Photocatalytic Degradation of 4BS Dye by N,S-Codoped TiO2 Pillared MontmorillonitePhotocatalysts under Visible-Light Irradiation

Gaoke Zhang,*,†,‡ Xinmiao Ding,† Yanjun Hu,† Baibiao Huang,‡ Xiaoyang Zhang,‡Xiaoyan Qin,‡ Jin Zhou,† and Junwei Xie†

School of Resources and EnVironmental Engineering, Wuhan UniVersity of Technology, 122 Luoshi Road,Wuhan 430070, People’s Republic of China, and State Key Laboratory of Crystal Materials, ShandongUniVersity, Jinan 250100, People’s Republic of China

ReceiVed: May 5, 2008; ReVised Manuscript ReceiVed: September 21, 2008

The photocatalytic activities of nitrogen and sulfur codoped TiO2 pillared montmorillonite (N,S-TiO2-PILM)for the degradation of 4BS dye under visible-light irradiation (λ >400nm) were studied. The catalysts weresynthesized by impregnating doped titania sol into the interlayer of montmorillonite (MMT) and characterizedby the UV-vis diffuse reflectance spectra, transmission electron microscopy, and the FT-IR absorption spectra.The ordered structure of MMT was destroyed to some extent and the size of TiO2 particles is about 2-6 nm.The absorption edge of the doped samples shows a red-shift as compared to that of pure TiO2. The photocatalyicactivity of N,S-TiO2-PILM with a mole ratio of Ti:S of 1:4 and obtained at 350 °C for 2 h is higher than thatof the other samples and Degussa P25 under visible light irradiation. The chromophore in the molecularstructure of 4BS was destroyed completely by the photocatalytic reaction and the naphthalene rings andbenzene rings were also decomposed partly.

1. Introduction

Photocatalysis technology with titanium dioxide (TiO2)photocatalyst has attracted extensive attention because of its lowcost, nontoxicity, and structural stability. However, the pollutantsin wastewater are usually much diluted, and those are thoughtto be harmful even if they are present in an extremely lowconcentration in the environment. On the one hand, TiO2

photocatalyst could be activated only by UV light due to itslarge energy band gap (ca. 3.2 eV for anatase). On the otherhand, TiO2 has a small surface area and low adsorbability, whichresults in low photocatalytic efficiency in much diluted solu-tions.1-5

To effectively utilize visible light, which represents about42% of the energy of the solar spectrum, much attention hasbeen paid to improve the photocatalytic property and visiblelight response of TiO2. Among all the methods, a main approachis to dope transition metals into TiO2.6-10 However, the metalion doped TiO2 suffers from thermal instability and the increaseof carrier-recombination centers.11 Recently, it was found thatdoping of a nonmetallic element such as carbon,12,13 nitrogen,14-18

sulfur,19-21 etc. into TiO2 may be more appropriate for theextension of photocatalytic activity of TiO2 into the visibleregion than other methods because their impurity states are nearthe valence band edge, and their role as recombination centersmight be minimized as compared to metal cation doping.2

Meanwhile, codoped titania with double nonmetal elements hasattracted more attention, such as N,S-codoped TiO2,21 TiO2-xAy

(A )N, S).22 Although the nonmetals doping can narrow theband gap of TiO2 photocatalysts and improve the utilization ofthe solar spectrum, the surface area and adsorbability of thecatalyst cannot be increased in this process.

TiO2 pillared clays have been studied by many researchersbecause of their high adsorbability and photocatalytic activity.23,24

In our previous research, we found that the clays in thecomposites not only can increase the specific surface area ofthe catalysts but also lower the forming temperature of anataseTiO2 in it.25 In the present work, we synthesized N,S-codopedTiO2-PILM composites through the method reported in ourprevious researches26 using TiCl4 as a precursor and studiedthe photocatalytic degradation of 4BS dye solution by the as-synthesized samples under visible light irradiation. A consider-able improvement of the photocatalytic activity was found forall the doped samples in comparison to the undoped TiO2-PILMcatalyst.

