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Applied Catalysis A: General 467 (2013) 335–341

Contents lists available at ScienceDirect

Applied Catalysis A: General

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

ynthesis of La-doped Ag1.4K0.6Ta4O11 nanocomposites as efficienthotocatalysts for hydrogen production and organic pollutantsegradation

uwei Wanga,b,c, Yufeng Zhub, Guijian Liua,c,∗, Tai-Chu Laua,b,∗

Advanced Laboratory of Environmental Research and Technology (ALERT), Joint Advanced Research Center, USTC-CityU, Suzhou, Jiangsu 215123, ChinaDepartment of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, ChinaCAS Key Laboratory of Crust-Mantle and the Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui30026, China

r t i c l e i n f o

rticle history:eceived 8 May 2013eceived in revised form 11 July 2013ccepted 20 July 2013vailable online 31 July 2013

eywords:antalum oxide

a b s t r a c t

A new silver tantalate material has been synthezied by heating a mixture of AgNO3, Ta2O5 and KCl at850 ◦C for 20 h. XRD, EDX, XPS, SEM, TEM and HRTEM show that the material consists of Ag1.4K0.6Ta4O11

nanoplates, with Ag nanoparticles present on the surface, which is consistent with enhanced absorptionof the material in the visible region. Addition of 0.5–5 mol% of La2O3 in the preparation of Ag1.4K0.6Ta4O11

results in significant change in the morphology from nanoplates to nanoplates/nanowire composites. Thephotocatalytic activities of Ag1.4K0.6Ta4O11 and La-Ag1.4K0.6Ta4O11 have been evaluated by degradationof the organic pollutants rhodamine B (RhB) and pentachlorophenol (PCP) in water under visible light

athanum dopinghotodegradationrganic pollutantsydrogen generation

(� > 420 nm), as well as by photocatalytic reduction of water to H2 at � > 390 nm. The photocatalyticactivity of Ag1.4K0.6Ta4O11 is significantly enhanced by La-doping; the optimal La content is 1 mol% fordegradation of organic substrates and 5 mol% for H2 evolution, with photocatalytic activity significantlyhigher than that of P25 TiO2. The enhancement of photocatalytic activity upon La doping is attributedto trapping of excited electrons by La3+, and the formation of nanowires which further promote chargeseparation.

. Introduction

The use of semiconductor photocatalysts for solar waterplitting and degradation of organic pollutants has received consid-rable attention in recent years [1–4]. One of the most challengingasks in this area is to develop a photocatalyst that can work effi-iently under sunlight [5].

Titanium dioxide (TiO2) has been extensively used as a pho-ocatalyst because of its photostability, natural abundance, andontoxicity [6]. The main disadvantage of TiO2 is that it is activenly under UV light due to its wide band gap (3.2 eV). Conse-uently, various strategies have been used to develop visible-lightesponsive photocatalysts, either by exploring new materials, or

y enhancing the photocatalytic activity of existing materials usingethods such as non-metal-ion substitution, as in TiO2−xNx [7–9],

m2Ti2S2O5 [10,11] and N-La2Ti2O7 [12]; metal-ion substitution,

∗ Corresponding author at: Department of Biology and Chemistry, City Universityf Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China. Tel.: +852 34427811;ax: +852 34420522.

E-mail address: bhtclau@cityu.edu.hk (T.-C. Lau).

926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apcata.2013.07.041

© 2013 Elsevier B.V. All rights reserved.

as in (V-, Fe- or Mn-)TiO2 and In1−xNixTaO4 [13,14]; semicon-ductor/graphene heterojunction, as in �-Fe2O3/reduced grapheneoxide [15]. Other methods such as solid-solution fabrication, as in(Ga1−xZnx)(N1−xOx) [16], ZnS-CuInS2-AgInS2 and �-AgAl1−xGaxO2[17,18] have also been reported.

The use of silver-based oxides for a variety of photocatalyticapplications has also received much attention in recent years.Examples of Ag-containing catalysts include AgNbO3 and AgTaO3[19], AgInW2O8 [20], Ag3VO4 [21], AgInZn7S9 [22], AgGaS2[23,24], AgIn5S8 [25], AgGa0.9In0.1S2 [26], Ag0.7Na0.3NbO3 [27],AgLi1/3Ti2/3O2 and AgLi1/3Sn2/3O2 [28], (AgNbO3)0.75(SrTiO3)0.25[29], Ag2ZnSnS4 [30], AgGa2In3S8 [31], Ag3PO4 [32], �-AgAl1−xGaxO2 [18], Ag/AgX (X = Br, Cl) nanocomposites [33,34],Ag2V4O11 [35], and Ag-modified TiO2 [36]. Ag can enhance thephotocatalytic activity of metal oxides through the followingmechanisms: (1) the contribution of Ag 4d orbitals to the valenceband may narrow the band gap of the metal oxide [2]. (2) Excitedelectrons can be trapped by silver ions and so the recombination

of electron–hole pairs may be inhibited [37–40]. (3) The existenceof metallic Ag nanoparticles will give rise to distinct plasmonicresonance in the visible region, and this will enhance the responseof the metal oxide to visible light [41–44].

