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Chemical Engineering Journal 168 (2011) 455–460

Contents lists available at ScienceDirect

Chemical Engineering Journal

journa l homepage: www.e lsev ier .com/ locate /ce j

reparation and photocatalytic properties of CdS/La2Ti2O7 nanocomposites underisible light

ui Wanga, Di Xua, JingBing Liua, KunWei Lib, Hao Wanga,∗

The College of Materials Science and Engineering, Beijing University of Technology, Beijing, 100124, PR ChinaChina National Institute of Standardization, Beijing, 100088, PR China

r t i c l e i n f o

rticle history:eceived 21 October 2010

a b s t r a c t

A novel CdS/La2Ti2O7 nanocomposite photocatalyst working under visible light was prepared by a sim-ple sonochemical coupled method. The catalyst was characterized by powder X-ray diffraction, scanning

eceived in revised form7 December 2010ccepted 7 January 2011

eywords:dS/La2Ti2O7

electron microscopy (SEM), transmission electron microscopy (TEM), and UV–vis diffuse reflectance spec-troscopy. The nanocomposite exhibited a strong photocatalytic activity for decomposition of methylorange (MO) under UV and visible light irradiation. Moreover, the nanocomposite with La/Cd = 1:3 showsthe highest photocatalytic activity. The enhanced photocatalytic performance might be attributed tothe layered structure of CdS/La2Ti2O7 nanocomposites and the matched band potentials of the two

anocompositehotocatalytic properties

semiconductors.

. Introduction

Photocatalytic degradation of organic pollutants is one of theromising processes for environmental purification [1]. Unfortu-ately, the number of photocatalysts working under visible light

rradiation is still limited [2]. So great efforts have been made toevelop efficient visible light active photocatalysts to utilize theisible light which accounts for the largest proportion of the solarpectrum and artificial light sources [3].

As is well known, few materials can provide suitable photo-xidation power and photo-reduction power at the same time,hich are necessary for complete decomposition of harmful

rganics. And it is also very difficult for single photocatalysto absorb visible light efficiently, so composite photocatalystsith two or more components have been extensively explored.

mproved photocatalytic performance have been obtained fromarious nanocomposite photocatalysts such as WO3/TiO2 [4],dS/TiO2 [5], CdS/Au/TiO2 [6], CdS/LaMnO3 [7], CdS/mesoporousirconium titanium phosphate (ZTP) [8], CaFe2O4/WO3 [3], and

O3/W/PbBi2Nb1.9Ti0.1O9 [9]. The general concept is that the com-ined semiconductors should have different band gaps so thatharge separation can be enhanced by the internal electric field

riving force.

Among all of the visible-light-sensitive photocatalytic materialseveloped so far, CdS is one of the most active photocatalysts, owingo its suitable band gap (2.3 eV) that corresponds well with the spec-

∗ Corresponding author. Tel.: +86 10 67392733; fax: +86 10 67392445.E-mail address: haowang@bjut.edu.cn (H. Wang).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.01.035

© 2011 Elsevier B.V. All rights reserved.

trum of sunlight [8]. However, CdS is prone to photocorrosion inphotocatalytic reactions and attempts have been made to improveits stability. Among various methods, coupling CdS with anotherwide-band-gap semiconductor (such as TiO2) seems to be a goodchoice [10–12]. Besides, other materials which could be coupledwith CdS are still being investigated.

As one of the layered compounds, lanthanide titanate (La2Ti2O7)has attracted widespread attention [13–15] in the photocatalyticdomain due to its unique layered structure and chemical activ-ity, which is ascribed to the peculiar electronic structure, i.e.the hypervalency of the La atom constructing the layered per-ovskite structure. Recently, we obtained La2Ti2O7 two-dimensional(2D) nanosheets by a simple hydrothermal synthesis method [16].Experiment results show that the prepared La2Ti2O7 nanosheetspossess superior UV photocatalytic properties in degrading methylorange (MO) solutions and the evolution of H2 from water–ethanolsolution. However, the band gap energy of La2Ti2O7 is ca. 3.8 eV,which makes it impossible to absorb any visible light (>400 nm).

