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Chemical Engineering Journal 180 (2012) 330– 336

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

jo u r n al hom epage: www.elsev ier .com/ locate /ce j

ybrid nanocomposite with visible–light photocatalytic activity: CdS–pillareditanate

ie Fu, Gengnan Li, Fengna Xi, Xiaoping Dong ∗

epartment of Chemistry, School of Sciences, Zhejiang Sci-Tech University, 5 Second Avenue, Xiasha Higher Education Zone, Hangzhou 310018, China

r t i c l e i n f o

rticle history:eceived 5 July 2011eceived in revised form 23 October 2011ccepted 31 October 2011

a b s t r a c t

CdS–pillared titanate as an efficient visible–light active photocatalyst has been synthesized through asimple reassembling reaction between negatively charged titanate sheets and positively charged CdScolloid nanoparticles. The formation of the heterostructure with the interlayer spacing of 2.7 nm wasclearly evidenced by powder X–ray diffraction and high–resolution transmission electron microscopic

eywords:onolayered titanate

dS colloidsxfoliation–restackingisible–light photodegradation

analysis. The diffuse reflectance UV–visible measurement demonstrated that the nanocomposite dis-plays dual band–gap character with Eg = 3.47 and 2.24 eV, corresponding to the values for titanate hostlayers and CdS nanoparticles, respectively, and the optical feature of CdS nanoparticles make them asthe sensitizers to photosensitize titanate host layers under visible–light irradiation. The test of photocat-alytic activity revealed that the methylene blue molecules could be photodegraded effectively under thevisible–light irradiation (� > 420 nm).

. Introduction

Zeolites have been widely studied and applied in the processesf drying, separation, sorption and selective catalysis due to theirharacteristic properties of the microporous structures [1,2]. There-ore, intense researches have been devoted to synthesize a seriesf materials with similar porous structure. Pillaring of sol particlesnto layered clays provides a useful method for creating relatedorous materials with zeolite properties, where the silicate layersf clays are pillared by particles with nano- to subnanometer size3–6]. However, both zeolites and clays with silicate networks arensulators and do not show photocatalytic activities. Thus, manynterests have been focused on porous materials with semiconduct-ng or magnetic networks. A number of transition metal oxysalts,uch as K2Ti4O9 [7–10], KNb3O8 [8,11], NaMnO2 [12], RbTaO3 [13],s0.7Ti1.825O4 [9,14,15], and other materials which are derived fromhem, e.g., K0.8Fe0.8Ti1.2O4 [16], and K0.8Ti(5.2−2x)/3Li(0.8−x)/3MnxO417], have typical layered structure. They are ideal precursors asost layers to obtain pillared porous structure with semiconductingr magnetic properties. However, most of these layered compoundso not swell like clays because of their higher charge density inost layers compare to the clays, which results in a useless of the

irect ion–exchange method applied to the pillaring of these lay-red materials.

∗ Corresponding author. Tel.: +86 571 86843228; fax: +86 057 86843228.E-mail address: xpdong@zstu.edu.cn (X. Dong).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.10.098

© 2011 Elsevier B.V. All rights reserved.

To overcome the above problem, several new methods havebeen developed. Landis et al. [18] firstly intercalated organic ammo-nium ions into the interlayer galleries of layered compoundsby ion–exchange to increase the interlayer distance, and thenhydrolyze silicon alkoxides between the host layers. Finally, thepillared structure was formed by calcining the product to removeorganic species. Following this way, the silica–pillared layeredstructures from K0.8Fe0.8Ti1.2O4 [16], Na2Ti3O7 [18], KCa2Nb3O10[19], and Rb0.75Mn0.75Ti1.25O4 [20] were successively reported.The intercalated organic ammonium ions could also be replacedby other large inorganic cations, such as [Al13O4(OH)24(H2O)12]7+

[21,22], and Cr3(OAc)72+ [23]. After calcining the ion–exchanged

products, the metal oxide pillared materials could be obtained.Besides the above–mentioned, the exfoliation–restacking processas another effective method has attracted considerable atten-tion. A series of layered metal oxides, e.g., K0.8Li0.27Ti1.73O4[24–26], RbTaO3 [13,27], KCa2Nb3O10 [28], K4Nb6O17 [29], andK0.45MnO2 [30,31], can be delaminated into their elemental lay-ers by treating their protonic forms with tetraalkyl ammoniumhydroxide, e.g., TBAOH. Since the resultant individual sheets arenegatively charged, it is expected to reassemble them with posi-tively charged nanoparticles or nanoclusters easily. By using thistechnique, alumina–[32], chromia–[33], iron oxide–[34,35], nickeloxide–[35,36], titanium oxide–[37–39] pillared layered titanate,and titanium oxide–pillared layered manganese oxide [40], layered

niobate [41], have been prepared successfully. These pillared mate-rials, especially those which own a semiconductor–semiconductorpillared structure, exhibit high specific surface areas and enhancedphotocatalytic activities due to their porous structure.