3 sis A:

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those of the standard card for the K2Ta4O11 crystal (PDF#12-0092),with a very small amount of K2Ta2O6 impurity (PDF#35-1464).The ionic radii of Ag+ (129 pm) and K+ (133 pm) are similar,suggesting that replacement of K+ by Ag+ is favorable [36,47].

36 R. Wang et al. / Applied Cataly

We report here the synthesis and characterization of a newilver tantalate nanomaterial, Ag1.4K0.6Ta4O11, via a molten saltethod. Tantalum oxide based materials are promising photo-

atalysts for water splitting, since the conduction bands of theseaterials consist of Ta 5d orbitals which are located at a more

egative potential than titanates (3d) and niobates (4d), and thisould facilitate the reduction of H2O to H2 [2,45,46]. AlthoughgTaO3 is known, it shows photocatalytic activity only under UV

rradiation due to its wide band gap (3.4 eV) [19]. On the otherand, Ag1.4K0.6Ta4O11 shows enhanced absorption in the visibleegion due to plasmonic resonance from Ag nanoparticles presentn the surface of the samples. In addition, the addition of justfew % of La2O3 in the preparation of Ag1.4K0.6Ta4O11 results

n a remarkable change in the morphology of the material fromanoplates to nanoplate/nanowire composites, with also enhancedhotocatalytic activity under visible light. The La-Ag1.4K0.6Ta4O11omposites exhibit significantly higher photocatalytic activityoward water reduction and degradation of organic pollutants thaniO2 under visible light.

. Experimental

.1. Synthesis

Ag1.4K0.6Ta4O11 was synthesized by the following procedure.ground mixture of AgNO3 (0.68 mmol, Johnson Matthey 99%),

a2O5 (0.68 mmol, Aldrich 99%) and KCl (4 g, Aldrich 99%) in aorcelain crucible was heated in a muffle furnace in air to 850 ◦C,t a heating rate of 20 ◦C/h. After 20 h, the mixture was cooled tooom temperature, washed several times with distilled water andhen dried at 70 ◦C. La-doped Ag1.4K0.6Ta4O11 was synthesized byhe same procedure except that 0.5–5 mol% of La2O3 (Aldrich, 99%)as added to the reaction mixture.

.2. Characterization

Power X-ray diffraction (XRD) was performed on a Rigaku/max-2500 X-ray diffractometer with Cu K� irradiation

� = 1.5406 ´̊A) at a scanning speed of 0.02◦/s over the 2� range of0–90◦. The accelerating voltage and the applied current were0 kV and 40 mA, respectively. The average crystal size of theg1.4K0.6Ta4O11 and x%La-Ag1.4K0.6Ta4O11 samples were calcu-

ated using the Scherer equation: Dc = K�/ˇ cos �, where Dc is theverage crystal size, K is the Scherer constant (equal to 0.89),

is the X-ray wavelength (0.15406 nm), ˇ is the full width atalf maximum (FWHM), and � is the diffraction angle (2� = 36.0◦

as used in this work). The morphologies of the samples werexamined by using a Philips XL30 environmental scanning electronicroscope (ESEM) at an accelerating voltage of 10 kV. Analysis of

he catalyst surface was done by X-ray photoelectron spectroscopyXPS) using a Leybold Heraeus-Shengyang SKL-12 electron spec-rometer equipped with a VG CLAM 4 MCD electron energynalyzer. Mg-K� X-ray radiation (h� = 1253.6 eV) was generatedt 10 kV and 15 mA. The spectrometer chamber had a residual gasressure close to 2 × 10−9 mbar during data acquisition. UV–visiffuse reflectance spectra (DRS) of the samples were obtainedrom a Perkin-Elmer Lambda 750 UV/vis spectrophotometer.itrogen sorption isotherms were measured at −196 ◦C by usingMicromeritics Tristar II 3020 system. The standard multipoint

runauer–Emmett–Teller (BET) method was used to calculate

he specific surface area using the adsorption data in the P/P0ange from 0.07 to 0.22. Pore size distributions curves wereomputed from the desorption branches of the isotherms usinghe Barrett–Joyner–Halenda (BJH) model.