Herein, a novel nanocomposite CdS/La2Ti2O7 photocatalystshave been synthesized and characterized. The photocatalytic prop-erties of nanocomposites were investigated under both UV andvisible light irradiation. The mechanism of the enhanced photo-catalytic activity of this nanocomposite was discussed.

2. Experimental details

2.1. Sample preparation

All reagents were of analytical grade and purchased from Bei-jing Chemical Reagent Ltd without further purification. La2Ti2O7

456 R. Wang et al. / Chemical Engineering Journal 168 (2011) 455–460

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Fig. 1. The schematic diagram for the formation process of the nanocomposites.

owders were synthesized in aqueous medium as reported by ourrevious paper [16]: The equivalent molar quantities (5 mmol) ofa(NO3)3·6H2O and Ti(SO4)2 were dissolved in deionized water.hen 5 ml NaOH solution (8 M) was dropped into the above solu-ion to form white precipitation mixtures. The mixture was stirredor 10 min and then sealed in a 50 ml Teflon lined stainless-steelutoclave and hydrothermal reaction proceeded at 200 ◦C for 24 h.

The nanocomposite photocatalyst with CdS were prepared byhe sonochemical method from an aqueous solution of cadmiumhloride (CdCl2) and thioacetamide (H3CCSNH2) (Fig. 1). In a typ-cal procedure, 1.0 mmol pre-prepared La2Ti2O7 was added into0 ml deionized water and dispersed with the help of ultrasoundadiation for 20 min. Then 3 mmol of CdCl2 was added (The La/Cdolar ratio is thus 1:1.5). After CdCl2 was dissolved completely,ml of H3CCSNH2 solution (containing 3 mmol thioacetamide) was

ntroduced. Subsequently, the mixture was exposed to ultrasoundrradiation in the air for another 30 min. During this sonochemicalrocess, a yellow precipitate was formed. After that the temper-ture was raised to 60 ◦C for 3 h. The resulting precipitates wereollected by centrifugation at 3000 rpm several times, washed witheionized water and ethanol thoroughly, and dried at 60 ◦C in anven for 5 h before further characterization. In the next procedure,he La3+/Cd2+ molar ratio was altered to 1:3 and 1:6 by increasinghe Cd2+ dosage, and the amount of H3CCSNH2 was twice as larges CdCl2 in each experiment. In addition, single CdS was preparednder the same sonochemical condition for comparison.

.2. Characterization

Crystallographic phase of the prepared samples was investi-ated by X-ray diffraction at room temperature with a Germanruker AXS D8 ADVANCE diffractometer using CuK�1 radiation� = 1.5405 A). The accelerating voltage, emission current, andcanning speed were 40 kV, 40 mA and 0.2◦/s, respectively. Theorphologies and microstructures of the samples were observed

sing a Hitachi S4800 field emission scanning electron microscopeFESEM) and Hitachi-H8100 transmission electron microscopeTEM) with an accelerating voltage of 200 kV. Optical absorptiontudies were carried out using an ultraviolet–visible–near-nfrared (UV–vis–NIR) spectrophotometer (ShimadzuV-3101PC).

.3. Photocatalytic activities

The decolourization of MO solution was carried out in our home-ade instruments. At first, 0.1 g of product was dispersed intobeaker which was filled with 100 ml of MO solutions with a

oncentration of 1 × 10−5 M. Prior to irradiation, the suspensionsere magnetically stirred in a dark condition for 15 min to estab-

ish adsorption/desorption equilibrium. The suspensions were then

rradiated under both ultraviolet light by using a 400 W high-ressure Hg lamp (� < 280 nm) or visible light by a 500-W tungstenalogen lamp (� > 400 nm) for different times, and then the con-entrations of MO solution were determined by measuring thebsorbance at 464 nm with the UV–vis spectrophotometer.

Fig. 2. XRD patterns of La2Ti2O7, CdS, and CdS/La2Ti2O7 nanocomposites: (a)La2Ti2O7; (b) La/Cd = 1:1.5; (c) La/Cd = 1:3; (d) La/Cd = 1:6; (e) CdS.

