Photocatalytic Degradation of RhB over TiO2 Bilayer Films Effect of Defects and its Location

https://www.sodocs.net/doc/ba16043115.html,/Langmuir ?2010American Chemical Society

Photocatalytic Degradation of RhB over TiO 2Bilayer Films:Effect of

Defects and Their Location

Jiandong Zhuang,Wenxin Dai,Qinfen Tian,Zhaohui Li,Liyan Xie,Jixin Wang,and Ping Liu*

State Key Laboratory Breeding Base of Photocatalysis,Research Institute of Photocatalysis,Fuzhou University,

Fuzhou 350002,China

Xicheng Shi and Donghui Wang

Research Institute of Chemical Defence,Beijing 100191,China Received January 21,2010.Revised Manuscript Received March 15,2010

TiO 2bilayer films with a normal surface (Ns-TiO 2),surface defects (Sd-TiO 2),and interface defects (Id-TiO 2)were successfully prepared by a combination of cold plasma treatment (CPT)and sol -gel dip-coating technology.The photodegradation of rhodamine B (RhB)over these as-prepared TiO 2films was investigated via UV -vis irradiation.Results indicate that the three kinds of films exhibit very different photodegradation processes for RhB.A mainly N-deethylation reaction over the Ns-TiO 2films,whereas an efficient degradation (cycloreversion)of RhB occurs over the Sd-TiO 2films.In the RhB/Id-TiO 2system,however,efficient N-deethylation concomitant with the highly efficient cycloreversion of RhB is observed.The efficiency of the complete mineralization of RhB dye follows the order of Id-TiO 2>Sd-TiO 2>Ns-TiO 2.It is proposed that the defect sites at the surface or the interface of TiO 2films promote the separation of photogenerated electron -holes,leading to a higher photoactivity of defective TiO 2films.Moreover,the higher stability over Id-TiO 2as compared to Sd-TiO 2indicates that the interface defect sites in TiO 2could be applied in environmental photocatalysis.

1.Introduction

Photocatalysis is a “green”technology for the treatment of environment pollutants with solar energy.1,2The application has been focused on the oxidative decomposition of volatile organic compounds (VOCs)and the purification of wastewater.3-7In the past three decades,titanium dioxide (TiO 2)has become one of the most extensively studied metal oxides because of its excellent photocatalytic activity and photoinduced hydrophilicity.8-10These unusual properties make TiO 2suitable for a variety of thin film applications.11,12Compared with TiO 2powder,the applica-tion of such films in the treatment of wastewater is beneficial for some reasons:allowing for the more efficient distribution of light,dispensing with stirring during the reaction,and omitting separa-tion procedures for the dispersed catalyst particles after reaction.However,the photocatalytic performance of TiO 2film is still low because of its small loading amount of TiO 2.Therefore,further improving the photocatalytic efficiency of TiO 2film is one of the

most imperative tasks for the application of heterogeneous photocatalysis in the future.

For photocatalytic systems,it is well known that the competi-tion among the recombination,trapping,and transfer of photo-generated electron -hole pairs determines the overall quantum efficiency of the photocatalyst and impacts the photocatalytic activity.Hence,an improved separation of this electron -hole pair is essential for enhancing the photocatalytic efficiency of the heterogeneous catalytic process.2,13-15

The catalytic property of the material is greatly affected by its surface microstructure,including its morphology,surface -OH contents,surface electronic states,defects,and so on.8-10,15,16Surface defects,important adsorption sites,and active sites for heterogeneous catalysis are believed to influence the reactivity of oxide surfaces strongly.17,18The generation of surface defects will create a broad distribution of surface electronic states mediating the exchange of electrons between the surface of the oxide catalyst and the adsorbed reacting molecule.19Moreover,defect sites are presumed to affect the electron -hole recombination process in photocatalysts,causing a change in chemical rates that depends on charge transfer from either electrons or holes.2,15,20Surface defects (such as oxygen vacancies),usually created and observed

*To whom correspondence should be addressed.E-mail:liuping@https://www.sodocs.net/doc/ba16043115.html,.Fax:t86-(0)591-83779239.

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Zhuang et al.Article in vacuum,are very unstable and readily removed under common

circumstances or in aqueous media.9,21-23In contrast to surface

defects,subsurface defects(such as bulk defects)may exist more

stably in heterogeneous catalytic processes.However,the role of

bulk defects in heterogeneous photocatalytic activity is still

disputed,although it has been reported to play a major role in

a variety of surface phenomena of materials.24In general,the bulk

defects are considered to be the captured sites of photogenerated

electron-hole pairs25,26whereas some investigations indicate that

the bulk defects can promote the separation of photogenerated

electron-hole pairs in oxides.27,28It is very important to illumi-

nate the effects of subsurface defects on the photoactivity of

oxides,especially its difference with respect to that of surface

defects.

