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Synthesis of La-doped Ag1.4K0.6Ta4O11 nanocomposites as efficient photocatalysts for hydrogen production and organic pollutants degradation
Applied Catalysis A: General 467 (2013) 335–341
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Synthesis of La-doped Ag1.4 K0.6 Ta4 O11 nanocomposites as efficient photocatalysts for hydrogen production and organic pollutants degradation Ruwei Wang a,b,c , Yufeng Zhu b , Guijian Liu a,c,∗ , Tai-Chu Lau a,b,∗ a
Advanced Laboratory of Environmental Research and Technology (ALERT), Joint Advanced Research Center, USTC-CityU, Suzhou, Jiangsu 215123, China Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China c CAS Key Laboratory of Crust-Mantle and the Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China b
a r t i c l e
i n f o
Article history: Received 8 May 2013 Received in revised form 11 July 2013 Accepted 20 July 2013 Available online 31 July 2013 Keywords: Tantalum oxide Lathanum doping Photodegradation Organic pollutants Hydrogen generation
a b s t r a c t A new silver tantalate material has been synthezied by heating a mixture of AgNO3 , Ta2 O5 and KCl at 850 ◦ C for 20 h. XRD, EDX, XPS, SEM, TEM and HRTEM show that the material consists of Ag1.4 K0.6 Ta4 O11 nanoplates, with Ag nanoparticles present on the surface, which is consistent with enhanced absorption of the material in the visible region. Addition of 0.5–5 mol% of La2 O3 in the preparation of Ag1.4 K0.6 Ta4 O11 results in significant change in the morphology from nanoplates to nanoplates/nanowire composites. The photocatalytic activities of Ag1.4 K0.6 Ta4 O11 and La-Ag1.4 K0.6 Ta4 O11 have been evaluated by degradation of the organic pollutants rhodamine B (RhB) and pentachlorophenol (PCP) in water under visible light ( > 420 nm), as well as by photocatalytic reduction of water to H2 at > 390 nm. The photocatalytic activity of Ag1.4 K0.6 Ta4 O11 is significantly enhanced by La-doping; the optimal La content is 1 mol% for degradation of organic substrates and 5 mol% for H2 evolution, with photocatalytic activity significantly higher than that of P25 TiO2 . The enhancement of photocatalytic activity upon La doping is attributed to trapping of excited electrons by La3+ , and the formation of nanowires which further promote charge separation. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The use of semiconductor photocatalysts for solar water splitting and degradation of organic pollutants has received considerable attention in recent years [1–4]. One of the most challenging tasks in this area is to develop a photocatalyst that can work efficiently under sunlight [5]. Titanium dioxide (TiO2 ) has been extensively used as a photocatalyst because of its photostability, natural abundance, and nontoxicity [6]. The main disadvantage of TiO2 is that it is active only under UV light due to its wide band gap (3.2 eV). Consequently, various strategies have been used to develop visible-light responsive photocatalysts, either by exploring new materials, or by enhancing the photocatalytic activity of existing materials using methods such as non-metal-ion substitution, as in TiO2−x Nx [7–9], Sm2 Ti2 S2 O5 [10,11] and N-La2 Ti2 O7 [12]; metal-ion substitution,
∗ Corresponding author at: Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China. Tel.: +852 34427811; fax: +852 34420522. E-mail address: [email protected] (T.-C. Lau). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.07.041
as in (V-, Fe- or Mn-)TiO2 and In1−x Nix TaO4 [13,14]; semiconductor/graphene heterojunction, as in ␣-Fe2 O3 /reduced graphene oxide [15]. Other methods such as solid-solution fabrication, as in (Ga1−x Znx )(N1−x Ox ) [16], ZnS-CuInS2 -AgInS2 and -AgAl1−x Gax O2 [17,18] have also been reported. The use of silver-based oxides for a variety of photocatalytic applications has also received much attention in recent years. Examples of Ag-containing catalysts include AgNbO3 and AgTaO3 [19], AgInW2 O8 [20], Ag3 VO4 [21], AgInZn7 S9 [22], AgGaS2 [23,24], AgIn5 S8 [25], AgGa0.9 In0.1 S2 [26], Ag0.7 Na0.3 NbO3 [27], AgLi1/3 Ti2/3 O2 and AgLi1/3 Sn2/3 O2 [28], (AgNbO3 )0.75 (SrTiO3 )0.25 [29], Ag2 ZnSnS4 [30], AgGa2 In3 S8 [31], Ag3 PO4 [32], AgAl1−x Gax O2 [18], Ag/AgX (X = Br, Cl) nanocomposites [33,34], Ag2 V4 O11 [35], and Ag-modified TiO2 [36]. Ag can enhance the photocatalytic activity of metal oxides through the following mechanisms: (1) the contribution of Ag 4d orbitals to the valence band may narrow the band gap of the metal oxide [2]. (2) Excited electrons can be trapped by silver ions and so the recombination of electron–hole pairs may be inhibited [37–40]. (3) The existence of metallic Ag nanoparticles will give rise to distinct plasmonic resonance in the visible region, and this will enhance the response of the metal oxide to visible light [41–44].