General 467 (2013) 335–341

2.3. Photocatalysis

A 200 W xenon arc lamp (Newport, Model 71232) was usedas the light source. The incident radiation intensity was mea-sured using a laser power meter (Molectron Detector, USA), andthe average light intensity was 48.3 mW/cm2 with no cutoff fil-ter; 29.5 mW/cm2 with 390 nm cutoff filter; and 18.9 mW/cm2 with420 nm cutoff filter.

Photodegradation experiments were carried out as follows.A mixture of rhodamine B (RhB) or pentachlorophenol (PCP)(20 mg/L) and photocatalyst (50 mg) in 30 mL water was put in aquartz tube reactor (12 mm in diameter and 200 mm in length)and the mixture was sonicated for 5 min to disperse the cat-alyst in the aqueous solution. The distance between the liquidsurface and the light source was about 11 cm. Before photoirra-diation, the suspension was stirred in the dark for one hour toallow adsorption–desorption equilibrium to be established. Thelight emitted from the Xe-lamp was passed through a water jacketand a cutoff filter before reaching the sample solution. Aliquotswere taken at regular time intervals and centrifuged before anal-ysis. The concentration of the substrates was monitored with aShimadzu UV-1700 UV–vis spectrophotometer.

Photocatalytic hydrogen generation experiments were con-ducted under argon at room temperature in water containing 20%methanol as sacrificial donor. The photocatalyst was sonicated for5 min in water prior to irradiation with a 200 W xenon arc lamp.The evolved gas was measured by a gas chromatograph (HP 5890Series II) equipped with a thermal conductivity detector (TCD).

3. Results and discussion

3.1. Characterization of Ag1.4K0.6Ta4O11

Ag1.4K0.6Ta4O11 was synthesized by heating a mixture of AgNO3with one mol equivalent of Ta2O5 in KCl at 850 ◦C for 20 h. La-doped Ag1.4K0.6Ta4O11 was prepared by adding 0.5–5 mol% ofLa2O3 to the mixture before heating. The powder X-ray diffrac-tion (XRD) patterns of the tantalate and La-doped tantalate samplesare shown in Fig. 1. The diffraction patterns are consistent with

Fig. 1. XRD patterns of (a) Ag1.4K0.6Ta4O11; (b) 0.5%La-Ag1.4K0.6Ta4O11; (c)1%La-Ag1.4K0.6Ta4O11; (d) 2%La-Ag1.4K0.6Ta4O11; (e) 3%La-Ag1.4K0.6Ta4O11; (f) 4%La-Ag1.4K0.6Ta4O11; and (g) 5%La-Ag1.4K0.6Ta4O11. The standard XRD pattern ofAg1.4K0.6Ta4O11 is also included for reference. The impurity peak marked by (�)is due to K2Ta2O6. The peak marked by (↓) corresponds to La0.33TaO3.

R. Wang et al. / Applied Catalysis A:

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the increase in surface area.

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Fig. 2. EDX spectrum of Ag1.4K0.6Ta4O11.

he observed K2Ta4O11 crystal structure for the Ag1.4K0.6Ta4O11nd La-Ag1.4K0.6Ta4O11 samples indicate the Ag+ ions can occupyhe K+ lattice sites and do not change the crystal structure ofhe sample. If synthesis was carried out without AgNO3 or KCl,he XRD patterns of the product correspond to K2Ta4O11 org2Ta4O11, respectively (Fig. S1). Energy-dispersive X-ray spec-

roscopy (EDX) reveals that the ratio of Ag/K is 1.37:0.64 (Fig. 2).pon doping with La (0.5–5 mol%), the diffraction patterns ofg1.4K0.6Ta4O11 remains essentially unchanged, except that a verymall amount of La0.33TaO3 is present (PDF#42-0061), as evidencedy the appearance of its characteristic peaks at 2� 22.49◦ and2.665◦. The intensity of these peaks increases with the La doping

evel.The average crystallite sizes of the samples, which were calcu-

ated from the Debye-Scherrer formula, are shown in Table 1. It cane seen that La doping does not cause a significant change in theverage crystallite size of the products, which is consistent with theRD analysis.

The SEM photographs of Ag1.4K0.6Ta4O11 (Fig. 3a and b) showhat it consists of hexagonal nanoplates of rather uniform sizediameter, 200–650 nm) and thickness (ca. 70 nm). Remarkably, theddition of just 0.5–5 mol% of La2O3 results in significant changesn the morphology of the samples, which consist of a mixture ofanoplates and nanowires that are 35–125 nm in diameter and sev-ral �m in length (Fig. 3c). The relative amount of the nanowiresncreases with increasing La2O3 content (Fig. S2). The transmis-ion electron micrograph (TEM, Fig. 3d) clearly shows the presencef metallic Ag nanoparticles on the surface of the Ag1.4K0.6Ta4O11anoplates, with the particle size ranging from 20–30 nm. The1 1 1) planes of the single-crystalline Ag can be seen in the HRTEMFig. 3e).