Cold plasma treatment(CPT)is among a variety of techniques

that can create defects on metal oxide surfaces at low temperature

without affecting the bulk material.9,21-23,29,30In the present

investigation,TiO2films with defects at different locations,

including surface defects and interface defects,were prepared by

a combination of CPT and sol-gel dip-coating technology.To

make clear how the photocatalysis of TiO2films is affected by the

existence and location of defects,the photodegradation of rho-

damine B(RhB)as a simulate reaction was investigated on these

as-prepared TiO2films under irradiation.This work may provide new insights into understanding the relationship among the defects,photoactivity,and heterogeneous catalytic reaction me-chanisms.

2.Experimental Section

2.1.Materials.Titanium tetraisopropoxide was obtained from the Sinopharm Chemical Reagent Co.5,5-Dimethyl-1-pyrroline-N-oxide(DMPO)was purchased from Alfa Aesar. Rhodamine-B dye(N,N,N0,N0-tetraethyl rhodamine,RhB)and other chemicals were of analytical reagent grade and were used without further purification.Deionized water was used through-out this study.For reference,the structure of RhB is shown below.

2.2.Preparation of the TiO2Anatase Sol.The anatase TiO2sol was prepared via modified sol-gel processing of titanium tetraisopropoxide.31HNO3(1.1mL,68vol%)was diluted with H2O(150mL),and then titanium tetraisopropoxide was added slowly under vigorous stirring for hydrolysis.The suspension formed was stirred at313K for48h and then dialyzed to obtain a TiO2sol at pH

3.5.

2.3.Preparation of TiO2Bilayer Samples.TiO2thin films were deposited onto the carefully cleaned quartz slides(25mm?75mm?1.2mm)by dip coating the TiO2anatase sol at a uniform draw rate of2cm/min under the ambient atmosphere.After being dried at393K for30min and cooled to room temperature in air, the obtained TiO2monolayer film was transferred to the vacuum chamber and treated with a plasma discharge(200W)under 5Torr of He for5min.Then the treated film was again immersed in the TiO2sol immediately to be coated with another TiO2layer on its surface via repeating the coating and drying processes. Finally,a TiO2bilayer film,called the interface defective TiO2 bilayer film,was prepared.To compared with this,a normal TiO2 bilayer film sample was prepared by dip coating twice but without the above cold plasma treatment.Moreover,a surface-defective TiO2bilayer film sample was also prepared by cold plasma treatment over the surface of a normal TiO2bilayer film.The schematic diagram of the preparation process of TiO2bilayer samples is shown in Scheme1.The resulting transparent TiO2 bilayer films with interface defects,surface defects,and a normal surface were denoted as Id-TiO2,Sd-TiO2,and Ns-TiO2,respec-tively.

2.4.Characterization.X-ray diffraction(XRD)patterns were collected on a Bruker D8Advance X-ray diffractometer with Cu K R radiation operated at40kV and40mA.The data were recorded in the2θrange of20-70°with a step width of0.02°. Scanning electron microscopy(SEM)investigations were carried out on an FEI Nova NanoSEM230field-emission scanning electron microscope.Transmission electron microscopy(TEM) investigations were carried out on a Tecnai G2F20S-TWIN(FEI company)with a field emission gun at200kV.X-ray photoelec-tron spectroscopy(XPS)analysis was conducted on an ESCA-LAB250photoelectron spectrometer(Thermo Fisher Scienctific) at1.2?10-9mbar using an Al K R X-ray beam(1486.6eV). Before XPS measurements,the film surfaces were etched with an argon ion gun(1kV,0.5μA)for5s.A Bruker model A300 spectrometer equipped with a xenon lamp(with254nm filter)was used for measurements of the electron-spin resonance(ESR) signals of radicals spin trapped by DMPO.The settings were center field,3512.48G;microwave frequency,9.86GHz;and power,6.35mW.The TiO2powders used for TEM and ESR tests were carefully scratched off of the TiO2-coated quartz slides before the experiments.