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We report here the synthesis and characterization of a new silver tantalate nanomaterial, Ag1.4 K0.6 Ta4 O11 , via a molten salt method. Tantalum oxide based materials are promising photocatalysts for water splitting, since the conduction bands of these materials consist of Ta 5d orbitals which are located at a more negative potential than titanates (3d) and niobates (4d), and this would facilitate the reduction of H2 O to H2 [2,45,46]. Although AgTaO3 is known, it shows photocatalytic activity only under UV irradiation due to its wide band gap (3.4 eV) [19]. On the other hand, Ag1.4 K0.6 Ta4 O11 shows enhanced absorption in the visible region due to plasmonic resonance from Ag nanoparticles present on the surface of the samples. In addition, the addition of just a few % of La2 O3 in the preparation of Ag1.4 K0.6 Ta4 O11 results in a remarkable change in the morphology of the material from nanoplates to nanoplate/nanowire composites, with also enhanced photocatalytic activity under visible light. The La-Ag1.4 K0.6 Ta4 O11 composites exhibit significantly higher photocatalytic activity toward water reduction and degradation of organic pollutants than TiO2 under visible light.
2. Experimental 2.1. Synthesis Ag1.4 K0.6 Ta4 O11 was synthesized by the following procedure. A ground mixture of AgNO3 (0.68 mmol, Johnson Matthey 99%), Ta2 O5 (0.68 mmol, Aldrich 99%) and KCl (4 g, Aldrich 99%) in a porcelain crucible was heated in a muffle furnace in air to 850 ◦ C, at a heating rate of 20 ◦ C/h. After 20 h, the mixture was cooled to room temperature, washed several times with distilled water and then dried at 70 ◦ C. La-doped Ag1.4 K0.6 Ta4 O11 was synthesized by the same procedure except that 0.5–5 mol% of La2 O3 (Aldrich, 99%) was added to the reaction mixture.
2.2. Characterization Power X-ray diffraction (XRD) was performed on a Rigaku D/max-2500 X-ray diffractometer with Cu K␣ irradiation ´˚ at a scanning speed of 0.02◦ /s over the 2 range of ( = 1.5406 A) 10–90◦ . The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. The average crystal size of the Ag1.4 K0.6 Ta4 O11 and x%La-Ag1.4 K0.6 Ta4 O11 samples were calculated using the Scherer equation: Dc = K/ˇ cos , where Dc is the average crystal size, K is the Scherer constant (equal to 0.89), is the X-ray wavelength (0.15406 nm), ˇ is the full width at half maximum (FWHM), and is the diffraction angle (2 = 36.0◦ was used in this work). The morphologies of the samples were examined by using a Philips XL30 environmental scanning electron microscope (ESEM) at an accelerating voltage of 10 kV. Analysis of the catalyst surface was done by X-ray photoelectron spectroscopy (XPS) using a Leybold Heraeus-Shengyang SKL-12 electron spectrometer equipped with a VG CLAM 4 MCD electron energy analyzer. Mg-K␣ X-ray radiation (h = 1253.6 eV) was generated at 10 kV and 15 mA. The spectrometer chamber had a residual gas pressure close to 2 × 10−9 mbar during data acquisition. UV–vis diffuse reflectance spectra (DRS) of the samples were obtained from a Perkin-Elmer Lambda 750 UV/vis spectrophotometer. Nitrogen sorption isotherms were measured at −196 ◦ C by using a Micromeritics Tristar II 3020 system. The standard multipoint Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface area using the adsorption data in the P/P0 range from 0.07 to 0.22. Pore size distributions curves were computed from the desorption branches of the isotherms using the Barrett–Joyner–Halenda (BJH) model.