The X-ray photoelectron spectra of Ag1.4K0.6Ta4O11 and La-oped Ag1.4K0.6Ta4O11 are shown in Fig. 4. No significantontamination, besides carbon, is found in the spectra. The bind-ng energy was determined by reference to C 1s line at 284.6 eV.

n the whole energy range spectrum shown in Fig. 4a, the ele-

ents K, Ag, Ta and O can be observed in Ag1.4K0.6Ta4O11nd 1%La-Ag1.4K0.6Ta4O11. However, La is present only in the

able 1hysicochemical properties of Ag1.4K0.6Ta4O11 and La-Ag1.4K0.6Ta4O11 samples.

La-dopedAg1.4K0.6Ta4O11

Average crystallitesize (nm)a

BET surfacearea (m2/g)b

Por(cm

0% 48.2 2.59 0.000.5% 48.5 2.38 0.001% 47.9 2.47 0.002% 48.5 2.86 0.014% 49.1 3.53 0.015% 49.1 3.68 0.01TiO2

a The average crystallite size was calculated by using the Scherer equation.b Obtained from N2 adsorption data in the P/P0 range from 0.07 to 0.22.c Single-point pore volume calculated from the adsorption isotherm at P/P0 = 0.97.d k is the rate constant for the photodegradation of RhB or PCP at � > 420 nm.e H2 evolution from 20% methanol in water at � > 390 nm.

General 467 (2013) 335–341 337

1%La-Ag1.4K0.6Ta4O11 sample. The atom concentrations were cal-culated by the following equation,

Cn = In/Sn∑iIi/Si

where Cn is the element concentration, In and Ii are the peakintensities of the element and other elements, respectively; Sn

and Si are the relative sensitivity of the element and other ele-ments, respectively. For Ag1.4K0.6Ta4O11 nanoplates, the atom ratioof Ag:K:Ta:O was calculated to be 1.4:0.7:3.7:11.7, which agreesreasonably well with results from XRD and EDX. Two bands at ca.367.5 and 373.6 eV, ascribed to Ag 3d5/2 and Ag 3d3/2 bindingenergies, respectively (Fig. 4b), are observed [48–50]. Each bandcould be further deconvoluted into two peaks at 367.5, 368.6 and373.6, 374.8 eV, respectively. The peaks at 367.5 and 373.5 eV areattributed to the Ag+ of Ag1.4K0.6Ta4O11, and those at 368.6 and374.8 eV are ascribed to metallic Ag [35]. The calculated surfacemole ratio of Ag+ to Ag0 is 12:1. In 1%La-Ag1.4K0.6Ta4O11, the Laconcentration is calculated to be 1.3 atom%. There are two peakscentered at 852.2 eV and 835.1 eV, which are attributed to La 3d5/2and La 3d3/2, respectively (Fig. 4c) [51]. Apart from the presence ofLa peaks, the positions and relative intensities of the other peaks in1%La-Ag1.4K0.6Ta4O11 are similar to those in Ag1.4K0.6Ta4O11.

The specific surface areas and pore volumes of the samples weremeasured using the nitrogen gas sorption technique, and typicalisotherms are shown in Fig. 5. For the Ag1.4K0.6Ta4O11 nanoplates(Fig. 5 0%), a type IV isotherm along with one small, but obvious hys-teresis loop at relative pressures of P/P0 = 0.88–0.98 was observed,which is characteristic of mesoporous materials [52].

The adsorption isotherm of the Ag1.4K0.6Ta4O11 nanoplatesexhibits a sharp increase at P/P0 = 0.88–1.00, corresponding tocapillary condensation within mesopores, indicating a narrowpore-size distribution. The pore-size is centered at 12.5 nm in thesample. Apparently La doping did not cause significant changein the mesoporous structure of Ag1.4K0.6Ta4O11 (Fig. 5 0.5–5%).However, there are additional peaks on the pore-size distributioncurves, which presumably arise from the pores in the nanowiresformed after La2O3 doping.