2.5.Photocatalytic Activity Measurements.The photo-catalytic degradation of RhB in the liquid phase was conducted in a quartz tube with a

3.3cm inner diameter and a1

4.5cm length. The tube was illuminated by four surrounding wideband lamps (4W,Philips TL/05)with a predominant wavelength at365nm.It is assumed that both TiO2and the RhB dye can be excited in this experiment because the lamps emit both ultraviolet and visible light and the wavelength coverage is from350to450nm.The light intensity at different wavelengths has been measured with a light meter(Supporting Information SI-1).Two TiO2bilayer films

Chart1.Rhodamine B

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M.Phys.Rev.B1985,31,3369–3371.

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Article Zhuang et al.

(TiO 2content ～1.2mg,Supporting Information SI-2)were immersed in 80mL of an RhB solution.Prior to irradiation,the solution was magnetically stirred in the dark for 30min to ensure the establishment of an adsorption/desorption equilibrium be-tween the TiO 2film and the RhB dye.An aliquot (2mL)of the solution was taken at a certain time interval during the experiment and analyzed on a Varian UV -vis spectrophotometer (Cary-50).After every assay,the analyzed aliquot was quickly poured back into the reactor to ensure a roughly equivalent volume of solution.The change in RhB absorbance in the solution was used to monitor the extent of reaction at given irradiation time intervals.Total organic carbon (TOC)assays of the degraded solution after 5h of irradiation were carried out on a TOC analyzer (TOC-V CPH ,Shimadzu).The efficiency of RhB mineralization (E M )was calculated with the following equation (eq 1)

E M ?

TOC 0-TOC F

TOC 0

?100%

e1T

where TOC 0is the calculated TOC value of the initial RhB solution and TOC F is the final measured TOC value of the degraded RhB solution.

3.Results and Discussion

3.1.Characterization Results. 3.1.1.SEM and TEM.Figure 1shows the cross-sectional SEM image of the Id-TiO 2film.From the SEM image,we can identify the bilayer structure of the as-prepared film.The inner layer is well kept after the cold plasma treatment (CPT).Moreover,the inner layer integrates with the outer layer and the TiO 2film is observed to be compact and uniform.The thickness of the TiO 2monolayer is in the range of 70-100nm.The as-obtained Id-TiO 2film,containing ca.7nm TiO 2nanoparticles with lattice fringes of d =0.351nm that are well matched with the crystallographic plane of anatase TiO 2(101),is observed in Figure 2.This result also agrees well with the XRD results of the Ns-TiO 2film (Supporting Information SI-3).It confirms that the CPT is just a surface-treatment technology and will not affect the crystal form of the bulk material.

3.1.2.XPS.To investigate surface states of the TiO 2films after CPT for 5min,Ti 2p core levels were measured by X-ray photoelectron spectroscopy (XPS).The comparison of the Ti 2p spectra taken on the Ns-TiO 2and the Sd-TiO 2films is shown in Figure 3.The peak of Ti 2p for the untreated surface (Ns-TiO 2)is symmetric,indicating fully coordinated Ti 4tions.22However,a shoulder 1.5eV to the right of the main Ti 2p 2/3peak appears after the sample was treated with plasma for 5min,indicating the creation of Ti 3tdefects on the surface.It was presumed that the surface defects were created by the removal of surface oxygen

atoms or,most likely,bridging oxygen atoms on those crystal facets that have bridging species.22,32The principal conclusion from the XPS result is that the CPT method can indeed introduce defects onto the surface of the TiO 2film.However,no shoulder peak has been observed for the Ti 2p 2/3peak (Ti 3tsites)over the Id-TiO 2sample.The absence of the Ti 3tsites in Id-TiO 2can be explained by the fact that the inner TiO 2layer with defect sites is coated with another normal TiO 2layer and XPS testing occurs only on the outer surface of the sample.

3.2.Photocatalytic Degradation of RhB on As-Prepared TiO 2Films.The photocatalytic activities of the as-prepared samples are evaluated by the degradation of rhodamine B (RhB)under UV -vis light illuminations.The temporal adsorption spectral changes of the RhB solution (10μM)taking place during irradiation are illustrated in Figure

4.As a comparison,an RhB photolysis process without TiO 2films was also performed under the same conditions.It can be seen in Figure 4a that RhB has a major absorption band at 554nm.During irradiation,small changes in both the absorption peak intensity and wavelength shifts are observed with increasing time,indicating that the RhB solution is fairly stable to irradiation.

The photodegradation of RhB assisted by Ns-TiO 2films is shown in Figure 4b.After 180min of irradiation,the maximum absorption band of the solution gradually shifted from 554to 499nm.The gradual hypsochromic shifts of the absorption maximum are caused by the N-deethylation of RhB during irradiation,which has been confirmed by Watanabe and co-workers.33This

Figure 1.Cross-sectional SEM image of the Id-TiO 2bilayer film.Figure 2.HRTEM image of TiO 2.