2.3. Photocatalysis A 200 W xenon arc lamp (Newport, Model 71232) was used as the light source. The incident radiation intensity was measured using a laser power meter (Molectron Detector, USA), and the average light intensity was 48.3 mW/cm2 with no cutoff filter; 29.5 mW/cm2 with 390 nm cutoff filter; and 18.9 mW/cm2 with 420 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 a quartz tube reactor (12 mm in diameter and 200 mm in length) and the mixture was sonicated for 5 min to disperse the catalyst in the aqueous solution. The distance between the liquid surface and the light source was about 11 cm. Before photoirradiation, the suspension was stirred in the dark for one hour to allow adsorption–desorption equilibrium to be established. The light emitted from the Xe-lamp was passed through a water jacket and a cutoff filter before reaching the sample solution. Aliquots were taken at regular time intervals and centrifuged before analysis. The concentration of the substrates was monitored with a Shimadzu UV-1700 UV–vis spectrophotometer. Photocatalytic hydrogen generation experiments were conducted under argon at room temperature in water containing 20% methanol as sacrificial donor. The photocatalyst was sonicated for 5 min in water prior to irradiation with a 200 W xenon arc lamp. The evolved gas was measured by a gas chromatograph (HP 5890 Series II) equipped with a thermal conductivity detector (TCD). 3. Results and discussion 3.1. Characterization of Ag1.4 K0.6 Ta4 O11 Ag1.4 K0.6 Ta4 O11 was synthesized by heating a mixture of AgNO3 with one mol equivalent of Ta2 O5 in KCl at 850 ◦ C for 20 h. Ladoped Ag1.4 K0.6 Ta4 O11 was prepared by adding 0.5–5 mol% of La2 O3 to the mixture before heating. The powder X-ray diffraction (XRD) patterns of the tantalate and La-doped tantalate samples are shown in Fig. 1. The diffraction patterns are consistent with those of the standard card for the K2 Ta4 O11 crystal (PDF#12-0092), with a very small amount of K2 Ta2 O6 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].
Fig. 1. XRD patterns of (a) Ag1.4 K0.6 Ta4 O11 ; (b) 0.5%La-Ag1.4 K0.6 Ta4 O11 ; (c) 1%La-Ag1.4 K0.6 Ta4 O11 ; (d) 2%La-Ag1.4 K0.6 Ta4 O11 ; (e) 3%La-Ag1.4 K0.6 Ta4 O11 ; (f) 4%LaAg1.4 K0.6 Ta4 O11 ; and (g) 5%La-Ag1.4 K0.6 Ta4 O11 . The standard XRD pattern of Ag1.4 K0.6 Ta4 O11 is also included for reference. The impurity peak marked by () is due to K2 Ta2 O6 . The peak marked by (↓) corresponds to La0.33 TaO3 .
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1%La-Ag1.4 K0.6 Ta4 O11 sample. The atom concentrations were calculated by the following equation, Cn =
Fig. 2. EDX spectrum of Ag1.4 K0.6 Ta4 O11 .
The observed K2 Ta4 O11 crystal structure for the Ag1.4 K0.6 Ta4 O11 and La-Ag1.4 K0.6 Ta4 O11 samples indicate the Ag+ ions can occupy the K+ lattice sites and do not change the crystal structure of the sample. If synthesis was carried out without AgNO3 or KCl, the XRD patterns of the product correspond to K2 Ta4 O11 or Ag2 Ta4 O11 , respectively (Fig. S1). Energy-dispersive X-ray spectroscopy (EDX) reveals that the ratio of Ag/K is 1.37:0.64 (Fig. 2). Upon doping with La (0.5–5 mol%), the diffraction patterns of Ag1.4 K0.6 Ta4 O11 remains essentially unchanged, except that a very small amount of La0.33 TaO3 is present (PDF#42-0061), as evidenced by the appearance of its characteristic peaks at 2 22.49◦ and 22.665◦ . The intensity of these peaks increases with the La doping level. The average crystallite sizes of the samples, which were calculated from the Debye-Scherrer formula, are shown in Table 1. It can be seen that La doping does not cause a significant change in the average crystallite size of the products, which is consistent with the XRD analysis. The SEM photographs of Ag1.4 K0.6 Ta4 O11 (Fig. 3a and b) show that it consists of hexagonal nanoplates of rather uniform size (diameter, 200–650 nm) and thickness (ca. 70 nm). Remarkably, the addition of just 0.5–5 mol% of La2 O3 results in significant changes in the morphology of the samples, which consist of a mixture of nanoplates and nanowires that are 35–125 nm in diameter and several m in length (Fig. 3c). The relative amount of the nanowires increases with increasing La2 O3 content (Fig. S2). The transmission electron micrograph (TEM, Fig. 3d) clearly shows the presence of metallic Ag nanoparticles on the surface of the Ag1.4 K0.6 Ta4 O11 nanoplates, with the particle size ranging from 20–30 nm. The (1 1 1) planes of the single-crystalline Ag can be seen in the HRTEM (Fig. 3e). The X-ray photoelectron spectra of Ag1.4 K0.6 Ta4 O11 and Ladoped Ag1.4 K0.6 Ta4 O11 are shown in Fig. 4. No significant contamination, besides carbon, is found in the spectra. The binding energy was determined by reference to C 1s line at 284.6 eV. In the whole energy range spectrum shown in Fig. 4a, the elements K, Ag, Ta and O can be observed in Ag1.4 K0.6 Ta4 O11 and 1%La-Ag1.4 K0.6 Ta4 O11 . However, La is present only in the