Table 1 shows the physicochemical properties of theAg1.4K0.6Ta4O11 nanoplates and the x%La-Ag1.4K0.6Ta4O11nanocomposites. Upon La doping, the surface area of x%La-Ag1.4K0.6Ta4O11 first decreases slightly (0.5–1%), then graduallyincreases (2–5%). The decrease in surface area at low La dopingcould be due to the slight increase in size of the nanoplates uponLa doping (see Figs. 3 and S2). However, as La doping increases,nanowires of much smaller sizes are formed, which contribute to

The UV–visible diffuse reflectance spectra (DRS) of theAg1.4K0.6Ta4O11 nanoplates and the x%La-Ag1.4K0.6Ta4O11nanocomposites are shown in Fig. S3. All samples exhibit a

e volume3/g)c

RhB k (h−1)d PCP k (h−1)d H2 rate(�mol/h/g)e

7 0.08 0.46 0.76 0.11 0.33 1.07 0.19 0.97 1.4

0.14 0.54 4.00.12 0.35 5.4

2 0.13 0.32 8.60.09 0.11 6.0

338 R. Wang et al. / Applied Catalysis A: General 467 (2013) 335–341

F tion FA age of

bacn[ss

ig. 3. (a) Low-magnification SEM image of Ag1.4K0.6Ta4O11, (b) high-magnificag1.4K0.6Ta4O11, (d) TEM image of Ag1.4K0.6Ta4O11 nanoparticles, and (e) HRTEM im

and gap absorption threshold below 400 nm. In addition, theylso show absorptions in the visible region (400–800 nm) that areentered at around 510 nm, which is attributed to plasmonic reso-

ance from Ag nanoparticles present on the surface of the samples35,41,42], as observed by XPS. The absorptions of the La-dopedamples are slightly red-shifted relative to the undoped sample,uggesting electronic interaction between La3+ and tantalate.

Fig. 4. XPS spectra of (a) survey spectrum; (b) Ag 3d; (c) L

ESEM image of Ag1.4K0.6Ta4O11, (c) low-magnification SEM image of the 1%La-Ag nanoparticle.

Generally, the surface plasmon resonance (SPR) wavelengthdepends strongly on the size and shape of the nanoparticles,the interparticle distance, and the dielectric property of the sur-

rounding medium [53]. The relationship between the highestplasmon absorption wavelength of silver and its particle size wasshown in Table S1. In the current study, the Ag particle sizewas determined to be 20–30 nm by TEM (as shown in Fig. 3d).

a 3d for Ag1.4K0.6Ta4O11 and 1%La-Ag1.4K0.6Ta4O11.

R. Wang et al. / Applied Catalysis A: General 467 (2013) 335–341 339

F distrix

Te

3

3

ad(ioirAm

ig. 5. Nitrogen adsorption–desorption isotherms and the corresponding pore size%La-Ag1.4K0.6Ta4O11.

hus the large red-shift of the plasmon resonance to 510 nm wasxpected.

.2. Photocatalytic activity

.2.1. Photocatalytic degradation of organic pollutants in waterThe photocatalytic activities of the Ag1.4K0.6Ta4O11 nanoplates

nd the x%La-Ag1.4K0.6Ta4O11 nanocomposites were evaluated byegradation of two common organic pollutants, rhodamine BRhB) and pentachlorophenol (PCP), in water under visible lightrradiation (� > 420 nm). Before light irradiation, the mixture ofrganic pollutant and photocatalyst in water was stirred for 60 min

n the dark to achieve sorption equilibrium. RhB and PCP wereeadily degraded upon visible light irradiation in the presence ofg1.4K0.6Ta4O11 and La-Ag1.4K0.6Ta4O11 (Fig. 6). Control experi-ents showed that both light and photocatalyst are required for the

bution curve calculated from adsorption branch of the nitrogen isotherm (inset) of

degradation of the organic substrates. The rates of the photodegra-dation of the organic substrates could be approximately describedby the first-order equation, ln(C/C0) = −kt, where k is the rateconstant, C and C0 are the concentration of organic pollutant attime t and 0, respectively. Kinetic data are summarized in Table 1.

The rate of photodegradation of RhB by Ag1.4K0.6Ta4O11(k = 0.08 h−1) is slightly slower than that of P25 TiO2 (k = 0.09 h−1).On the other hand, Ag1.4K0.6Ta4O11 is four times faster than TiO2in the photodegradation of PCP (0.46 vs. 0.11 h−1). Doping with Laenhances the photocatalytic activity of Ag1.4K0.6Ta4O11 (Table 1 andFig. 7), with the 1% La-doped sample showing the highest activity(two and nine times faster than TiO2 for the photodegradation of

RhB and PCP, respectively). In the case of PCP, samples with >3%La are actually less active than the undoped sample. In general, thephotocatalytic activity of metal oxide semiconductors depends onthe surface area, optical absorption capability and diffusion rates