Figure 3.Ti 2p XPS spectra of TiO 2thin films:(a)Ns-TiO 2and

(b)Sd-TiO 2.

(32)Lu,G.;Linsebigler,A.;Yates,J.T.,

Jr.J.Phys.Chem.B 1994,98,11733–11738.

(33)Watanabe,T.;Takizawa,T.;Honda,K.J.Phys.Chem.1977,81,1845–1851.Takizawa,T.;Watanabe,T.;Honda,K.J.Phys.Chem.1978,82,1391–1396.

Zhuang et al.Article

hypsochromic shift in λmax of RhB corresponds to a step-by-step deethylation of RhB to give N ,N ,N 0-triethyl rhodamine (TER,539nm),N ,N 0-diethyl rhodamine (DER,522nm),N -ethyl rhodamine (MER,510nm),and rhodamine at 498nm.The molar absorption coefficients ?max of these five compounds are 11.5?10-4,5.5?10-4,7.2?10-4,6.1?10-4,and 8.4?10-4M -1cm -1,respectively.The final photodegraded product of the RhB/Ns-TiO 2system is presumed to be the completely N-deethy-lated compound (rhodamine),which has a characteristic absorp-tion peak at 498nm in aqueous solution.The yield of rhodamine from RhB was estimated to be ca.50%by a calculation of the absorption spectra using the ?max of these two compounds.Moreover,the extent of mineralization of dye was examined by determining the total organic carbon (TOC)in the solution (eq 1).TOC values reflect the general concentration of organics in the solution bulk;therefore,changes in the TOC reflect the degree of mineralization of an organic substrate during the irradiation period.After UV irradiation for 5h,the TOC value of the final solution is determined to be 2.53ppm,which is about 0.83ppm less than the calculated value of the initial RhB solution (3.36ppm).The results suggest that deethylation dominates in the catalytic process whereas the mineralization efficiency is evaluated to be only ca.25%.

By contrast,when the Sd-TiO 2film is used,the adsorption peak of the dye at around 554nm undergoes a fairly large decrease with irradiation time whereas the hypsochromic shifts of the absorp-tion band are considerably insignificant,as shown in Figure 4c.It is presumed that there is negligible deethylation and that the cleavage of the whole chromophore structure (cycloreversion)of RhB occurs preferentially over the Sd-TiO 2film.Concomitantly,no new absorption peaks appear,which confirms the mineraliza-tion of RhB on the Sd-TiO 2film.The mineralization efficiency is

determined to be ca.60%by a calculation based on the TOC values of RhB before and after 5h of illumination.

For the RhB/Id-TiO 2system (Figure 4d),however,the absorp-tion bands of RhB show two types of spectral changes.One is a hypsochromic shift in the absorbance maximum;the other is a decrease in absorbance.The absorbance maximum of RhB shifts gradually from 554to 510nm;concurrently,the absorption peaks of the dye remarkably fade away.Moreover,absorption peaks located at wavelengths lower than 400nm also decrease rapidly during the illumination period.The product after 5h of illumina-tion,which has an absorption peak at 510nm,can be identified as an incompletely N-deethylated outcome of RhB,N -ethyl rhoda-mine (MER).The final TOC value of the solution of Id-TiO 2is 0.87ppm,demonstrating the highest mineralization efficiency (ca.75%)among these three specimens.

Figure 5summarizes the corresponding wavelength shifts and the absorbance changes in the major absorption bands of the RhB solution over those in the irradiated TiO 2films.As shown in Figure 5a,a similar tendency can be found in the spectral changes in RhB over both Ns-TiO 2and Id-TiO 2films.The absorbance of RhB decreases quickly in the first 60min;meanwhile,the absorbance maximum also shifts dramatically after 90min of illumination.These drastic changes can be assigned to the N-deethylation of RhB,which takes place in a stepwise manner and predominates during the initial irradiation period in both systems of RhB/Ns-TiO 2and RhB/Id-TiO 2.When the Sd-TiO 2film is used,steady degradation but negligible deethylation of RhB is perceived during irradiation.Note that in Figure 5b,when the irradiation time is >90min,the photodegradation rate of RhB over the Sd-TiO 2film is similar to that over the Id-TiO 2film.It can be inferred from the above results that the N-deethylation of RhB is closely related to the surface of the TiO 2film whereas the

Figure 4.Temporal UV -visible adsorption spectral changes for the RhB solution (10μM)as a function of UV (365nm)irradiation time:

photolysis of (a)RhB,(b)the Ns-TiO 2bilayer,(c)the Sd-TiO 2bilayer,and (d)the Id-TiO 2

bilayer.