I /Sn
n
I /Si i i
where Cn is the element concentration, In and Ii are the peak intensities of the element and other elements, respectively; Sn and Si are the relative sensitivity of the element and other elements, respectively. For Ag1.4 K0.6 Ta4 O11 nanoplates, the atom ratio of Ag:K:Ta:O was calculated to be 1.4:0.7:3.7:11.7, which agrees reasonably 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 binding energies, respectively (Fig. 4b), are observed [48–50]. Each band could be further deconvoluted into two peaks at 367.5, 368.6 and 373.6, 374.8 eV, respectively. The peaks at 367.5 and 373.5 eV are attributed to the Ag+ of Ag1.4 K0.6 Ta4 O11 , and those at 368.6 and 374.8 eV are ascribed to metallic Ag [35]. The calculated surface mole ratio of Ag+ to Ag0 is 12:1. In 1%La-Ag1.4 K0.6 Ta4 O11 , the La concentration is calculated to be 1.3 atom%. There are two peaks centered at 852.2 eV and 835.1 eV, which are attributed to La 3d5/2 and La 3d3/2, respectively (Fig. 4c) [51]. Apart from the presence of La peaks, the positions and relative intensities of the other peaks in 1%La-Ag1.4 K0.6 Ta4 O11 are similar to those in Ag1.4 K0.6 Ta4 O11 . The specific surface areas and pore volumes of the samples were measured using the nitrogen gas sorption technique, and typical isotherms are shown in Fig. 5. For the Ag1.4 K0.6 Ta4 O11 nanoplates (Fig. 5 0%), a type IV isotherm along with one small, but obvious hysteresis 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 Ag1.4 K0.6 Ta4 O11 nanoplates exhibits a sharp increase at P/P0 = 0.88–1.00, corresponding to capillary condensation within mesopores, indicating a narrow pore-size distribution. The pore-size is centered at 12.5 nm in the sample. Apparently La doping did not cause significant change in the mesoporous structure of Ag1.4 K0.6 Ta4 O11 (Fig. 5 0.5–5%). However, there are additional peaks on the pore-size distribution curves, which presumably arise from the pores in the nanowires formed after La2 O3 doping. Table 1 shows the physicochemical properties of the Ag1.4 K0.6 Ta4 O11 nanoplates and the x%La-Ag1.4 K0.6 Ta4 O11 nanocomposites. Upon La doping, the surface area of x%LaAg1.4 K0.6 Ta4 O11 first decreases slightly (0.5–1%), then gradually increases (2–5%). The decrease in surface area at low La doping could be due to the slight increase in size of the nanoplates upon La doping (see Figs. 3 and S2). However, as La doping increases, nanowires of much smaller sizes are formed, which contribute to the increase in surface area. The UV–visible diffuse reflectance spectra (DRS) of the Ag1.4 K0.6 Ta4 O11 nanoplates and the x%La-Ag1.4 K0.6 Ta4 O11 nanocomposites are shown in Fig. S3. All samples exhibit a
Table 1 Physicochemical properties of Ag1.4 K0.6 Ta4 O11 and La-Ag1.4 K0.6 Ta4 O11 samples. La-doped Ag1.4 K0.6 Ta4 O11
Average crystallite size (nm)a
BET surface area (m2 /g)b
Pore volume (cm3 /g)c
RhB k (h−1 )d
PCP k (h−1 )d
H2 rate (mol/h/g)e
0% 0.5% 1% 2% 4% 5% TiO2
48.2 48.5 47.9 48.5 49.1 49.1
2.59 2.38 2.47 2.86 3.53 3.68
0.007 0.006 0.007 0.01 0.01 0.012
0.08 0.11 0.19 0.14 0.12 0.13 0.09
0.46 0.33 0.97 0.54 0.35 0.32 0.11
0.7 1.0 1.4 4.0 5.4 8.6 6.0
a b c d e
The average crystallite size was calculated by using the Scherer equation. Obtained from N2 adsorption data in the P/P0 range from 0.07 to 0.22. Single-point pore volume calculated from the adsorption isotherm at P/P0 = 0.97. k is the rate constant for the photodegradation of RhB or PCP at > 420 nm. H2 evolution from 20% methanol in water at > 390 nm.
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Fig. 3. (a) Low-magnification SEM image of Ag1.4 K0.6 Ta4 O11 , (b) high-magnification FESEM image of Ag1.4 K0.6 Ta4 O11 , (c) low-magnification SEM image of the 1%LaAg1.4 K0.6 Ta4 O11 , (d) TEM image of Ag1.4 K0.6 Ta4 O11 nanoparticles, and (e) HRTEM image of Ag nanoparticle.
band gap absorption threshold below 400 nm. In addition, they also show absorptions in the visible region (400–800 nm) that are centered at around 510 nm, which is attributed to plasmonic resonance from Ag nanoparticles present on the surface of the samples [35,41,42], as observed by XPS. The absorptions of the La-doped samples are slightly red-shifted relative to the undoped sample, suggesting electronic interaction between La3+ and tantalate.