340 R. Wang et al. / Applied Catalysis A: General 467 (2013) 335–341

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ofpplAd(tnetithi

3

nwslAthw1

Ft�

ig. 6. Plot of C/C0 (C = concentration at time t, C0 = initial concentration) vs. time fo%La-Ag1.4K0.6Ta4O11 under visible light irradiation (� > 420 nm).

f charge carriers [54–57]. In the present case, increasing the sur-ace area apparently has no advantageous effect on RhB and PCPhotodegradation; the 2–5% La-Ag1.4K0.6Ta4O11 samples, whichossess larger surface areas than the 1%La-Ag1.4K0.6Ta4O11, show

ower photocatalytic activity than that of 1% La sample (Table 1).lso it can be seen that all the La-doped Ag1.4K0.6Ta4O11 samplesisplay very similar absorption both in the UV and visible regionsFig. S3), so the band gap effect is not expected to be large. Hencehe main effects of La3+ could be due to its ability to inhibit recombi-ation of photogenerated electrons–holes pairs by trapping excitedlectrons [58,59]. Moreover, the addition of La3+ leads to the forma-ion of nanowires (Figs. 3c and S2), which should be more efficientn light scattering and absorption, as well as in charge separa-ion [60–62]. The optimal La content is 1%, presumably because itsetero-nanostructure is most efficient for separation of the photo-

nduced electron–hole pairs at � > 420 nm [63].

.2.2. Photocatalytic H2 evolutionThe photocatalytic H2 production efficiency of Ag1.4K0.6Ta4O11

anoplates, x%La-Ag1.4K0.6Ta4O11 nanocomposites and P25 TiO2ere investigated at � > 390 nm with methanol (20 vol%) as the

acrificial donor (Fig. 7, Table 1). Control experiments showed thatight, catalyst and sacrificial agent are all required for H2 generation.g1.4K0.6Ta4O11 itself is a rather inefficient photocatalyst. However,

he H2 evolution rate is significantly increased with La doping. Theighest H2 evolution rate was obtained from 5%La-Ag1.4K0.6Ta4O11,hich is approximately 10 times that of the undoped sample and

.6 times that of P25 TiO2. This is in contrast to photodegradation

ig. 7. Plot of amount of H2 evolved vs. time for photocatalytic water reduc-ion by Ag1.4K0.6Ta4O11 and x%La-Ag1.4K0.6Ta4O11 in aqueous methanol (20 vol%) at> 390 nm.

photodegradation of RhB (left panel) and PCP (right panel) by Ag1.4K0.6Ta4O11 and

of organic substrates, in which 1%La-Ag1.4K0.6Ta4O11 is the mostactive photocatalyst. This could be due to the difference in thewavelengths of light used; in water reduction � > 390 nm lightwas used, while in photodegradation � > 420 nm light was used.Apparently 5%La-Ag1.4K0.6Ta4O11 is a more efficient photocatalystat lower wavelengths. In addition, the increased surface area mayalso contribute to this enhancement (Table 1).

4. Conclusions

A new silver tantalate photocatalyst, Ag1.4K0.6Ta4O11, has beenprepared. This material makes use of Ag nanoparticles on the sur-face to enhance absorption in the visible region. The photocatalyticactivity of this material is significantly enhanced by doping witha few % of La3+. The rates of photocatalytic degradation of organicpollutants and H2 generation from water by La-Ag1.4K0.6Ta4O11 arehigher than that of P25 TiO2. The beneficial effects of La dopingare attributed to trapping of excited electrons by La3+ as well asto change in morphology from nanoplates to nanoplates/nanowirecomposites, which would enhance the separation of photo-inducedelectron/hole pairs.

Acknowledgements

The work described in this paper was supported by the Univer-sity Grants Committee (UGC) of Hong Kong Special AdministrativeRegion (AoE/P-04/04) and the State Key Laboratory in Marine Pollu-tion (SKLMP). The photochemical equipment used in this work wassupported by a Special Equipment Grant from UGC (SEG CityU02).The XRD and XPS used in this work were provided by the Instituteof Advanced Materials of the Hong Kong Baptist University. Fund-ing for the XRD and XPS were from the Special Equipment Grantfrom the University Grants Committee of the Hong Kong SpecialAdministrative Region, China (SEG HKBU06).”

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.apcata.2013.07.041.

References

[1] T.L. Thompson, J.T. Yates Jr., Chem. Rev. 106 (2006) 4428–4453.

[2] A. Kudo, Y. Miseki, Chem. Soc. Rev. 38 (2009) 253–278.[3] M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995)

69–96.[4] X.B. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891–2959.[5] R. Abe, M. Higashi, K. Domen, J. Am. Chem. Soc. 132 (2010) 11828–11829.