Article Zhuang et al.

promoted cycloreversion has something to do with the existence of defects.

It has been well known that surface defects may influence the adsorption of molecules on oxide surfaces.34To confirm the effect of surface defects on RhB adsorption,all three types of TiO 2films were immersed in RhB solution for 30min.The comparison of the absorption spectra of these TiO 2films (adsorbed RhB)is shown in Figure 6.Note that no significant change is observed for the absorption spectra of Sd-TiO 2as contrasted with the original spectra,whereas an obvious absorption at ～560nm appears on the spectra of Ns-TiO 2and Id-TiO 2films.The absorption peak can be identified as RhB,and the slightly red-shifted absorption mainly corresponds to the dye associated with the surface of TiO 2.35Apparently,the RhB dye adsorbs more poorly on the Sd-TiO 2surface than on the untreated TiO 2surfaces of Id-TiO 2and Ns-TiO 2.

Several reasons may make it hard for the dye molecules to approach the surface of the Sd-TiO 2film.One is the unavoidable adsorption of molecules (such as O 2and H 2O)on the defect-enriched surface when the Sd-TiO 2film was exposed to the atmosphere.Covered by adsorbates,the surface of the oxide will be unfavorable for the approach of dye molecules.Furthermore,the electrostatic repulsion between the cationic RhB dye and the positive Sd-TiO 2surface (t TiOH 2t)due to the surface oxygen vacancy also prevents the RhB molecule from adsorbing onto the TiO 2surface efficiently.

The difference in the adsorption ability of Ns-TiO 2,Sd-TiO 2,and Id-TiO 2samples for RhB may account for the different degradation processes of RhB over these three samples during the initial irradiation.Considerable investigations aimed at the me-chanism of the photosensitized oxidative transformation of RhB over the semiconductor surface have been carried out.Watanabe and co-workers 33investigated the N-deethylation of RhB ad-sorbed on CdS.He noticed that electron transfer from the adsorbed dye in its singlet excited state to the conduction band of the semiconductor is the principal pathway as the initiation step of the observed photochemical N-deethylation of RhB.The strong adsorption of dyes on the semiconductor surface is an important criterion for efficient charge transfer.16,36In the RhB/Sd-TiO 2system,RhB molecules cannot approach the TiO 2sur-face and the electrons can no longer be injected from excited RhB states into the conduction band of the semiconductor.In other words,charge transfer between excited RhB molecules and a semiconductor is less efficient on the surface of Sd-TiO 2than on the surface of Ns-TiO 2.Therefore,the N-deethylation of RhB cannot effectually take place on Sd-TiO 2films.

3.3.Primary Active Species in the RhB/TiO 2Systems.It is generally accepted that the photocatalytic degradation of organic compounds proceeds via two routes:direct hole oxidation or ?OH radical oxidation.To elucidate the paths involved in the photocatalytic degradation of RhB on different TiO 2samples,the effects of radicals and hole scavengers are evaluated.

Isopropanol has been described as the best hydroxyl radical quencher because of its high-rate-constant reaction with the radical.37It has been widely used in photocatalysis in order to discriminate the direct oxidation of substrates by holes or by ?OH radicals.38,39When isopropanol is added to the RhB solution at a concentration of 10mM,extensive inhibitions in RhB degradation are observed in all three heterogeneous systems.The result suggests that the hydroxyl radical plays an essential role in the reaction mechanism of RhB oxidation,which might be the only oxidation route in our experiments.With this viewpoint,the higher degrada-tion efficiency of RhB in the Id-TiO 2system compared to that in the Ns-TiO 2system can be attributed to the higher concentration and generation rate of ?OH in homogeneous media.

Figure 6.UV -vis absorption spectra of TiO 2films before (---)

and after (-)adsorbing RhB:(a)Ns-TiO 2,(b)Sd-TiO 2,and (c)Id-TiO 2.

(34)Hollins,

P.Surf.Sci.Rep.1992,16,51–94.