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Titanium oxides have received much attention owing to theirotential applications in terms of photocatalytic activities andptical properties, including solar energy conversion and envi-onmental purification [42–48]. However, titanium oxides onlybsorb the UV light. To efficiently utilize solar energy, numerousfforts have been carried out to improve their visible–light pho-ocatalytic activities, including doping with metals [49–52] andonmetal atoms [53–57], sensitizing with dyes [58,59], and cou-ling with other narrow band gap semiconductors [60–64]. It is aity that stable and efficient dyes are rare, and the photocatalyticctivities would be reduced in many cases of doped TiO2 due tohe high recombination rate of charge carriers [65]. Therefore, its more interested in designing and developing the narrow bandap semiconductors–TiO2 composites. Cadmiun sulfide is an idealemiconductor to sensitize TiO2 for visible–light response owingo its narrow band gap of 2.42 eV and the bottom of conductionand higher than that of TiO2 [44]. In the present work, we havettempted to prepare an efficient visible–light active photocata-yst with a CdS–pillared titanate heterostructure via restacking thexfoliated titanate nanosheets in the presence of CdS nanoparticleshich were positively charged by modifying with CTAB at room

emperature. The photocatalytic activity of the nanocomposite wasxamined by employing the photodegradation of methylene bluender visible–light irradiation.

. Materials and methods

.1. Materials

All chemicals were of analytical grade and were used withouturther purification. All water used throughout the experimentsas purified to a resistivity of ≥16.5 M� cm by an Arium 611 (Sar-

orius) reagent water system.

.2. Sample preparation

The starting layered titanate, K0.8Ti1.73Li0.27O4 (KTLO), was syn-hesized by solid–state reaction as previously described [66]. Theesulting potassium titanate was converted into its protonic phase,1.07Ti1.73O4·H2O (HTO), by treating the powder (∼1 g) with 100 mLf 1 M HCl solution for 3 days at room temperature, and the HClolution was renewed every 24 h. The resulting acid–exchangehase was filtered and then washed with copious amounts of dis-illed water. 0.4 g of the acid–exchange product was immersed in00 mL of 0.0025 M TBAOH solution at room temperature [24,25].fter one week of vigorous shaking, a turbid colloidal suspensionf Ti0.87O2 nanosheets was obtained.

Nanocrystallite cadmium sulfide was synthesized throughdding 40 mL of 0.25 M Cd(NO3)2 solution very slowly to 50 mLf 0.25 M Na2S solution in the presence of 2 M sodium silicatender violent stirring. After being stirred for several hours, therecipitate was collected by centrifuging and then washed withopious amounts of distilled water to remove the sodium silicateompletely. The CdS colloid nanoparticles can be prepared accord-ng to a reported process with some modification [67]. Briefly,.5 mmol of premade CdS nanoparticles were added to 100 mL of.01 M CTAB surfactant solution, and then ultrasonicated for severalours to produce monodispersed CdS colloid nanoparticles whichere stabilized by CTAB surfactant. After the ultrasonic treatment,

small fraction of incompletely dispersed particles was removedy decantation.

The incorporation of CdS colloid nanoparticles into the inter-ayer space of titanate sheets was achieved as follows: the premadei0.87O2 nanosheets colloidal suspension was introduced into thebove CdS colloid nanoparticles system, and kept static for 24 h at

ournal 180 (2012) 330– 336 331

room temperature. Finally, a yellowish product was obtained andwashed by absolute ethanol and distilled water successively forseveral times, and then dried at room temperature.

2.3. Characterization

The X–ray diffraction (XRD) patterns of powder samples weretaken by a Bruker D8 Advance diffractometer using Cu K� radiation(� = 0.15418 nm). The SEM images were observed using a Hitachi S-4800 field emission scanning electron microscope. High–resolutiontransmission electron microscope (HR–TEM) images were obtainedusing a JEOL JEM–2100 with an accelerating voltage of 200 kV.UV–visible diffuse reflectance spectra of samples were collected bya Shimadzu 2501–PC spectrophotometer equipped with an inte-grating sphere 30 mm in diameter using BaSO4 as a reference.

2.4. Photocatalytic reactivity test

Visible–light photocatalytic activity of the photocatalyst wasexamined by the degradation of methylene blue (MB) in aqueoussolution. 50 mg of the nanocomposite was added into 100 mL ofMB aqueous solution (10 mg L−1), and then magnetically stirredfor 2 h in the dark to establish absorption–desorption equilibriumbefore illumination. The light from a 300–W Xe arc lamp was passedthrough the condensed water and a UV light filter glass (to cutoffradiation with � < 420 nm) and then focused onto the reactor. Dur-ing the irradiation, stirring was maintained to keep the mixturein reaction suspension. Sample aliquots (4 mL) were removed atgiven time intervals and centrifuged to remove the photocatalystparticles. The concentration change of MB was analyzed by mea-suring the absorbance at � = 664.5 nm using a Shimadzu 2501–PCUV–visible spectrophotometer. The degradation efficiency at time twas determined from the value of C/C0, where C0 is the initial con-centration before illumination and C is the concentration of MBaqueous solution at the irradiation time t. For comparison, P25,KLTO, and N–doped TiO2 [68] were also examined using the sametesting procedure.