3. Results and discussion

3.1. Powder characterization by XRD, FESEM and TEM

The powder XRD patterns of La2Ti2O7, CdS, and CdS/La2Ti2O7nanocomposites are shown in Fig. 2. In the nanocomposites(Fig. 2b–d), the characteristic diffraction peaks at 2� of 29.8, 32.2,and 40.0◦ are attributed to La2Ti2O7 with a perovskite structureconforming to the P21 space group (JCPDF 81-1066). Meanwhile,there are several additional diffraction peaks at 2� of 26.5, 28.2and 43.6, which can be attributed to (0 0 2), (1 0 1) and (1 1 0)crystal planes of hexagonal CdS phase (JCPDF 41-1049) with aspace group of P63mc. From Fig. 2b–d, the peaks represented thegreenockite phase of CdS are becoming stronger and stronger, indi-cating that the content of CdS is increasing. It should be pointedout that compared with CdS synthesized under the same sono-chemical conditions (Fig. 2e), these peaks in the CdS/La2Ti2O7nanocomposites are rather weak. The other diffraction peaksof CdS cannot be clearly identified because they overlap withthe peaks of La2Ti2O7. Moreover, in the nanocomposite sam-ples, only the diffraction peaks of CdS and La2Ti2O7 are found,indicating that no new phase appears in the experimental proce-dure.

In Fig. 3, the SEM images show the morphology of La2Ti2O7,CdS, and CdS/La2Ti2O7 nanocomposites. From Fig. 3a, it can beseen that the La2Ti2O7 plates are transparent even in SEM image,indicating that the plates are very thin, which is in accordancewith our previous results [16]. In Fig. 3b, the morphology of CdSis irregular spherical particles of about 200–300 nm, and theseCdS particles are agglomerates of nanosized particles of 50 nmwith no distinctive morphological features. Fig. 3c–e show theSEM images of CdS/La2Ti2O7 nanocomposites, and these threesamples exhibit a similar morphology. The CdS nanoparticles areuniformly distributed on the surface of La2Ti2O7 nanosheets, andno obvious change is observed in the morphology of La2Ti2O7 afterbeing composed with CdS, which indicating that the ultrasonicprocessing did not damage the surface morphology of La2Ti2O7.With the ratio of La/Cd increasing from 1:1.5 to 1:6, the size of

the CdS particles increases from approximately 20 nm to 50 nm,and the number of CdS particles is also on the rise. These CdSparticles are also constructed by many tiny nanocrystallites, how-ever, it is noticeable that they are much smaller than the pure

R. Wang et al. / Chemical Engineering Journal 168 (2011) 455–460 457

F La/Cd

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ig. 3. SEM images of (a) La2Ti2O7; (b) CdS; and CdS/La2Ti2O7 nanocomposites, (c)

dS particles (Fig. 3b) obtained by the same ultrasonic pro-essing. Fig. 3f is the TEM image of the nanocomposite witha/Cd = 1:3. The SAED pattern is also shown in the inserted picture,hich is obtained by aligning the electron beam perpendicu-

ar to the face of this plate. The irregular spot pattern indicateshe overlapping of the two crystal lattices, and both of the two

emiconductors have crystalline structure. The morphology of theanocomposites is in accordance with the SEM images: La2Ti2O7

s transparent nanosheet with rectangular shape under the elec-ron beam, and CdS particles are uniformly distributed on itsurface.

= 1:1.5; (d) La/Cd = 1:3; (e) La/Cd = 1:6; (f) TEM (SAED inserted) with La/Cd = 1:3.