Generally, the surface plasmon resonance (SPR) wavelength depends strongly on the size and shape of the nanoparticles, the interparticle distance, and the dielectric property of the surrounding medium [53]. The relationship between the highest plasmon absorption wavelength of silver and its particle size was shown in Table S1. In the current study, the Ag particle size was determined to be 20–30 nm by TEM (as shown in Fig. 3d).
Fig. 4. XPS spectra of (a) survey spectrum; (b) Ag 3d; (c) La 3d for Ag1.4 K0.6 Ta4 O11 and 1%La-Ag1.4 K0.6 Ta4 O11 .
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Fig. 5. Nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curve calculated from adsorption branch of the nitrogen isotherm (inset) of x%La-Ag1.4 K0.6 Ta4 O11 .
Thus the large red-shift of the plasmon resonance to 510 nm was expected. 3.2. Photocatalytic activity 3.2.1. Photocatalytic degradation of organic pollutants in water The photocatalytic activities of the Ag1.4 K0.6 Ta4 O11 nanoplates and the x%La-Ag1.4 K0.6 Ta4 O11 nanocomposites were evaluated by degradation of two common organic pollutants, rhodamine B (RhB) and pentachlorophenol (PCP), in water under visible light irradiation ( > 420 nm). Before light irradiation, the mixture of organic pollutant and photocatalyst in water was stirred for 60 min in the dark to achieve sorption equilibrium. RhB and PCP were readily degraded upon visible light irradiation in the presence of Ag1.4 K0.6 Ta4 O11 and La-Ag1.4 K0.6 Ta4 O11 (Fig. 6). Control experiments showed that both light and photocatalyst are required for the
degradation of the organic substrates. The rates of the photodegradation of the organic substrates could be approximately described by the first-order equation, ln(C/C0 ) = −kt, where k is the rate constant, C and C0 are the concentration of organic pollutant at time t and 0, respectively. Kinetic data are summarized in Table 1. The rate of photodegradation of RhB by Ag1.4 K0.6 Ta4 O11 (k = 0.08 h−1 ) is slightly slower than that of P25 TiO2 (k = 0.09 h−1 ). On the other hand, Ag1.4 K0.6 Ta4 O11 is four times faster than TiO2 in the photodegradation of PCP (0.46 vs. 0.11 h−1 ). Doping with La enhances the photocatalytic activity of Ag1.4 K0.6 Ta4 O11 (Table 1 and Fig. 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, the photocatalytic activity of metal oxide semiconductors depends on the surface area, optical absorption capability and diffusion rates
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Fig. 6. Plot of C/C0 (C = concentration at time t, C0 = initial concentration) vs. time for the photodegradation of RhB (left panel) and PCP (right panel) by Ag1.4 K0.6 Ta4 O11 and x%La-Ag1.4 K0.6 Ta4 O11 under visible light irradiation ( > 420 nm).
of charge carriers [54–57]. In the present case, increasing the surface area apparently has no advantageous effect on RhB and PCP photodegradation; the 2–5% La-Ag1.4 K0.6 Ta4 O11 samples, which possess larger surface areas than the 1%La-Ag1.4 K0.6 Ta4 O11 , show lower photocatalytic activity than that of 1% La sample (Table 1). Also it can be seen that all the La-doped Ag1.4 K0.6 Ta4 O11 samples display very similar absorption both in the UV and visible regions (Fig. S3), so the band gap effect is not expected to be large. Hence the main effects of La3+ could be due to its ability to inhibit recombination of photogenerated electrons–holes pairs by trapping excited electrons [58,59]. Moreover, the addition of La3+ leads to the formation of nanowires (Figs. 3c and S2), which should be more efficient in light scattering and absorption, as well as in charge separation [60–62]. The optimal La content is 1%, presumably because its hetero-nanostructure is most efficient for separation of the photoinduced electron–hole pairs at > 420 nm [63]. 3.2.2. Photocatalytic H2 evolution The photocatalytic H2 production efficiency of Ag1.4 K0.6 Ta4 O11 nanoplates, x%La-Ag1.4 K0.6 Ta4 O11 nanocomposites and P25 TiO2 were investigated at > 390 nm with methanol (20 vol%) as the sacrificial donor (Fig. 7, Table 1). Control experiments showed that light, catalyst and sacrificial agent are all required for H2 generation. Ag1.4 K0.6 Ta4 O11 itself is a rather inefficient photocatalyst. However, the H2 evolution rate is significantly increased with La doping. The highest H2 evolution rate was obtained from 5%La-Ag1.4 K0.6 Ta4 O11 , which is approximately 10 times that of the undoped sample and 1.6 times that of P25 TiO2 . This is in contrast to photodegradation