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Zhuang et al.Article

This speculation can be confirmed by an ESR experiment in which the concentration of ?OH radicals is tested by the DMPO-?OH adduct signature.Figure 7illustrates the electron-spin resonance spectra of the DMPO-?OH spin adduct under ambient conditions at different irradiation times.The character-istic four peaks of the DMPO-?OH adduct with intensity 1:2:2:1can obviously be observed after irradiation.40The ESR signal intensity increases in the first 5min of irradiation and subse-quently decays.Note that the intensity of the DMPO-?OH adduct signal is much stronger in irradiated Id-TiO 2than in irradiated Ns-TiO 2under similar conditions.Moreover,the lifetime of DMPO-?OH formed in the Id-TiO 2suspension is much longer than that in Ns-TiO 2.The ESR results provides a solid indication that the photogenerated charge carriers in Id-TiO 2not only possess high separation efficiency but also are long-lived enough to diffuse to the surface and subsequently react with the adsorbed H 2O and hydroxyl groups.It is postulated that the defect site can act as a shallow trapped site to promote the separation of photogenerated electron -hole pairs and subsequently accelerate the formation of ?OH radicals.

However,the results in Figure 4shows that the photooxidation of RhB proceeds by two different routes:a main N-deethylation reaction on the Ns-TiO 2film and a main cycloreversion on the Sd-TiO 2film.In fact,Zhao et al.41found that the N-deethylation of RhB by ?OH radicals is mostly induced by the ?OH radicals at the surface of the TiO 2film (?OH surf )whereas oxidative degrada-tion of the dye chromophore (cycloreversion of RhB)is mainly caused by the ?OH radicals in the bulk solution (?OH sol ).There-fore,the difference in activity between Ns-TiO 2and Sd-TiO 2films may be attributed to the different reaction routes induced by the different kinds of ?OH radicals in the two TiO 2systems (i.e.,the photoinduced N-deethylation of RhB over Ns-TiO 2films is mainly dependent on the ?OH surf radicals whereas the photo-induced cycloreversion of RhB is mainly dependent on the ?OH sol radicals over Sd-TiO 2films).

Hydroxyl radicals can be formed via both two pathways,including electron-transfer mediation and the photogenerated holes reaction.To make clear the generation route of ?OH radicals over Ns-TiO 2and Sd-TiO 2films,methanol,which acts as an efficient hole scavenger,42-44was added to the solutions at the

beginning of the photocatalytic reaction to quench the photo-generated holes in the irradiated TiO 2films.Figure 8summarizes the time-dependent degradation of RhB over different TiO 2films with methanol addition (10mM).The addition of methanol does not suppress the degradation of RhB on the Ns-TiO 2film (curve a).This means that the formation of ?OH radicals at the surface of the Ns-TiO 2film is mainly dependent on electron-transfer mediation.Because the photogenerated holes are sca-venged by methanol,the photogenerated electrons can easily diffuse to the surface of the Ns-TiO 2film to form ?OH radicals,leading to the degradation of RhB.

For the RhB/Sd-TiO 2system,a remarkable inhibitory effect is observed (curve b).The result suggests that the photogenerated holes play a predominant part in the photooxidation of RhB on the defective surface of Sd-TiO 2films.Because neither the concentration of RhB is high nor the RhB molecules are strongly adsorbed on the Sd-TiO 2surface under the experimental condi-tions (Figure 6),there is little possibility of a direct reaction of RhB molecules with holes.A logical conclusion is that the holes that diffuse to the semiconductor surface mainly react with adsorbed hydroxyl groups or H 2O molecules.Therefore,distinct from that in the RhB/Ns-TiO 2system,the ?OH radicals in the bulk solution are mainly formed via the photogenerated holes reactions in the RhB/Sd-TiO 2system.

Although the addition of methanol to the RhB/Id-TiO 2system also brings about an obvious inhibitory effect,the degradation of RhB to some extent can still be observed after 5h of irradiations (curve c).The result indicates that a great number of the ?OH radicals formed in the RhB/Id-TiO 2system come from reactions between the photogenerated holes and the surface-adsorbed H 2O

Figure 8.Effect of methanol addition (10mM)in the photocata-lyzed degradation of RhB (10μM)with irradiated TiO 2films:(a)Ns-TiO 2,(b)Sd-TiO 2,and (c)Id-TiO 2.

(40)Kochany,J.;Bolton,J.

R.J.Phys.Chem.1991,95,5116–5120.

(41)Wu,T.;Liu,G.;Zhao,J.;Hidaka,H.;Serpone,N.J.Phys.Chem.B 1998,102,5845–5851.

(42)Spanhel,L.;Weller,H.;Henglein,A.J.Am.Chem.Soc.1987,109,6632–6635.

(43)Howe,R.F.;Gratzel,M.J.Phys.Chem.1985,89,4495–4499.