3.2. Photoabsorption properties

Photoabsorbance of the prepared samples is examined by usinga UV–vis spectrometer. Fig. 4 shows the typical diffuse reflectionspectra of CdS/La2Ti2O7 nanocomposites compared to the spec-trum of pure La2Ti2O7 and CdS. The band-gap of the products can

be estimated from the onset of the absorption edge [17]. For a crys-talline semiconductor, the optical absorption near the band edgefollows the equation ah� = A(h� − Eg)n/2, where a, �, Eg, and A areabsorption coefficient, light frequency, band gap, and a constant,respectively. According to the equation, the value of n for La2Ti2O7

458 R. Wang et al. / Chemical Engineering Journal 168 (2011) 455–460

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Fig. 5. a: The degradation rate (C/C0) of MO as a function of irradiation time (C0

and C are the equilibrium concentration of MO before and after UV light irradiation,respectively): (a) a blank experiment (in the absence of samples); (b) a dark exper-iment (sample e without irradiation); (c) CdS; (d) La2Ti2O7; (e) Degussa P25 TiO2

and CdS/La Ti O nanocomposites: (f) La/Cd = 1:1.5; (g) La/Cd = 1:3; (h) La/Cd = 1:6.

ig. 4. UV–vis absorption spectra of (a) La2Ti2O7; (b) CdS; and CdS/La2Ti2O7

anocomposites, (c) La/Cd = 1:1.5; (d) La/Cd = 1:3; (e) La/Cd = 1:6.

s 1. The band gap of the La2Ti2O7 nanosheets can be estimated toe 2.92 eV from the onset of the absorption edge. Therefore, thishotocatalyst only has absorption in the UV region.

However, the three CdS/La2Ti2O7 nanocomposites exhibit aroad absorption bands from 400 to 600 nm, which undergo thebvious red shift in the absorption edge [5,8,18–20]. Their energyaps are estimated to be 2.17–2.25 eV, which are slightly higherhan single CdS (2.0 eV), indicating the effective photoabsorp-ion property. It might thus be ascribed to the coupling of thewo semiconductors. So we can estimate that the CdS/La2Ti2O7anocomposites should have visible light response characteristic.

.3. Photocatalytic activities

CdS and La2Ti2O7 have both been used as semiconductor-ype photocatalysts for the photoreductive dehalogenation ofalogenated benzene derivatives, photocatalytic degradation ofater pollutants and photocatalytic reduction of toxic metal ions.

o we investigate photocatalytic activity of CdS, La2Ti2O7 anddS/La2Ti2O7 nanocomposites with the photocatalytic degradationf MO as a test reaction.

First the photocatalytic activity of the samples are tested underV light irradiation, the results are shown in Fig. 5a. As one wouldave expected, Fig. 5a (a) (blank experiment) demonstrates thatlmost no MO degradation occurs, while Fig. 5a (b) (dark experi-ent) shows that only a small quantity of MO is degraded (less than

0%, it can be interpreted by the photolysis effect). For the purposesf comparison, the degradation of MO is also carried out by usingdS, La2Ti2O7 and Degussa P25 TiO2 under the same condition. Theegradation rate by CdS reaches to nearly 84% after 60 min (Fig. 5ac)). Meanwhile, La2Ti2O7 (Fig. 5a (d)) exhibits higher photocat-lytic activity than CdS, which is almost the same as Degussa P25iO2 (Fig. 5a (e)).

Furthermore, CdS/La2Ti2O7 nanocomposites with differenta/Cd ratios (Fig. 5a (f)–(h)) show an even higher photocatalyticctivity. After 60 min, the degradation degree of all MO solutionss up to nearly 100%, and the nanocomposite with La/Cd = 1:3hows the highest photocatalytic activity (Fig. 5a (g)): around 81%f degradation is achieved at 20 min, and complete degradation

ccurred at 60 min. The results show that coupling of the two semi-onductors could improve the photocatalytic activity. However, ashe UV light irradiation has higher energy, the properties of degra-ation of MO have no obvious improvement.

2 2 7

b: Degradation curves of methyl orange solutions irritated under ultraviolet light(a) without sample for 50 min, with sample for (b) 10 min, (c) 20 min, (d) 30 min, (e)40 min, and (f) 50 min.