3. Results and discussion

3.1. The formation mechanism of the CdS–pillared titanatenanostructure

The formation process of the CdS–pillared titanate nanocom-posite by exfoliation–restacking technique is illustrated in Fig. 1.The positively charged CdS nanoparticle colloidal suspension andnegatively charged titanate nanosheets colloidal suspension wereobtained by treating the CdS nanoparticles under ultrasonicationin the presence of CTAB surfactant and chemical exfoliation of theparent layered protonic titanate in the TBAOH solution, respec-tively. Obviously, both of the suspensions exhibited clear Tyndalllight scattering effect when a side–incident light beam was used,demonstrating the presence of exfoliated titanate nanosheets andCTAB stabled CdS nanoparticles dispersed in the two aqueous sus-pensions, respectively. After simply mixing the two suspensions,a yellowish flocculated sample (the picture at the right–bottom ofFig. 1) with pillared structure was obtained.

3.2. Morphology analyses

Fig. 2(a) and (b) demonstrates the SEM images of as–flocculatedproduct which was performed by simply mixing the negatively

charged titanate nanosheet suspension and positively chargedCdS colloid nanoparticle suspension. The CdS–pillared titanatenanocomposite exhibits a plate–like morphology with the lateralsize about several hundred micrometers (see Fig. 2(a)), which is

332 J. Fu et al. / Chemical Engineering Journal 180 (2012) 330– 336

Fig. 1. Schematic model illustrates an exfoliation–reassembling route to construct the CdS–pillared titanate nanocomposite. The three photographs represent the picturesof exfoliated titanate nanosheet suspension, CdS colloid nanoparticle suspension, and CdS–pillared titanate nanocomposite, respectively. The light beam was incident fromt

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uch larger than those of the starting titanites and delaminatedheets [26,66]. This can be ascribed to the crumpling and overlap-ing of nanosheets during the process of flocculation, which can belearly observed in the TEM image (Fig. 2(c)). On the other hand,occulation always produces a randomly restocked lamellar mate-ial without a well–defined interlayer registry, as Fig. 2(b) shows.dditional, the restacked product shows a high surface area, whichan be observed in Fig. 2(b). This is beneficial to the adsorption ofrganic dyes and finally improves the degradation of organic pol-utants. Fig. 2(d) exhibits the EDS spectra of the nanocomposite,

hich shows distinct peaks attributed to Ti, O, Cd and S, confirm-ng the chemical identities of the corresponding elements in theanocomposite.

ig. 2. SEM images of CdS–pillared layered titanate at a low magnification (a) and high m

3.3. Structure analyses

The powder XRD patterns of the layered potassium titanateKTLO, acid–exchanged form HTO and CdS–pillared titanate arerepresented in Fig. 3. The XRD features for the KTLO (Fig. 3(a))correspond to the lepidocrocite–like layered structure withorthorhombic symmetry and C base–centered lattice type, whichhas been reported in the previous literature [66]. After acid treat-ment of the pristine KTLO, the (0 2 0) reflection shifts down to alower angle side (Fig. 3(b)). The corresponding d value is increased

from 0.77 nm for KTLO to 0.92 nm for HTO, which is attributed tothe incorporation of a monolayer of water molecules. Strong andsharp (0 2 0) diffraction peaks of both KTLO and HTO indicate that

agnification (b). TEM image (c) and EDS spectra (d) of CdS–pillared layered titanate.

J. Fu et al. / Chemical Engineering Journal 180 (2012) 330– 336 333

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ig. 3. Powder XRD patterns of KTLO (a), HTO (b), and CdS–pillared titanate (c).

he well ordered layered structure were formed. Upon reacting withositively charged CdS colloid nanoparticles, the exfoliated titanateanosheets were restacked, leading to the formation of nanocom-osite with pillared architecture. Furthermore, the diffraction peak,2 0 0), related to in–pillared diffraction, can be observed, indicat-ng that the titanate sheets in pillared nanocomposite have retainedhe 2D structure of the original Ti0.87O2 upon the exfoliation andestacking processes. In the low–angle range, the nanocompositeas 5 broad diffraction peaks at 3.3◦, 6.5◦, 9.8◦, 13.1◦, and 16.4◦,ith corresponding d values of 2.7, 1.33, 0.9, 0.67, and 0.54 nm,

ndexed as (0 1 0), (0 2 0), (0 3 0), (0 4 0), and (0 5 0) reflections,espectively. Such well–developed (0 k 0) peaks suggest that theesulting CdS–pillared titanate is fairly well ordered along the c-xis (perpendicular to the layer plans). Thus, the interlayer spacingf the pillared nanocomposite can be determined as 2.7 nm. Ashe thickness of the titanate monolayer is about 0.75 nm [15,69],ccordingly, the height of gallery or the diameter of CdS is about.95 nm.

In order to further confirm the formation of pillared structuref CdS–Ti0.87O2 nanocomposite, the HR–TEM analysis was used.s shown in Fig. 4, the cross–sectional view of the nanocompos-

te exhibits an assembly of parallel dark lines representing theitanium oxide layers, confirming the formation of an alternately

rranged structure of the titanate sheets and CdS nanoparticles.he basal spacing of the nanocomposite could be estimated to be

Fig. 4. HR–TEM image of the CdS–pillared titanate.