2. Experimental Section

2.1. Preparation of the Catalysts. The N,S-codoped TiO2-PILM (N,S-TiO2-PILM) powders were prepared by a modifiedsolution method. The detailed processes were described in ourprevious research.26 The obtained wet cakes were dried undervacuum at 80 °C and then calcined at 300, 350, 400, and 500°C in air for 2 h, identified as sample A, B, C and D,respectively. The TiO2-PILM was obtained by similar proce-dures without adding thiourea solution and calcined at 350 °Cfor 2 h, identified as sample E. The catalysts with different moleratios of Ti:S such as 1:2 and 1:6 were also prepared by theabove procedures and calcined at 350 °C for 2 h in air. Theundoped TiO2 was obtained by adding ammonia into the titaniasol slowly with vigorous stirring for several minutes thenfollowing the same procedure as above.

2.2. Photocatalytic Reaction. The photocatalytic activitiesof the as-prepared samples were evaluated on the basis of thephotodegradation of 4BS aqueous solution as a model dyewastewater. A 100 mL sample of 4BS aqueous solution (30mg/L) and 0.20 g of catalyst powders were added into a 500mL beaker. The suspension was stirred magnetically in the darkfor 10 min to disperse the catalyst before irradiating. The

* Corresponding author. E-mail: gkzhang@whut.edu.cn.† Wuhan University of Technology.‡ Shandong University.

J. Phys. Chem. C 2008, 112, 17994–1799717994

10.1021/jp803939z CCC: $40.75 2008 American Chemical SocietyPublished on Web 10/28/2008

irradiation was performed by a 300 W Dy lamp with a lightfilter (wavelength G400 nm) 25 cm above the liquid surface.At a defined time interval, the absorbance of the 4BS solutionwas analyzed by using a UV-vis spectrophotometer (UV751GD)at 500 nm.

3. Results and Discussion

3.1. Characterization. 3.1.1. TEM Analysis. Crystallitesizes, phases, and shapes were observed with transmissionelectron microscopy (TEM) and high-resolution transmissionelectron microscopy (HRTEM) (H-600 STEM/EDX PV9100,Japan), operated at 200 kV. From Figure 1a,b, it can be seenthat the intercalation of TiO2 nanoparticles into the interlayersof MMT destroyed the ordered structure of MMT to someextent, resulting in some exfoliated one-layer and multilayersheets. TiO2 nanoparticles were formed in the interlayers ofMMT and on the surface of MMT during the hydrolysis processof the precursor. The photograph in Figure 1b clearly showsthe lattice fringe spacing of the TiO2 (anatase) nanoparticles isabout 0.352 nm and the size of TiO2 particles is about 2-6 nmand some of them are marked in Figure 1b.

3.1.2. UV-Vis Diffuse Reflectance Spectra. The diffusereflectance spectra (Figure 2) of sample B and TiO2 were carriedout on a UV-vis spectrophotometer (UV2550, Shimadzu,Japan) and BaSO4 was used as a reflectance standard. FromFigure 2, the absorption edge of N,S-TiO2-PILM shows astronger absorption in the UV-vis light region and a shift tothe visible light region as compared to that of undoped TiO2.The wavelength of the absorption edge of sample E and TiO2

is 394.31 and 422.57 nm, respectively. Thus, the band gapenergy estimated from the absorption edges is about 3.14 and2.93 eV, respectively. The red-shift in the N,S-TiO2-PILMsample is ascribed to the synergetic effects of substitution ofthe crystal lattice oxygen by nitrogen during the TiO2 nitridationand the presence of an impurity state of S3p on the upper edgeof the valence band.20

3.1.3. FT-IR Absorption Spectra. Figure 3 shows the FT-IR absorption spectra of TiO2-PILM and N,S-TiO2-PILMobtained on Fourier transform infrared spectroscopy (FT-IR)(Nexus, Thermo Nicolet). It can be seen that there were no

significant differences between the two spectra of before andafter doping the N and S in the spectra. The FT-IR spectrum ofthe N,S-codoped sample shows three new absorption peaks at2919, 2850, and 1400 cm-1, which correspond to the dissym-metrical stretching vibrations, symmetrical stretching vibrations,and bending vibrations of the -CH2- group, respectively.27

Figure 1. (a) TEM image of sample B; (b) HRTEM photograph of sample B.