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R. Wang et al. / Applied Cataly

[6] A. Fujishima, K. Honda, Nature 238 (1972) 37–38.[7] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271.[8] J. Wang, D.N. Tafen, J.P. Lewis, Z.L. Hong, A.K. Manivannan, M.J. Zhi, M. Li, N.Q.

Wu, J. Am. Chem. Soc. 131 (2009) 12290–12297.[9] Z.Z. Zhang, X.X. Wang, J.L. Long, Q.A. Gu, Z.X. Ding, X.Z. Fu, J. Catal. 276 (2010)

201–214.10] A. Ishikawa, T. Takata, J.N. Kondo, M. Hara, H. Kobayashi, K. Domen, J. Am. Chem.

Soc. 124 (2002) 13547–13553.11] F.X. Zhang, K. Maeda, T. Takata, K. Domen, J. Catal. 280 (2011) 1–7.12] F.K. Meng, J.T. Li, Z.L. Hong, M.J. Zhi, A. Sakla, C.C. Xiang, N.Q. Wu, Catal. Today

199 (2013) 48–52.13] H. Yamashita, M. Harada, J. Misaka, M. Takeuchi, K. Ikeue, M. Anpo, J. Pho-

tochem. Photobiol. A 148 (2002) 257–261.14] Z.G. Zou, J.H. Ye, K. Sayama, H. Arakawa, Nature 414 (2001) 625–627.15] F.K. Meng, J.T. Li, S.K. Cushing, J. Bright, M.J. Zhi, J.D. Rowley, Z.L. Hong, A.

Manivannan, A.D. Bristow, N.Q. Wu, ACS Catal. 3 (2013) 746–751.16] K. Maeda, K. Teramura, D.L. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen, J. Phys.

Chem. B 110 (2006) 13753–13758.17] I. Tsuji, H. Kato, A. Kudo, Angew. Chem. Int. Ed. 44 (2005) 3565–3568.18] S.X. Ouyang, J.H. Ye, J. Am. Chem. Soc. 133 (2011) 7757–7763.19] H. Kato, H. Kobayashi, A. Kudo, J. Phys. Chem. B 106 (2002) 12441–12447.20] J.W. Tang, Z.G. Zou, J.H. Ye, J. Phys. Chem. B 107 (2003) 14265–14269.21] R. Konta, H. Kato, H. Kobayashi, A. Kudo, Phys. Chem. Chem. Phys. 5 (2003)

3061–3065.22] I. Tsuji, H. Kato, H. Kobayashi, A. Kudo, J. Am. Chem. Soc. 126 (2004)

13406–13413.23] J.S. Jang, S.H. Choi, N. Shin, C. Yu, J.S. Lee, J. Solid State Chem. 180 (2007)

1110–1118.24] J.S. Jang, D.W. Hwang, J.S. Lee, Catal. Today 120 (2007) 174–181.25] D. Chen, J.H. Ye, J. Phys. Chem. Solids 68 (2007) 2317–2321.26] J.S. Jang, P.H. Borse, J.S. Lee, S.H. Choi, H.G. Kim, J. Chem. Phys. 128 (2008)

154717–154722.27] G.Q. Li, D.F. Wang, Z.G. Zou, J.H. Ye, J. Phys. Chem. C 112 (2008) 20329–20333.28] Y. Hosogi, H. Kato, A. Kudo, J. Mater. Chem. 18 (2008) 647–653.29] D. Wang, T. Kako, J.H. Ye, J. Phys. Chem. C 113 (2009) 3785–3792.30] I. Tsuji, Y. Shimodaira, H. Kato, H. Kobayashi, A. Kudo, Chem. Mater. 22 (2010)

1402–1409.31] H. Kaga, K. Saito, A. Kudo, Chem. Commun. 46 (2010) 3779–3781.32] Z.G. Yi, J.H. Ye, N. Kikugawa, T. Kako, S.X. Ouyang, H.S. Williams, H. Yang, J.Y.

Cao, W.J. Luo, Z.S. Li, Y. Liu, R.L. Withers, Nat. Mater. 9 (2010) 559–564.

33] M.S. Zhu, P.L. Chen, M.H. Liu, ACS NANO 5 (2011) 4529–4536.34] H. Zhang, X.F. Fan, X. Quan, S. Chen, H.T. Yu, Environ. Sci. Technol. 45 (2011)

5731–5736.35] H.F. Shi, Z.S. Li, J.H. Kou, J.H. Ye, Z.G. Zou, J. Phys. Chem. C 115 (2011) 145–

151.