Fig. 5. Diffuse reflection UV–visible spectra of KTLO (a), HTO (b) and theCdS–pillared layered titanate (c).

2.7 nm. This result is in good accordance with the d value of the(0 1 0) refection (see Fig. 3(c)) obtained from XRD analysis.

3.4. Optical properties

Fig. 5 compares the UV–visible diffuse reflectance spectra ofKLTO, HTO and CdS–pillared titanate nanocomposite. The absorbedcurve of KTLO is quite similar to those of the layered alkali metalstitanates, such as Na2Ti3O7 and K2Ti4O9 [8]. Analogously, an appar-ent red shift of ∼30 nm in absorption edge occurs in HTO comparesto KTLO. Even so, it is still not large enough to make HTO showspectral response in the visible–light region. In contrast with theabsorption of pure titanates, the CdS intercalated titanate has anapparently different absorption feature, which shows a significantabsorption in the visible–light region from 400 to 600 nm. This extraabsorption feature can be attributed to the CdS nanoparticles.

By analyzing the UV–visible absorption spectra of these mate-rials, the optical band–gap energies of them can be calculated byusing the function (aEp)2 = k(Ep − Eg)n [70], where Eg, Ep, a, k rep-resent the optical band–gap energy, photon energy, absorptioncoefficient, and a constant, respectively, and n is 1/2 for direct tran-sitions. Fig. 6 displays the plots of (aEp)2 versus Ep for KTLO, HTOand the CdS pillared titanate nanocomposite based on the spec-tral response in Fig. 5. The optical band–gaps Eg are given by theextrapolate values of Ep at (aEp)2 = 0. As shown in Fig. 6(a), thevalue of KTLO is Eg = 3.75 eV, which is quite larger than the 3.2 eVof anatase. Upon the acid–exchange, the value of HTO decreasesto 3.40 eV (see Fig. 6(b)), which is similar to the case of H2Ti3O7and H2Ti4O9·H2O [8]. The CdS intercalated titanate nanocompos-ite displays dual band–gap character with Eg = 3.47 and Eg = 2.24 eV(Fig. 6(c)), corresponding to the values for titanate host layers andCdS nanoparticles, respectively. In addition, the Eg value for titanatelayers in the CdS pillared titanate nanocomposite is close to that ofHTO, demonstrating that the intercalation of CdS into titanate lay-ers has no obvious effect on the optical band–gap energy of HTO.These results, suggest that the titanate host layers could be photo-sensitized by CdS nanoparticles in the interlayer gallery, which willbe described below in detail.

3.5. Photocatalytic activity and mechanism of CdS–pillaredlayered titanate

The optical properties of CdS–pillared titanate discussedabove indicate that the material should exhibit photocatalyticactivity under visible–light irradiation. Fig. 7(a) displays thevisible–light photodegradation efficiencies of MB in the presence of

334 J. Fu et al. / Chemical Engineering Journal 180 (2012) 330– 336

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Fig. 7. Photodegradation of methylene blue over blank, P25, KTLO, N–doped TiO2,and CdS–pillared titanate under visible–light irradiation with � > 420 nm (a), and

Once the pillared composite is illuminated by visible–light, the CdSphotosensitizer with a narrow energy (2.24 eV) can be easily acti-vated and induced charge carriers, including electrons (e−) andpositive holes (h+). The e− and h+ could move to the surface of

ig. 6. The optical absorption edges (eV) of KTLO (a), HTO (b) and the CdS–pillareditanate (c).

ifferent photocatalysts, which had establishedbsorption–desorption equilibrium before visible–light irradi-tion. Apparently, MB concentration very gradually decreasedn the presence of KTLO under visible–light. And it was seenhat the MB is very stable and almost no decomposition in thebsence of catalyst can be observed. However, the photocatalyticctivity was significantly enhanced after intercalating with CdSanoparticles. The inset in Fig. 7(a) illustrates the variations inb absorbance around 664 nm using CdS–pillared titanate, and a

learly decreased of the absorbance was observed with increased

rradiation time. Fig. 7(b) shows the decomposition rate of MB byifferent photocatalysts after visible–light exposure for 8 h. Theegradation rates were 4.8%, 23.2%, 9.11%, and 72.8% for the P25,

decomposition rates of blank, P25, KTLO, N–doped TiO2, and CdS–pillared titanateafter visible–light irradiation for 8 h (b). Inset of (a) shows the absorption change at664 nm of MB solution under visible–light irradiation over CdS–pillared titanate.

KTLO, N–doped TiO2, and CdS–pillared titanate, respectively. Thus,compared to that of the layered KTLO, the photocatalytic activityof CdS–pillared titanate is improved greatly and much higher thanthat of N–doped TiO2 and P25.

The mechanism of the enhanced photocatalytic property of 2Dtitanate nanosheets coupled with CdS can be depicted in Fig. 8.

Fig. 8. Energy level diagram of CdS–pillared titanate nanostructure with theproposed photocatalytic mechanism. The direction of arrows on the thin lines cor-responds to the transfer of electrons.