In most literatures, the time-dependent UV–vis absorptionspectra of a dye solution (such as MO, rhodamine B, etc.) duringthe photodegradation is an important factor to evaluate the perfor-mance of photocatalysts [1,16,21,22]. Fig. 5b shows the evolutionof MO degradation curves in the presence of 0.1 g of CdS/La2Ti2O7nanocomposites (La/Cd = 1:3) under UV light irradiation. The char-acteristic absorption of MO at 465 nm is chosen as the monitoredparameter for the photocatalytic degradation process. From theabsorption spectra we can see that the intensity of the adsorptionpeaks (from Fig. 5b (a)–(f)) diminishes gradually as the exposuretime increases under UV light irradiation. No new absorption peaksappears in either the visible or ultraviolet regions, indicating thephotodegradation of MO is effective. The degradation ratio of MOsolution is up to 99%.

To test whether or not the composed samples have photocat-alytic activity under visible light, we repeated the photocatalysisexperiment with a 500-W tungsten halogen lamp (� > 400 nm).

The results are shown in Fig. 6a. The solution degradation in blankexperiment (Fig. 6a (a)) and dark experiment (Fig. 6a (b)) are alsovery limited. As La2Ti2O7 (Fig. 6a (c)) is a wide band gap semicon-ductor which cannot response to visible light well, only a slight

R. Wang et al. / Chemical Engineering Journal 168 (2011) 455–460 459

0 20 40 60 80 100 120 140

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Fig. 6. a: The degradation rate (C/C0) of MO as a function of irradiation time (C0

and C are the equilibrium concentration of MO before and after visible light irradia-tion, respectively): (a) a blank experiment; (b) a dark experiment (sample g withoutirradiation); (c) La2Ti2O7; (d) Degussa P25 TiO2; (e) CdS; and CdS/La2Ti2O7 nanocom-pmw1

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osites: (f) La/Cd = 1:1.5; (g) La/Cd = 1:3; (h) La/Cd = 1:6. b: Degradation curves ofethyl orange solutions irritated under visible light (a) without sample for 140 min,ith sample for (b) 20 min, (c) 40 min, (d) 60 min, (e) 80 min, (f) 120 min, and (g)

40 min.

ecrease in the concentration of MO is detected. The degradationn the presence of La2Ti2O7 could be mainly due to surface adsorp-ion and self-degradation of MO due to visible light illumination.ig. 6a (d) represents the degradation of MO in the presence of P25nder visible light. It can be seen that the decreasing rate is a littleaster than La2Ti2O7, which is mostly for the reason that the specificurface area of P25 is larger than La2Ti2O7, which inducing largerdsorption of MO.

Under visible light, the degradation rate of CdS (Fig. 6a (e))eaches to nearly 84% after 140 min. However, CdS/La2Ti2O7anocomposites with different La/Cd ratios (from Fig. 6a (f)–(h))how very high photocatalytic activity under visible light irradi-tion. After 140 min, the degradation degree of all MO solutionontaining CdS/La2Ti2O7 nanocomposites with different La/Cdatios is more than 90%, which is much higher than pure CdS par-icles. As shown in Fig. 6a (g), the photocatalyst with La/Cd = 1:3xhibits the highest photocatalytic activity, almost 99% degra-

ation degree after 140 min. It should be pointed out that thisample shows the strongest decomposition capability under bothV and visible light irradiation. The degradation percentages ofa/Cd = 1:1.5 (Fig. 6a (f)) and La/Cd = 1:6 (Fig. 6a (h)) are 88%nd 90%, respectively. The results show that coupling of the two

Fig. 7. Proposed mechanism for the visible light photodegradation of MO onCdS/La2Ti2O7 nanocomposite.

semiconductors can improve photocatalytic activity, and the degra-dation effect under visible light is more obvious than under UVlight. The time-dependent degradation curves of MO (La/Cd = 1:3)are also made in Fig. 6b, in order to make sure that MO dimin-ishes gradually as the exposure time increases under visible lightirradiation.