Figure 2. The UV-vis diffuse reflectance spectra of the samples.

Figure 3. The FT-IR absorption spectra of samples calcined at 350°C.

Photocatalytic Degradation of 4BS Dye J. Phys. Chem. C, Vol. 112, No. 46, 2008 17995

The reason may be caused by breaking up of the -CdS- and-CsN- and results in the organic species remaining on thesample surface. The S element substituted Ti4+ as a form ofS6+ and the substitutional N formed N-Ti-N and Ti-N-O,respectively.26 The two peaks at 526 and 469 cm-1 are ascribedto Si-O-Mg and Si-O-Fe bending vibrations, respectively.The intensity of the latter is stronger than that of the former,which indicates that the contents of Fe is higher than that ofMg and the basic layer structure of MMT remained afterpillaring.

3.2. Photocatalytic Properties. Our experimental resultsconfirm that the relationship between the absorbance and theconcentration of 4BS aqueous solution can be described as thestandard curve equation:

A ≈ 0.0231X (a)

where A is the absorbance and X is the concentration of the4BS solution.

Figure 4 shows the UV-vis absorption spectra of the 4BSsolution at different reaction times, in which the absorptionspectrum of the original solution shows four characteristic peaksat 208, 235, 337, and 500 nm.The two absorption peaks at 208and 235 nm belong to the structure of the benzene rings, theother two peaks correspond to the naphthalene rings andthe nitrogen-to-nitrogen double bond (-NdN-), respectively.The absorption peak at 500 nm decreased rapidly followingphotocatalysis, which confirmed the breakup of the chromophoreresponsible for the characteristic color of the azo dyes, ratherthan its discoloration or bleaching,28 and the -NdN- bond ofthe dyes in this study were the most active sites for oxidativeattack. The characteristic peaks of the naphthalene rings andthe benzene rings became smooth after visible-light irradiationfor 120 min, which illuminated that the catalyst not onlydestroyed the chromophore of 4BS, but also decomposed thenaphthalene ring and benzene ring partly. These results werealso confirmed by the comparative experiments without lightirradiation. The comparison experiment indicated that the fourdistinctive peaks still exist and the intensity of these peaksdecreases noticeably after 120 min without light irradiation andthe color of the recovered catalyst was red, which confirmedthat most of the 4BS dye was only adsorbed. It can be concludedthat the N,S-codoped TiO2-PILM powders have a strongeradsorbability and the light source is another key condition duringthe photocatalytic reaction. Therefore, the presence of both lightillumination and catalysts is necessary for the efficient degrada-

tion and the photocatalytic degradation of 4BS solution wascaused by photocatalytic oxidation reactions on the surface ofthe catalysts under visible light irradiation not by the adsorptionfunction.

The degradation rate of 4BS dye over the different catalystswas given in Figure 5. It can be seen that the degradation of4BS dye by the samples A, B, C, D, and P25 under visiblelight irradiation for 60 min is 91.4%, 95.6%, 74.46%, 53.54%,and 80.34%, respectively. It is noticeable that the degradationof 4BS over the sample E after 20 min hardly increased andthe recovered catalyst showed a red color, which indicates thedegradation of 4BS was mainly ascribed to the adsorptionbehavior of the undoped TiO2-PILM. From the results in Figure5, the calcination temperature has an obvious effect on thephotocatalytic activities of the catalysts. The sample calcinedat 300 °C shows decent photocatalytic activity. The photocata-lytic activity of the catalyst increases with increasing thecalcination temperature. The catalyst obtained at 350 °C showsthe highest photocatalytic activity. Usually, the specific surfacearea has a strong effect on the photocatalytic activity.22 Thedecrease of photocatalytic activity of the N,S-TiO2-PILMpowders calcined at above 400 °C was due to the sintering andthe growth of TiO2 crystallites, which results in a significantdecrease of specific surface area of the TiO2 powders,22,26 andmay lead to a decrease of nonmetal element atom entering intothe TiO2 lattice.21,29