of organic substrates, in which 1%La-Ag1.4 K0.6 Ta4 O11 is the most active photocatalyst. This could be due to the difference in the wavelengths of light used; in water reduction > 390 nm light was used, while in photodegradation > 420 nm light was used. Apparently 5%La-Ag1.4 K0.6 Ta4 O11 is a more efficient photocatalyst at lower wavelengths. In addition, the increased surface area may also contribute to this enhancement (Table 1). 4. Conclusions A new silver tantalate photocatalyst, Ag1.4 K0.6 Ta4 O11 , has been prepared. This material makes use of Ag nanoparticles on the surface to enhance absorption in the visible region. The photocatalytic activity of this material is significantly enhanced by doping with a few % of La3+ . The rates of photocatalytic degradation of organic pollutants and H2 generation from water by La-Ag1.4 K0.6 Ta4 O11 are higher than that of P25 TiO2 . The beneficial effects of La doping are attributed to trapping of excited electrons by La3+ as well as to change in morphology from nanoplates to nanoplates/nanowire composites, which would enhance the separation of photo-induced electron/hole pairs. Acknowledgements The work described in this paper was supported by the University Grants Committee (UGC) of Hong Kong Special Administrative Region (AoE/P-04/04) and the State Key Laboratory in Marine Pollution (SKLMP). The photochemical equipment used in this work was supported by a Special Equipment Grant from UGC (SEG CityU02). The XRD and XPS used in this work were provided by the Institute of Advanced Materials of the Hong Kong Baptist University. Funding for the XRD and XPS were from the Special Equipment Grant from the University Grants Committee of the Hong Kong Special Administrative Region, China (SEG HKBU06).” Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata. 2013.07.041. References
Fig. 7. Plot of amount of H2 evolved vs. time for photocatalytic water reduction by Ag1.4 K0.6 Ta4 O11 and x%La-Ag1.4 K0.6 Ta4 O11 in aqueous methanol (20 vol%) at > 390 nm.
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Synthesis of La-doped Ag1.4 K0.6 Ta4 O11 nanocomposites as efficient photocatalysts for hydrogen production and organic pollutants degradation Ruwei Wang a,b,c , Yufeng Zhu b , Guijian Liu a,c,∗ , Tai-Chu Lau a,b,∗ a
Advanced Laboratory of Environmental Research and Technology (ALERT), Joint Advanced Research Center, USTC-CityU, Suzhou, Jiangsu 215123, China Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China c CAS Key Laboratory of Crust-Mantle and the Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China b
a r t i c l e
i n f o
Article history: Received 8 May 2013 Received in revised form 11 July 2013 Accepted 20 July 2013 Available online 31 July 2013 Keywords: Tantalum oxide Lathanum doping Photodegradation Organic pollutants Hydrogen generation
a b s t r a c t A new silver tantalate material has been synthezied by heating a mixture of AgNO3 , Ta2 O5 and KCl at 850 ◦ C for 20 h. XRD, EDX, XPS, SEM, TEM and HRTEM show that the material consists of Ag1.4 K0.6 Ta4 O11 nanoplates, with Ag nanoparticles present on the surface, which is consistent with enhanced absorption of the material in the visible region. Addition of 0.5–5 mol% of La2 O3 in the preparation of Ag1.4 K0.6 Ta4 O11 results in significant change in the morphology from nanoplates to nanoplates/nanowire composites. The photocatalytic activities of Ag1.4 K0.6 Ta4 O11 and La-Ag1.4 K0.6 Ta4 O11 have been evaluated by degradation of the organic pollutants rhodamine B (RhB) and pentachlorophenol (PCP) in water under visible light ( > 420 nm), as well as by photocatalytic reduction of water to H2 at > 390 nm. The photocatalytic activity of Ag1.4 K0.6 Ta4 O11 is significantly enhanced by La-doping; the optimal La content is 1 mol% for degradation of organic substrates and 5 mol% for H2 evolution, with photocatalytic activity significantly higher than that of P25 TiO2 . The enhancement of photocatalytic activity upon La doping is attributed to trapping of excited electrons by La3+ , and the formation of nanowires which further promote charge separation. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The use of semiconductor photocatalysts for solar water splitting and degradation of organic pollutants has received considerable attention in recent years [1–4]. One of the most challenging tasks in this area is to develop a photocatalyst that can work efficiently under sunlight [5]. Titanium dioxide (TiO2 ) has been extensively used as a photocatalyst because of its photostability, natural abundance, and nontoxicity [6]. The main disadvantage of TiO2 is that it is active only under UV light due to its wide band gap (3.2 eV). Consequently, various strategies have been used to develop visible-light responsive photocatalysts, either by exploring new materials, or by enhancing the photocatalytic activity of existing materials using methods such as non-metal-ion substitution, as in TiO2−x Nx [7–9], Sm2 Ti2 S2 O5 [10,11] and N-La2 Ti2 O7 [12]; metal-ion substitution,
∗ Corresponding author at: Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China. Tel.: +852 34427811; fax: +852 34420522. E-mail address: [email protected] (T.-C. Lau). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.07.041
as in (V-, Fe- or Mn-)TiO2 and In1−x Nix TaO4 [13,14]; semiconductor/graphene heterojunction, as in ␣-Fe2 O3 /reduced graphene oxide [15]. Other methods such as solid-solution fabrication, as in (Ga1−x Znx )(N1−x Ox ) [16], ZnS-CuInS2 -AgInS2 and -AgAl1−x Gax O2 [17,18] have also been reported. The use of silver-based oxides for a variety of photocatalytic applications has also received much attention in recent years. Examples of Ag-containing catalysts include AgNbO3 and AgTaO3 [19], AgInW2 O8 [20], Ag3 VO4 [21], AgInZn7 S9 [22], AgGaS2 [23,24], AgIn5 S8 [25], AgGa0.9 In0.1 S2 [26], Ag0.7 Na0.3 NbO3 [27], AgLi1/3 Ti2/3 O2 and AgLi1/3 Sn2/3 O2 [28], (AgNbO3 )0.75 (SrTiO3 )0.25 [29], Ag2 ZnSnS4 [30], AgGa2 In3 S8 [31], Ag3 PO4 [32], AgAl1−x Gax O2 [18], Ag/AgX (X = Br, Cl) nanocomposites [33,34], Ag2 V4 O11 [35], and Ag-modified TiO2 [36]. Ag can enhance the photocatalytic activity of metal oxides through the following mechanisms: (1) the contribution of Ag 4d orbitals to the valence band may narrow the band gap of the metal oxide [2]. (2) Excited electrons can be trapped by silver ions and so the recombination of electron–hole pairs may be inhibited [37–40]. (3) The existence of metallic Ag nanoparticles will give rise to distinct plasmonic resonance in the visible region, and this will enhance the response of the metal oxide to visible light [41–44].
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R. Wang et al. / Applied Catalysis A: General 467 (2013) 335–341
We report here the synthesis and characterization of a new silver tantalate nanomaterial, Ag1.4 K0.6 Ta4 O11 , via a molten salt method. Tantalum oxide based materials are promising photocatalysts for water splitting, since the conduction bands of these materials consist of Ta 5d orbitals which are located at a more negative potential than titanates (3d) and niobates (4d), and this would facilitate the reduction of H2 O to H2 [2,45,46]. Although AgTaO3 is known, it shows photocatalytic activity only under UV irradiation due to its wide band gap (3.4 eV) [19]. On the other hand, Ag1.4 K0.6 Ta4 O11 shows enhanced absorption in the visible region due to plasmonic resonance from Ag nanoparticles present on the surface of the samples. In addition, the addition of just a few % of La2 O3 in the preparation of Ag1.4 K0.6 Ta4 O11 results in a remarkable change in the morphology of the material from nanoplates to nanoplate/nanowire composites, with also enhanced photocatalytic activity under visible light. The La-Ag1.4 K0.6 Ta4 O11 composites exhibit significantly higher photocatalytic activity toward water reduction and degradation of organic pollutants than TiO2 under visible light.
2. Experimental 2.1. Synthesis Ag1.4 K0.6 Ta4 O11 was synthesized by the following procedure. A ground mixture of AgNO3 (0.68 mmol, Johnson Matthey 99%), Ta2 O5 (0.68 mmol, Aldrich 99%) and KCl (4 g, Aldrich 99%) in a porcelain crucible was heated in a muffle furnace in air to 850 ◦ C, at a heating rate of 20 ◦ C/h. After 20 h, the mixture was cooled to room temperature, washed several times with distilled water and then dried at 70 ◦ C. La-doped Ag1.4 K0.6 Ta4 O11 was synthesized by the same procedure except that 0.5–5 mol% of La2 O3 (Aldrich, 99%) was added to the reaction mixture.