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dS nanoparticles, and then, the electron prefer to transfer fromonduction band (CB) of CdS to the CB of titanate host layers, whilehe holes remain on the valence band (VB) of CdS. As a result, theecombination between photoelectrons and holes could be effec-ively inhibited. The e− on the surface of titanate layers and h+ ondS nanoparticles could react with the absorbents, such as O2 and2O molecules, to produce highly active O2

− and OH• groups. Fur-hermore, the e− in the CB of titanate layers can be transferredo O2

− to form H2O2 and finally change into OH• [71]. The activeroups, OH• and h+ with high oxidation capacity would decomposeyes molecules directly. The following reactions would be occurreduring the photodegradation of organic dye molecules, which areimilar to the analyses reported by Xie et al. [71].

dS + h� (visible light) → CdS (e− + h+) (1)

dS (e−) + Ti0.87O2 → CdS + Ti0.87O2 (e−) (2)

i0.87O2 (e−) + O2 → Ti0.87O2 + O2− (3)

2 + Ti0.87O2 (e−) + 2H+ → Ti0.87O2 + H2O2 (4)

2O2 + O2− → OH• + OH− + O2 (5)

dS (h+) + H2O → OH• + H+ (6)

H• + dye → degraded products (e.g., CO2 and H2O) (7)

dS (h+) + dye → degraded products (8)

n the other hand, the high specific surface area can also provideore active sites to absorb water, O2, and organic dye molecules,hich would promote the reactions (Eqs. (3), (4), (6) and (8)), andnally, the photocatalytic activity of the CdS–pillared titanate wasnhanced.

. Conclusions

In summary, CdS–pillared titanate as a new visible–lightarvesting photocatalyst has been synthesized successfully bypplying the exfoliation–restacking technique. The resulting prod-ct has an interlayer spacing of 2.7 nm which is much largerhan the 0.92 nm of layered protonic titanate. It was also foundhat the nanocomposite displays dual band–gap character withg = 3.47 and 2.24 eV, corresponding to the values for titanateost layers and CdS nanoparticles, respectively, and the opti-al feature of CdS nanoparticles make them as the sensitizers tohotosensitize titanate host layers under visible–light irradiation.

n fact, the CdS–pillared titanate exhibited high photocatalyticctivity under visible–light (� > 420 nm), which was compared tohe inactivity of layered potassium titanate and P25. Moreover,he exfoliation–restacking method employed here would be anffective way to develop novel materials consisting of two dif-erent semiconductors with photochemical and electrochemicalroperties.

cknowledgments

The authors gratefully acknowledge the financial support fromhe National Natural Science Foundation of China (21001093),he Zhejiang Provincial Natural Science Foundation of ChinaY4090285, Y4110418), and the Science Foundation of Zhejiangci-Tech University (0913840-Y).

eferences

[1] A. Corma, Inorganic solid acids and their use in acid–catalyzed hydrocarbonreactions, Chem. Rev. 95 (1995) 559–614.

[2] R.S. Pillai, G. Sethia, R.V. Jasra, Sorption of CO, CH4, and N2 in alkali metal ionexchanged zeolite–X: grand canonical Monte Carlo simulation and volumetricmeasurements, Ind. Eng. Chem. Res. 49 (2010) 5816–5825.

[

ournal 180 (2012) 330– 336 335

[3] Y.S. Han, H. Matsumoto, S. Yamanaka, Preparation of new silica sol–based pil-lared clays with high surface area and high thermal stability, Chem. Mater. 9(1997) 2013–2018.

[4] J.H. Choy, J.H. Park, J.B. Yoon, Multilayered SiO2/TiO2 nanosol particles in two-dimensional aluminosilicate catalyst–support, J. Phys. Chem. B 102 (1998)5991–5998.

[5] J.H. Park, J.H. Yang, J.H. Yoon, S.J. Hwang, J.H. Choy, Intracrystalline structure andphysicochemical properties of mixed SiO2–TiO2 sol–pillared aluminosilicate, J.Phys. Chem. B 110 (2006) 1592–1598.

[6] G.K. Zhang, X.M. Ding, F.S. He, X.Y. Yu, J. Zhou, Y.J. Hu, J.W. Xie, Low–temperaturesynthesis and photocatalytic activity of TiO2 pillared montmorillonite, Lang-muir 24 (2008) 1026–1030.

[7] H. Izawa, S. Kikkawa, M. Koizumi, Ion exchange and dehydration oflayered titanates, Na2Ti3O7 and K2Ti4O9, J. Phys. Chem. 86 (1982)5023–5026.

[8] Y.I. Kim, S.J. Atherton, E.S. Brigham, T.E. Mallouk, Sensitized layered metal oxidesemiconductor particles for photochemical hydrogen evolution from nonsac-rificial electron donors, J. Phys. Chem. 97 (1993) 11802–11810.

[9] T. Sasaki, F. Izumi, M. Watanabe, Intercalation of pyridine in layered titanates,Chem. Mater. 8 (1996) 777–782.

10] M.R. Allen, A. Thibert, E.M. Sabio, N.D. Browning, D.S. Larsen, F.E. Osterloh, Evo-lution of physical and photocatalytic properties in the layered titanates A2Ti4O9

(A = K, H) and in nanosheets derived by chemical exfoliation, Chem. Mater. 22(2010) 1220–1228.