The visible-light photocatalytic activity of N,S-TiO2-PILMobtained at 350 °C was higher than that of the undoped TiO2-PILM and P25, which can be explained by N,S-codoping andits pillared structure. N-doping into TiO2 can narrow the bandgap and enhance the photocatalytic activities of the catalystsaccording to the previous studies.30-32 S-doping could also lowerthe band gap of TiO2 by the presence of an impurity state ofS3p on the upper edge of the valence band.21 Yu et al. foundthat mixing the N2p and S3p states with O2p states could narrowthe band gap of N,S-codoped TiO2 powders and lead to astronger absorption in the UV-vis light region.21 The result inFigure 2 indicates that the red-shift of TiO2 caused by the N,S-codoped results in an increase of the photo numbers absorbedand the efficiency of the band gap transition resulting fromintensive absorbance in the UV region.21 Moreover, N- andS-codoping also increased the number of photogeneratedelectrons and holes to participate in the photocatalytic reaction.The synergetic effects of codoping with double nonmetalelements can increase the visible-light photocatalytic activity

Figure 4. The UV-vis absorption spectra of 4BS solution during thedegradation by sample B. Figure 5. The degradation of 4BS dye by different catalysts.

17996 J. Phys. Chem. C, Vol. 112, No. 46, 2008 Zhang et al.

of the N,S-codoped TiO2 catalyst.21,22 As shown in Figure 4,the pillared structure of the as-prepared catalyst results in itsstronger adsorbabilitysthe enrichment of the reactant enhancedthe rate of photocatalytic degradation on the catalysts. Therefore,the high visible-light photocatalytic activity of the N,S-TiO2-PILM powders may be ascribed to the synergetic effects of thedoped nitrogen and sulfur atoms and the large specific surfacearea and mesoporous structure of the N,S-TiO2-PILM catalyst.26

In addition, the pillared structure and TiO2 dispersed in thestructure can provide more microenvironments and active sitesfor the photoreactions. The layered clays are able to spatiallycompartmentalize electron donors and acceptors and can separatephotoproducts. Some researches indicated that the photoinducedelectrons and holes could be separated more effectively con-sidering a charge transfer between the guest and the hostmaterials of composites.33-35 The mole ratio of Ti:S in thecatalyst has an important effect on its photocatalytic activity.The degradation of 4BS dye over the N,S-TiO2-PILM sampleswith different mole ratios of Ti:S is given in Figure 6. It can beseen that the as-prepared samples have the highest photocatalyticactivity when the mole ratio of Ti:S is 1:4. S-doping into TiO2

can restrain the growth of TiO2 crystallites, but an overdose ofS may cover the surface or exist in the interface of TiO2

crystallites, which can accelerate the aggregation of TiO2

particles and result in the growth of TiO2 crystallites becauseof the function of the bridge connection.36,37

4. Conclusion

N,S-TiO2-PILM catalysts were successfully synthesized bya sol method via impregnating doped titania sol into theinterlayers of MMT, using TiCl4 as a precursor of TiO2. Theordered structure of MMT was destroyed to some extent bythe intercalation of TiO2 nanoparticles into the interlayers ofMMT. The size of TiO2 particles is about 2-6 nm.Theabsorption edge of the N,S-TiO2-PILM had a red-shift ascompared to that of pure TiO2 because of the doping of N andS. The FT-IR spectra illuminate that N and S were successfullydoped into the TiO2-PILM and the basic layered structure ofMMT still remained after pillaring. The catalyst with a moleratio of 1:4 of Ti:S and calcined at 350 °C for 2 h shows thehighest photodegradation efficiency. The visible-light catalyticactivity of the as-prepared N,S-codoped catalyst for degrading4BS dye is higher than that of the undoped catalyst and DegussaP25 TiO2. The UV-vis absorption spectra of 4BS solutionanalyses suggested that the chromophore was destroyed com-

pletely and the naphthalene rings and benzene rings were alsodecomposed partly.

Acknowledgment. This work was supported by the NationalBasic Research Program of China (973 Program) 2007CB613302and Program for New Century Excellent Talents in University(NCET05-0662).

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JP803939Z

Figure 6. The degradation of 4BS dye over the catalysts with differentmole ratios of Ti:S.

Photocatalytic Degradation of 4BS Dye J. Phys. Chem. C, Vol. 112, No. 46, 2008 17997