(44)Tachikawa,T.;Tojo,S.;Kawai,K.;Endo,M.;Fujistuka,M.;Ohno,T.;Nishijima,K.;Miyamoto,Z.;Majima,T.J.Phys.Chem.B 2004,108,19299–19306.

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or OH groups,whereas the rest are formed from the photogen-erated electrons.

In a word,the photogenerated holes play a predominant role in the generation of hydroxyl radicals over defective TiO 2films.From what has been discussed above,we deduce that the defect sites,no matter whether they are present at the surface or at the interface of the TiO 2bilayer,can act as shallow trap sites to reduce the recombination of photogenerated electrons and holes.Con-sequently,more photogenerated holes can diffuse to the surface to react with surface-adsorbed H 2O and hydroxyl groups,leading to an accelerated generation rate of the ?OH radical.45,46

3.4.Degradation Mechanism of RhB over Different TiO 2Films.On the basis of the above explanations for the photo-induced degradation of RhB over the different TiO 2films,the

proposed photochemical processes in these three RhB/TiO 2systems are elucidated in Scheme 2.To illuminate the mechanism in concise form,only the outer layer of the TiO 2bilayer is taken into account.

For the Ns-TiO 2films,N-deethylation is the main reaction taking place during the UV -vis irradiation period in our experi-ments.Although the TiO 2semiconductor can be excited to produce electron -hole pairs,most of the charge carriers rapidly undergo recombinations,leading to the low photocatalytic effi-ciency of the TiO 2film (Scheme 2a1).When RhB molecules are chemisorbed onto TiO 2and excited,they can inject electrons into the conduction band of TiO 2.The electrons can be captured by the oxygen (electron acceptor)in solution to yield oxidant ?OH radicals (Scheme 2a2).Bai et al.47used an in situ STM method to study the adsorption of RhB in solution,and they found that the

Scheme 2.Electron Transfer between Excited TiO 2and RhB and Subsequent Formation and Reactions of ?OH Radicals on Different TiO 2Films

under UV -Vis Light Irradiation of RhB a

a

(a)RhB/Ns-TiO 2system,(b)RhB/Sd-TiO 2system,and (c)RhB/Id-TiO 2system.

(45)Ishibashi,K.;Fujishima,A.;Watanabe,T.;Hashimoto,

K.J.Photochem.Photobiol.,A 2000,134,139–142.

(46)Hirakawa,T.;Nosaka,https://www.sodocs.net/doc/ba16043115.html,ngmuir 2002,18,3247–3254.

(47)Wang,D.;Wan,L.J.;Wang,C.;Bai,C.L.J.Phys.Chem.B 2002,106,4223–4226.

Zhuang et al.Article

adsorbed RhB molecule stands by directly binding its two diethylamino (-N(C 2H 5)2)functional groups with the solid sur-face (Scheme 2a3).Therefore,the ?OH radicals yielded on the surface (?OH surf )readily attack the diethylamino groups,result-ing in the N-deethylation of RhB molecules.

Defect sites are presumed to affect the electron -hole recombi-nation process in the photocatalyst,causing a change in chemical reaction rates that depends on the charge transfer from either electrons or holes.In our experiment,bridging oxygen atoms at the two-coordinate bridging sites on the TiO 2surface can be removed by CPT,leading to the formation of oxygen vacancies (V O ??)on the surface of the TiO 2film.At room temperature,V O ??,which has an effective divalent positive charge with respect to the regular O 2-site,can easily recapture photoinduced electrons and enhance the photocatalytic reactions.27

Therefore,in the Sd-TiO 2film,the surface defect sites (V O ??)act as electron trap sites and shallowly trap the photogenerated electrons to reduce the recombination of electrons and holes (Scheme 2b1).Consequently,more holes diffuse to the surface to yield hydroxyl radicals (Scheme 2b2).In the RhB/Sd-TiO 2system,one possible adsorption of an RhB molecule on the defect-enriched TiO 2surface is shown in Scheme 2b3.With the opposed assistance provided by the repulsion between the cationic RhB molecule and the positively charged TiO 2surface,RhB can hardly adsorb on the surface.However,the RhB molecule may slightly adsorb on the TiO 2surface through the two oxygen atoms in the carboxylate group.Accordingly,N-deethylation of RhB on the Sd-TiO 2film is rather inefficient because the ?OH radicals generated on the surface cannot attack the diethylamino groups effectively.With the ceaseless formation of ?OH surf radicals,?OH radicals may diffuse into the bulk solution (?OH sol )48and then attack the chromophoric structure,leading to the cycloreversion of the RhB compounds.