11] A. Kudo, T. Sakata, Effect of ion exchange on photoluminescence of layeredniobate K4Nb6O17 and KNb3O8, J. Phys. Chem. 100 (1996) 17323–17326.

12] Y. Omomo, T. Sasaki, M. Watanabe, Preparation of protonic layered manganatesand their intercalation behavior, Solid State Ionics 151 (2002) 243–250.

13] K. Fukuda, I. Nakai, Y. Ebina, R.Z. Ma, T. Sasaki, Colloidal unilamellar layers oftantalum oxide with open channels, Inorg. Chem. 46 (2007) 4787–4789.

14] T. Sasaki, M. Watanabe, Y. Michiue, Y. Komatsu, F. Izymi, S. Takenouchi,Preparation and acid–base properties of a protonated titanate with thelepidocrocite–like layer structure, Chem. Mater. 7 (1995) 1001–1007.

15] T. Sasaki, M. Watanabe, H. Hashizume, H. Yamada, H. Nakazawa,Macromolecule–like aspects for a colloidal suspension of an exfoliatedtitanate. Pairwise association of nanosheets and dynamic reassemblingprocess initiated from it, J. Am. Chem. Soc. 118 (1998) 8329–8335.

16] J.N. Konda, S. Shibata, Y. Ebina, K. Domen, Preparation of a SiO2–pillaredK0.8Fe0.8Til.2O4 and IR study of N2 adsorption, J. Phys. Chem. 99 (1995)16043–16046.

17] X.P. Dong, M. Osada, H. Ueda, Y. Ebina, Y. Kotani, K. Ono, S. Ueda, K. Kobayashi,K. Takada, T. Sasaki, Synthesis of Mn-substituted titania nanosheets andferromagnetic thin films with controlled doping, Chem. Mater. 21 (2009)4366–4373.

18] M.E. Landis, B.A. Aufdembrink, P. Chu, I.D. Johnson, G.W. Kirker, M.K. Rubin,Preparation of molecular sieves from dense, layered metal oxides, J. Am. Chem.Soc. 113 (1991) 3189–3190.

19] Y. Ebina, A. Tanaka, J.N. Kondo, K. Domen, Preparation of silica pil-lared Ca2Nb3O10 and its photocatalytic activity, Chem. Mater. 8 (1996)2534–2538.

20] S. Yamanaka, K. Kunii, Z.L. Xu, Preparation and adsorption properties ofmicroporous manganese titanate pillared with silica, Chem. Mater. 10 (1998)1931–1936.

21] S.T. Wong, S. Cheng, Synthesis and characterization of pillared buserite, Inorg.Chem. 31 (1992) 1165–1172.

22] F. Kooli, T. Sasaki, M. Watanabe, Pillaring of a lepidocrocite–like titanate withaluminium oxide and characterization, Microporous Mesoporous Mater. 28(1999) 495–503.

23] Y. Ma, S.L. Suib, T. Ressler, J. Wong, M. Lovallo, M. Tsapatsis, Synthesis of porousCrOx pillared octahedral layered manganese oxide materials, Chem. Mater. 11(1999) 3545–3554.

24] T. Tanaka, Y. Ebina, K. Takada, K. Kurashima, T. Sasaki, Oversized titaniananosheet crystallites derived from flux-grown layered titanate single crystals,Chem. Mater. 15 (2003) 3564–3568.

25] T. Tanaka, K. Fukuda, Y. Ebina, K. Takada, T. Sasaki, Highly organized self-assembled monolayer and multilayer films of titania nanosheets, Adv. Mater.16 (2004) 872–875.

26] R. Besselink, T.M. Stawski, H.L. Castricum, D.H.A. Blank, J.E. Elshof, Exfoliationand restacking of lepidocrocite–type layered titanates studied by small–angleX–ray scattering, J. Phys. Chem. C 114 (2010) 21281–21286.

27] J.H. Huang, R.Z. Ma, Y. Ebina, K. Fukuda, K. Takada, T. Sasaki, Layer–by–layerassembly of TaO3 nanosheet/polycation composite nanostructures: multilayerfilm, hollow sphere, and its photocatalytic activity for hydrogen evolution,Chem. Mater. 22 (2010) 2582–2587.

28] Y. Ebina, T. Sasaki, M. Watanabe, Study on exfoliation of layeredperovskite–type niobates, Solid State Ionics 151 (2002) 177–182.

29] G.B. Saupe, C.C. Waraksa, H.N. Kim, Y.J. Han, D.M. Kaschak, D.M. Skinner, T.E.Mallouk, Nanoscale tubules formed by exfoliation of potassium hexaniobate,Chem. Mater. 12 (2000) 1556–1562.

30] Y. Omomo, T. Sasaki, L.Z. Wang, M. Watanabe, Redoxable nanosheet crystallitesof MnO2 derived via delamination of a layered manganese oxide, J. Am. Chem.

Soc. 125 (2003) 3568–3575.

31] L.Z. Wang, Y. Omomo, N. Sakai, K. Fukuda, I. Nakai, Y. Ebina, K. Takada, M.Watanabe, T. Sasaki, Fabrication and characterization of multilayer ultrathinfilms of exfoliated MnO2 nanosheets and polycations, Chem. Mater. 15 (2003)2873–2878.