The excellent photocatalytic performance in aqueous reactionsfor CdS/La2Ti2O7 nanocomposites can be attributed to its low bandgap energy, fast electron transfer velocity, large surface area, andlayered structures of La2Ti2O7. Low band gap energy makes theresponse wavelength range expand into visible spectrum range,indicating that the nanocomposite could serve as a promisingphotocatalyst for solar-driven applications. Fast electron transferbetween CdS and La2Ti2O7 may lead to higher quantum effi-ciency, supplying more generated electrons and photons to be usedin photocatalytic reactions. As La2Ti2O7 nanosheets have layeredstructure and very thin thickness, when they are coupled with CdS,a large surface area will not only supply more active sites for thedegradation reaction of organic compounds, but also effectivelypromote the separation efficiency of the electron–hole pairs [23].

The band structure of the photocatalysts plays a crucial role indetermining photocatalytic activity. The equation (1) [24] can beapplied for the approximate determination of the flat band poten-tial of the photocatalysts:

Vfb(NHE) = 2.94 − Eg. (1)

where Vfb and Eg represent a flat band potential and a bandgap,respectively. This equation can be applied for oxide semiconduc-tor photocatalysts consisting of a metal cation with a d0 or d10

configuration. According to this equation, the bottom of the con-duction band level and the top of the valence band potential ofLa2Ti2O7 nanosheets are estimated to be 0.02 and 2.94 eV (vs NHE),respectively.

On the basis of the above experimental results, a possible pho-tocatalysis process (Fig. 7) for the degradation of MO under visiblelight irradiation can conclude as follows [25]: (1) When visible lightis supplied to the CdS/La2Ti2O7 nanocomposite, the electrons on thevalence band of CdS are excited and then transfer to its conductionband, leaving behind electron holes on its valence band. And thenthe electrons and holes generated by CdS are separated. (2) Someelectrons are injected into La2Ti2O7 nanoparticles quickly since the

conduction band of CdS is more negative than that of La2Ti2O7.Moreover, the formed nanostructure on CdS/La2Ti2O7 nanocom-posite also leads to a more efficient interelectron transfer betweenthe two components [26]. (3) The photogenerated electrons arethen captured by O2 to yield O2

•− and H2O2, and then the OH• can

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e formed by reacting O2•− with H2O2 [27]. For its high reaction

bility to attack any organic molecule, the generation of •OH is aey factor in the reaction of the photo-oxidation of MO. (4) Thehotogenerated holes in CdS also may activate some unsaturatedrganic pollutants (e.g. MO), leading to subsequent decomposi-ion. Furthermore, the large specific surface area of CdS/La2Ti2O7anocomposite is also favorable for photocatalytic reaction.

To the best of our knowledge, there has been no report abouthotocatalytic properties of coupling La2Ti2O7 with other semicon-uctors in the previous studies. The catalytic activity is significantlynhanced by loading CdS on the surface of La2Ti2O7 for the degra-ation of MO, especially under visible light irradiation. To sump, the improvement of charge separation, efficient inter elec-ron transfer, the produced OH•, and large specific surface areare supposed to be responsible for the high efficient photocatalyticctivity of the CdS/La2Ti2O7 nanocomposite. Further research ofhe explicit mechanism for the strong photocatalytic activity ofhe CdS/La2Ti2O7 nanocomposite under both UV and visible lights now under investigation in our laboratory.

. Conclusions

By adopting CdS as the visible light absorbing semiconduc-or with a relatively narrow band gap, and La2Ti2O7 as theide band gap part, CdS/La2Ti2O7 nanocomposites have beenrepared by a simple sonochemical method, and the photocat-lytic behavior of the nanocomposites was investigated in detail.he prepared CdS/La2Ti2O7 photocatalyst showed enhanced visi-le light absorption and exhibited efficient photocatalytic activityor decomposition of MO under UV and visible light irradiation.

oreover, the nanocomposite with La/Cd = 1:3 shows the high-st photocatalytic activity. A visible light-induced photocatalyticegradation mechanism of MO on CdS/La2Ti2O7 nanocomposite isroposed.

cknowledgment

This work was supported by the Project for Academicuman Resources Development in Institutions of Higher Learn-

ng under the Jurisdiction of Beijing Municipality, PHR (IHLB)PHR201007101).

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