[[[[[

General 467 (2013) 335–341 341

36] Z.G. Xiong, J.Z. Ma, W.J. Ng, T.D. Waite, X.S. Zhao, Water Res. 45 (2011)2095–2103.

37] H. Tran, J. Scott, K. Chiang, R. Amal, J. Photochem. Photobiol. A 183 (2006) 41–52.38] E. Kowalska, H. Remita, C. Colbeau-Justin, J. Hupka, J. Belloni, J. Phys. Chem. C

112 (2008) 1124–1131.39] I. Bannat, K. Wessels, T. Oekermann, J. Rathousky, D. Bahnemann, M. Wark,

Chem. Mater. Chem. Mater. 21 (2009) 1645–1653.40] X. Wang, D.R.G. Mitchell, K. Prince, A.J. Atanacio, R.A. Caruso, Chem. Mater. 20

(2008) 3917–3926.41] S.K. Cushing, J.T. Li, F.K. Meng, T.R. Senty, S. Suri, M.J. Zhi, M. Li, A.D. Bristow,

N.Q. Wu, J. Am. Chem. Soc. 134 (2012) 15033–15041.42] J.T. Li, S.K. Cushing, J. Bright, F. Meng, T.R. Senty, P. Zheng, A.D. Bristow, N.Q.

Wu, ACS Catal. 3 (2013) 47–51.43] A.L. Linsebigler, G.Q. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735–758.44] X. Chen, Z.F. Zheng, X.B. Ke, E. Jaatinen, T.F. Xie, D.J. Wang, C. Guo, J.C. Zhao, H.Y.

Zhu, Green Chem. 12 (2010) 414–419.45] F.E. Osterloh, Chem. Mater. 20 (2008) 35–54.46] M. Kitano, M. Hara, J. Mater. Chem. 20 (2010) 627–641.47] G.K. Zhang, X. Zou, J. Gong, F.S. He, H. Zhang, S.X. Ouyang, H.X. Liu, Q. Zhang, Y.

Liu, X. Yang, B. Hu, J. Mol. Catal. A-Chem. 255 (2006) 109–116.48] P. Wang, B.B. Huang, X.Y. Qin, X.Y. Zhang, Y. Dai, M.H. Whangbo, Inorg. Chem.

48 (2009) 10697–10702.49] P. Wang, B. Huang, Z. Lou, X. Zhang, X. Qin, Y. Dai, Z. Zheng, X. Wang, Chem.-Eur.

J. 16 (2010) 538–544.50] P. Wang, B. Huang, Q. Zhang, X. Zhang, X. Qin, Y. Dai, J. Zhan, J. Yu, H. Liu, Z. Lou,

Chem.-Eur. J. 16 (2010) 10042–10047.51] B.M. Reddy, P.M. Sreekanth, E.P. Reddy, Y. Yamada, Q. Xu, H. Sakurai, T.

Kobayashi, J. Phys. Chem. B 106 (2002) 5695–5700.52] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T.

Siemieniewska, Pure Appl. Chem. 57 (1985) 603–619.53] Y. Tian, T. Tatsuma, J. Am. Chem. Soc. 127 (2005) 7632–7637.54] F.K. Meng, Z.L. Hong, J. Amdt, M. Li, M.J. Zhi, F. Yang, N.Q. Wu, Nano Res. 5 (2012)

213–221.55] B. Zhao, F. Chen, Y.C. Jiao, H.Y. Yang, J.L. Zhang, J. Mol. Catal. A-Chem. 348 (2011)

114–119.56] R.W. Wang, Y.F. Zhu, Y.F. Qiu, C.F. Leung, J. He, G.J. Liu, T.C. Lau, Chem.-Eur. J.

226 (2013) 123–130.57] D. Chen, J.H. Ye, Chem. Mater. 21 (2009) 2327–2333.58] K.T. Ranjit, I. Willner, S.H. Bossmann, A.M. Braun, Environ. Sci. Technol. 16

(2010) 10042–10047.

59] M.Y. Xing, D.Y. Qi, J.L. Zhang, F. Chen, Chem.-Eur. J. 17 (2011) 11432–11436.60] H. Kato, K. Asakura, A. Kudo, J. Am. Chem. Soc. 125 (2003) 3082–3089.61] Y.X. Li, G. Chen, H.J. Zhang, Z.H. Li, Mater. Res. Bull. 44 (2009) 741–746.62] H.Y. Wei, Y.S. Wu, N. Lun, F. Zhao, J. Mater. Sci. 39 (2004) 1305–1308.63] F.B. Li, X.Z. Li, M.F. Hou, Appl. Catal. B: Environ. 48 (2004) 185–194.