3 ering J

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[

[

36 J. Fu et al. / Chemical Engine

32] F. Kooli, T. Sasaki, V. Rives, M. Watanabe, Synthesis and characterization of anew mesoporous alumina pillared titanate with a double–layer arrangementstructure, J. Mater. Chem. 10 (2000) 497–501.

33] T.W. Kim, S.G. Hur, S.J. Hwang, H. Park, W. Choi, J.H. Choy, Heterostructuredvisible–light–active photocatalyst of chromia–nanoparticle–layered titanate,Adv. Funct. Mater. 17 (2007) 307–314.

34] T.W. Kim, H.W. Ha, M.J. Paek, S.H. Hyun, I.H. Baek, J.H. Choy, S.J. Hwang,Mesoporous iron oxide–layered titanate nanohybrids: soft–chemical synthe-sis: characterization, and photocatalyst application, J. Phys. Chem. C 112 (2008)14853–14862.

35] H.W. Ha, T.W. Kim, J.H. Choy, S.J. Hwang, Relationship between electrode per-formance and chemical bonding nature in mesoporous metal oxide–layeredtitanate nanohybrids, J. Phys. Chem. C 113 (2009) 21941–21948.

36] T.W. Kim, S.J. Hwang, S.H. Jhung, J.S. Chang, H. Park, W. Choi, J.H. Choy, Bifunc-tional heterogeneous catalysts for selective epoxidation and visible light drivenphotolysis: nickel oxide–containing porous nanocomposite, Adv. Mater. 20(2008) 539–542.

37] J.H. Choy, H.C. Lee, H. Jung, S.J. Hwang, A novel synthetic route to TiO2–pillaredlayered titanate with enhanced photocatalytic activity, J. Mater. Chem. 11(2001) 2232–2234.

38] J.H. Choy, H.C. Lee, H. Jung, H. Kim, H. Boo, Exfoliation and restacking routeto anatase–layered titanate nanohybrid with enhanced photocatalytic activity,Chem. Mater. 14 (2002) 2486–2491.

39] S.M. Paek, H. Jung, Y.J. Lee, M. Park, S.J. Hwang, J.H. Choy, Exfoliation andreassembling route to mesoporous titania nanohybrids, Chem. Mater. 18 (2006)1134–1140.

40] J.H. Kang, S.M. Paek, S.J. Hwang, J.H. Choy, Pre–swelled nanostructured elec-trode for lithium ion battery: TiO2–pillared layered MnO2, J. Mater. Chem. 20(2010) 2033–2038.

41] X. Pian, B.Z. Lin, Y.L. Chen, J.D. Kuang, K.Z. Zhang, L.M. Fu, Pillared nanocom-posite TiO2/Bi–doped hexaniobate with visible–light photocatalytic activity, J.Phys. Chem. C 115 (2011) 6531–6539.

42] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductorelectrode, Nature 238 (1972) 37–38.

43] P.V. Kamat, Photochemistry on nonreactive and reactive (semiconductor) sur-faces, Chem. Rev. 93 (1993) 267–300.

44] A. Hagfeldt, M. Grätzel, Light–induced redox reactions in nanocrystalline sys-tems, Chem. Rev. 95 (1995) 49–68.

45] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmen-tal applications of semiconductor photocatalysis, Chem. Rev. 95 (1995)69–96.

46] K.L. Yeung, A.J. Maira, J. Stolz, E. Hung, N.K.C. Ho, A.C. Wei, J. Soria,K.J. Chao, P.L. Yue, Ensemble effects in nanostructured TiO2 used in thegas–phase photooxidation of trichloroethylene, J. Phys. Chem. B 106 (2002)4608–4616.

47] A.J. Maira, W.N. Lau, C.Y. Lee, P.L. Yue, C.K. Chan, K.L. Yeung, Performance ofa membrane–catalyst for photocatalytic oxidation of volatile organic com-pounds, Chem. Eng. Sci. 58 (2003) 959–962.

48] S.L. Cao, K.L. Yeung, P.L. Yue, An investigation of trichloroethylene photocat-alytic oxidation on mesoporous titania–silica aerogel catalysts, Appl. Catal. B:Environ. 76 (2007) 64–72.

49] W. Choi, A. Termin, M.R. Hoffmann, The role of metal ion dopants inquantum–sized TiO2: correlation between photoreactivity and charge carrierrecombination dynamics, J. Phys. Chem. 98 (1994) 13669–13679.

50] D. Raftery, K. Sarah, Visible light driven V–doped TiO2 photocatalyst and its

photooxidation of ethanol, J. Phys. Chem. B 105 (2001) 2815–2819.

51] H. Yamashita, M. Harada, J. Misaka, M. Takeuchi, K. Ikeue, M. Anpo, Degrada-tion of propanol diluted in water under visible light irradiation using metalion-implanted titanium dioxide photocatalyst, J. Photochem. Photobiol. A 148(2002) 257–261.

[

ournal 180 (2012) 330– 336

52] J. Choi, H. Park, M.R. Hoffmann, Effects of single metal–ion doping on thevisible–light photoreactivity of TiO2, J. Phys. Chem. C 114 (2010) 783–792.

53] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible–light photocatalysisin nitrogen–doped titanium oxides, Science 293 (2001) 269–327.

54] S.U.M. Khan, M. Al-Shahry, W.B. Ingler Jr., Efficient photochemical water split-ting by a chemically modified n–TiO2, Science 297 (2002) 2243–2245.

55] T. Lindgren, J.M. Mwabora, E. Avendano, J. Jonsson, A. Hoel, C.G. Granqvist,S.E. Lindquist, Photoelectrochemical and optical properties of nitrogen dopedtitanium dioxide films prepared by reactive DC magnetron sputtering, J. Phys.Chem. B 107 (2003) 5709–5716.

56] O. Diwald, T.L. Thompson, T. Zubkov, E.G. Goralski, S.D. Walck, J.T. Yates Jr.,Photochemical activity of nitrogen–doped rutile TiO2 (1 1 0) in visible light, J.Phys. Chem. B 108 (2004) 6004–6008.

57] D. Zhao, C.C. Chen, Y.F. Wang, H.W. Ji, W.H. Ma, L. Zang, J.C. Zhao, Surface modi-fication of TiO2 by phosphate: effect on photocatalytic activity and mechanismimplication, J. Phys. Chem. C 112 (2008) 5993–6001.

58] W. Zhao, Y.L. Sun, F.N. Castellano, Visible–light induced water detoxifica-tion catalyzed by PtII dye sensitized titania, J. Am. Chem. Soc. 130 (2008)12566–12567.

59] Y.Z. Li, H. Zhang, X.L. Hu, X.J. Zhao, M. Han, Efficient visible–light–inducedphotocatalytic activity of a 3D–ordered titania hybrid photocatalyst with acore/shell structure of dye–containing polymer/titania, J. Phys. Chem. C 112(2008) 14973–14979.

60] W. Ho, J.C. Yu, J. Lin, J.G. Yu, P.S. Li, Preparation and photocatalytic behavior ofMoS2 and WS2 nanocluster sensitized TiO2, Langmuir 20 (2004) 5865–5869.

61] V.N.H. Nguyen, R. Amal, D. Beydoun, Photodeposition of CdSe using Se–TiO2

suspensions as photocatalysts, J. Photochem. Photobiol. A 179 (2006) 57–65.62] M. Shang, W.Z. Wang, L. Zhang, S.M. Sun, L. Wang, L. Zhou, 3D Bi2WO6/TiO2

hierarchical heterostructure: controllable synthesis and enhanced visiblephotocatalytic degradation performances, J. Phys. Chem. C 113 (2009)14727–14731.

63] Y.L. Lee, C.F. Chi, S.Y. Liau, CdS/CdSe co–sensitized TiO2 photoelectrode for effi-cient hydrogen generation in a photoelectrochemical cell, Chem. Mater. 22(2010) 922–927.

64] H.J. Lee, J. Bang, J. Park, S. Kim, S.M. Park, Multilayered semiconductor(CdS/CdSe/ZnS)–sensitized TiO2 mesoporous solar cells: all prepared by suc-cessive ionic layer adsorption and reaction processes, Chem. Mater. 22 (2010)5636–5643.

65] T. Tachikawa, Y. Takai, S. Tojo, M. Fujitsuka, H. Irie, K. Hashimoto, T. Majima,Visible light–induced degradation of ethylene glycol on nitrogen–doped TiO2

powders, J. Phys. Chem. B 110 (2006) 13158–13165.66] T. Sasaki, F. Kooli, M. Iida, Y. Michiue, S. Takenouchi, Y. Yajima, F. Izumi,

B.C. Chakoumakos, M. Watanabe, A mixed alkali metal titanate with thelepidocrocite–like layered structure. Preparation, crystal structure, pro-tonic form, and acid–base intercalation properties, Chem. Mater. 10 (1998)4123–4128.

67] X.C. Ma, F. Xu, L.Y. Chen, Y. Du, Z.D. Zhang, Fabrication of the stable adductCdS/CTAB/Clay with sandwich-like nanostructures, J. Nanopart. Res. 8 (2006)661–668.

68] B. Chi, L. Zhao, T. Jin, One–step template–free route for synthesis of mesoporousN–doped titania spheres, J. Phys. Chem. C 111 (2007) 6189–6193.

69] T. Sasaki, Y. Ebina, M. Watanabe, G. Decher, Multilayer ultrathin films of molec-ular titania nanosheets showing highly efficient UV–light absorption, Chem.Commun. 216 (2000) 3–2164.

70] N. Sakai, Y. Ebina, K. Takada, T. Sasaki, Electronic band structure of titania semi-

conductor nanosheets revealed by electrochemical and photoelectrochemicalstudies, J. Am. Chem. Soc. 126 (2004) 5851–5858.

71] Y. Xie, G. Ali, S.H. Yoo, S.O. Cho, Sonication–assisted synthesis of CdSquantum–dot–sensitized TiO2 nanotube arrays with enhanced photoelectro-chemical and photocatalytic activity, Appl. Mater. Interf. 2 (2010) 2910–2914.