It is evident that the interface defects can also promote the diffusion of photogenerated holes (Scheme 2c1).Moreover,as shown in Figure 6,RhB molecules adsorb lightly on the surface of the Id-TiO 2film.The electron transfer between dye molecules and semiconductors occurs easily (Scheme 2c2),and the subsequent N-deethylation of RhB molecules becomes the supreme reaction during the initial irradiation period.The concentration of ?OH sol radicals increases with the irradiation time and can oxidize the dye (Scheme 2c3).In this sense,the foregoing process involved in the N-deethylation of RhB over the Id-TiO 2film is similar to that over the Ns-TiO 2film,whereas the cycloreversion process also

shares many similarities with that over the Sd-TiO 2film.It is reasonable to think that the photochemical process occurring on the irradiated Id-TiO 2film is an integration of N-deethylation and the oxidization of RhB on the Ns-TiO 2and Sd-TiO 2films,respectively.

Furthermore,the specific catalytic process of RhB over Id-TiO 2films also leads to the highest mineralization efficiency in this heterogeneous https://www.sodocs.net/doc/ba16043115.html,bined with Figure 4d,we deduce that incomplete N-deethylation takes place in the initial irradia-tion period and then the ?OH radicals in the bulk solution will be more liable to attack the chromophore structure of incompletely deethylated RhB (MER)compounds.In other words,segmental deethylation may be beneficial for the cleavage of the aromatic chromophore and may promote the mineralization efficiency of the RhB dye in this case.

3.5.Stability of the Defects.Defects have long been thought to be a promotional factor in the catalytic reaction process.It would be significant if the defect sites were stable during the heterogeneous catalytic process.According to pre-vious studies,9,21-23the surface defects are metastable surface states and may be readily removed under common or aqueous circumstances.To investigate the stability of surface defects and interface defects,both Sd-TiO 2and Id-TiO 2films were treated in boiling water for 5min.The hot-water-treated Sd-TiO 2and Id-TiO 2films are denoted as W-Sd-TiO 2and W-Id-TiO 2,respec-tively.Figure 9shows the time-dependent absorption spectra of RhB during the photodegradation of RhB over water-treated TiO 2films under UV illumination.It is interesting that a significant hypsochromic shift of the absorption maximum caused by the N-deethylation of RhB is observed in the RhB/W-Sd-TiO 2system during the irradiation period,similar to that in RhB/Ns-TiO 2(Figure 4b).The phenomenon obviously in-dicates that the surface defects are unstable and readily remo-vable in boiling https://www.sodocs.net/doc/ba16043115.html,pared with the original Id-TiO 2films (Figure 4d),although the degradation rate of RhB on W-Id-TiO 2films decreased to some extent,only minute changes were observed in the RhB degradation process on the whole.The results reveal that the interface defects are more stable than the surface defects and almost survived during the hot-water treat-ment process.Moreover,the TOC result (1.54ppm)suggests that the remaining interface defects do promote the mineralization of RhB,and the mineralization efficiency is calculated to be 54%.

4.Conclusions

The photodegradation of RhB over the respective TiO 2bilayer films with surface defects and interface defects was studied by

Figure 9.Temporal UV -visible adsorption spectral changes for the RhB solution (10μM)on water-treated TiO 2films as a function of irradiation time:(a)W-Sd-TiO 2films and (b)W-Id-TiO 2films.

(48)Turchi,C.S.;Ollis,D.

E.J.Catal.1990,122,178–192.

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UV-vis irradiation.The defects,no matter whether they are located at the surface or at the interface,are considered to act as functional electron traps to reduce the recombination of electrons and holes,resulting in a promoter photoactivity of TiO2films for RhB degradation.However,the photocatalytic degradation pro-cesses of RhB over the two defective TiO2films are different.In contrast to the Sd-TiO2film,the irradiated Id-TiO2film leads to both the N-deethylation(incomplete)and cycloreversion of RhB, resulting in the highest mineralization efficiency of RhB.More-over,the interface defect in the TiO2film is proven to be more stable than the surface defect.

Acknowledgment.The work was supported by the National Natural Science Foundation of China(no.20773025),the Na-tional Basic Research Program of China(973program, 2007CB613306),and the Program for Changjiang Scholars and Innovative Research Team in University(PCSIRT0818). Supporting Information Available:Light intensities at different wavelengths,XRD results for the Ns-TiO2film, and pictures of the RhB solution before and after photode-gradation over different TiO2films.This material is available free of charge via the Internet at https://www.sodocs.net/doc/ba16043115.html,.