI-TiO2shows stronger adsorption and higher degradation capability than others.

TABLE II Kinetic equation and parameter of photocatalytic degradation reaction(t1/2in min and k in min-1).

2.MB degradation kinetics analysis

Figure 12 shows the relationship between ln(c0/ct) and reaction time t for MB degradation by pure TiO2and a series of synthesized photocatalysts.From the Fig.12,the CuAPTPP-TDI-TiO2microspheres possess the optimal photocatalytic performance.Furthermore, its degradation rate was higher than the either sensitized or modif i ed TiO2.The results show,the isocyanate groups of TDI on the surface of TiO2are able to f i x f i rmly the CuAPTPP molecules and inhibit the generation of inactive CuAPTPP dimer[38].As a result,the sensitization efficiency of CuAPTPP was improved signif i cantly.The dye molecules were able to inject the photo-generated electrons to the conduction band of TiO2as the dye molecules was irradiated.TDI molecule between CuAPTPP and TiO2,as a conjugated tunnel,could control the amounts of injected electrons, prohibit recombination of e--h+and enhance the coefficient of utilization for photo-generated electrons.Since the organic molecules on the surface of TiO2strengthened the compatibility with MB molecules,the adsorption rate increased greatly.

Table II shows the initial reaction rate of MB degradation kinetic equation,the linear correlation coefficient k,and the half-life t1/2.The reaction rate kinetics constant k of MB degradation by CuAPTPP-TDI-TiO2is 5.12×10-2min-1,which was seven times bigger than that of the pure TiO2.The degradation half-life t1/2is 11.3 min,which is the shortest degradation half-life among all catalysts tested.

3.Stability of the catalyst

FIG.11 The UV-Vis absorption curves of MB in the degradation process by(a)CuAPTPP-TDI-TiO2,(b)CuAPTPP-TiO2, (c)TDI-TiO2,and(d)pure TiO2.

FIG.12 Relationship curves between ln(c0/ct)and time t.

In order to investigate the stability of CuAPTPPTDI-TiO2and CuAPTPP-TiO2,the samples were recovered after each photocatalytic degradation experiment,and then were reused in the next photocatalytic experiment.Figure 13 shows the MB degradation curves by CuAPTPP-TDI-TiO2reused for 4 times, and degradation rate remained at about 90%.In comparison with CuAPTPP-TDI-TiO2,the degradation rate of MB by CuAPTPP-TiO2decreased rapidly after reusing,this result was due to the fall of fof CuAPTPP.Because the CuAPTPP on the surface of TiO2is merely physical adsorption and not chemical bond.This shows that,TDI could form a steady chemical brigdging bond linking between CuAPTPP and the surface of TiO2microspheres and enhanced the photocatalytic performance of sample.The catalytic activity of CuAPTPP-TDI-TiO2did not reduce after repeated utilization.This implies that as-prepared photocatalyst was reusable.

FIG.13 The MB degradation rate of CuAPTPP-TDI-TiO2photocatalytic microspheres repeatedly used.

IV.CONCLUSION

CuAPTPP-TDI-TiO2conjugated photocatalyst was successfully synthesized.The characterizations of the conjugated structure of the composite catalyst indicate that the bridging bond linking was formed to immobilize the dye sensitizer on the surface of TiO2.Two isocyanate groups-NCO of TDI molecules reacted with -NH2of CuAPTPP molecule and-OH of TiO2surface respectively.The immobilization of dye sensitizers overcame ef f ectively the fall of fof CuAPTPP,the utiliza-tion rate of dye sensitizer was improved.The size of CuAPTPP-TDI-TiO2photocatalytic microspheres was in the range of 4-10μm with loosen surface and exhibited excellent photocatalytic activity on the degradation of MB under visible-light irradation.The degradation rate of MB on CuAPTPP-TDI-TiO2was up to 98.7%under Xe lamp irradation within 120 min.The TDI linking on the surface of TiO2enhanced the compatibility of TiO2with MB,the adsorption properties of photocatalysts were strengthened considerably.The higher photo-response activity of CuAPTPP-TDI-TiO2may be due to the establishment of bridging bond linking between dye molecules and TiO2substrate.The kinetics of photocatalytic degradation of MB was investigated,suggesting a pseudo f i rst-order kinetics model. The CuAPTPP-TDI-TiO2sample was robust and able to use at least for four runs without obvious loss.

V.ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China(No.21276208),the Doctor Fundation of Education Ministry of China (No.20096118110008),the Special Research Fund of Shaanxi Provincial Department of Education of China (No.12JK0606),and the Research Fund for Excellent Doctoral Thesis of Xi’an University of Technology (No.207-002J1304).

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?Author to whom correspondence should be addressed.E-mail：bhyao@xaut.edu.cn,Tel.：+86-29-82066361,FAX：+86-29-82066361