2.2. Characterization Power X-ray diffraction (XRD) was performed on a Rigaku D/max-2500 X-ray diffractometer with Cu K␣ irradiation ´˚ at a scanning speed of 0.02◦ /s over the 2 range of ( = 1.5406 A) 10–90◦ . The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. The average crystal size of the Ag1.4 K0.6 Ta4 O11 and x%La-Ag1.4 K0.6 Ta4 O11 samples were calculated using the Scherer equation: Dc = K/ˇ cos , where Dc is the average crystal size, K is the Scherer constant (equal to 0.89), is the X-ray wavelength (0.15406 nm), ˇ is the full width at half maximum (FWHM), and is the diffraction angle (2 = 36.0◦ was used in this work). The morphologies of the samples were examined by using a Philips XL30 environmental scanning electron microscope (ESEM) at an accelerating voltage of 10 kV. Analysis of the catalyst surface was done by X-ray photoelectron spectroscopy (XPS) using a Leybold Heraeus-Shengyang SKL-12 electron spectrometer equipped with a VG CLAM 4 MCD electron energy analyzer. Mg-K␣ X-ray radiation (h = 1253.6 eV) was generated at 10 kV and 15 mA. The spectrometer chamber had a residual gas pressure close to 2 × 10−9 mbar during data acquisition. UV–vis diffuse reflectance spectra (DRS) of the samples were obtained from a Perkin-Elmer Lambda 750 UV/vis spectrophotometer. Nitrogen sorption isotherms were measured at −196 ◦ C by using a Micromeritics Tristar II 3020 system. The standard multipoint Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface area using the adsorption data in the P/P0 range from 0.07 to 0.22. Pore size distributions curves were computed from the desorption branches of the isotherms using the Barrett–Joyner–Halenda (BJH) model.
2.3. Photocatalysis A 200 W xenon arc lamp (Newport, Model 71232) was used as the light source. The incident radiation intensity was measured using a laser power meter (Molectron Detector, USA), and the average light intensity was 48.3 mW/cm2 with no cutoff filter; 29.5 mW/cm2 with 390 nm cutoff filter; and 18.9 mW/cm2 with 420 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 a quartz tube reactor (12 mm in diameter and 200 mm in length) and the mixture was sonicated for 5 min to disperse the catalyst in the aqueous solution. The distance between the liquid surface and the light source was about 11 cm. Before photoirradiation, the suspension was stirred in the dark for one hour to allow adsorption–desorption equilibrium to be established. The light emitted from the Xe-lamp was passed through a water jacket and a cutoff filter before reaching the sample solution. Aliquots were taken at regular time intervals and centrifuged before analysis. The concentration of the substrates was monitored with a Shimadzu UV-1700 UV–vis spectrophotometer. Photocatalytic hydrogen generation experiments were conducted under argon at room temperature in water containing 20% methanol as sacrificial donor. The photocatalyst was sonicated for 5 min in water prior to irradiation with a 200 W xenon arc lamp. The evolved gas was measured by a gas chromatograph (HP 5890 Series II) equipped with a thermal conductivity detector (TCD). 3. Results and discussion 3.1. Characterization of Ag1.4 K0.6 Ta4 O11 Ag1.4 K0.6 Ta4 O11 was synthesized by heating a mixture of AgNO3 with one mol equivalent of Ta2 O5 in KCl at 850 ◦ C for 20 h. Ladoped Ag1.4 K0.6 Ta4 O11 was prepared by adding 0.5–5 mol% of La2 O3 to the mixture before heating. The powder X-ray diffraction (XRD) patterns of the tantalate and La-doped tantalate samples are shown in Fig. 1. The diffraction patterns are consistent with those of the standard card for the K2 Ta4 O11 crystal (PDF#12-0092), with a very small amount of K2 Ta2 O6 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].
Fig. 1. XRD patterns of (a) Ag1.4 K0.6 Ta4 O11 ; (b) 0.5%La-Ag1.4 K0.6 Ta4 O11 ; (c) 1%La-Ag1.4 K0.6 Ta4 O11 ; (d) 2%La-Ag1.4 K0.6 Ta4 O11 ; (e) 3%La-Ag1.4 K0.6 Ta4 O11 ; (f) 4%LaAg1.4 K0.6 Ta4 O11 ; and (g) 5%La-Ag1.4 K0.6 Ta4 O11 . The standard XRD pattern of Ag1.4 K0.6 Ta4 O11 is also included for reference. The impurity peak marked by () is due to K2 Ta2 O6 . The peak marked by (↓) corresponds to La0.33 TaO3 .
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1%La-Ag1.4 K0.6 Ta4 O11 sample. The atom concentrations were calculated by the following equation, Cn =
Fig. 2. EDX spectrum of Ag1.4 K0.6 Ta4 O11 .
The observed K2 Ta4 O11 crystal structure for the Ag1.4 K0.6 Ta4 O11 and La-Ag1.4 K0.6 Ta4 O11 samples indicate the Ag+ ions can occupy the K+ lattice sites and do not change the crystal structure of the sample. If synthesis was carried out without AgNO3 or KCl, the XRD patterns of the product correspond to K2 Ta4 O11 or Ag2 Ta4 O11 , respectively (Fig. S1). Energy-dispersive X-ray spectroscopy (EDX) reveals that the ratio of Ag/K is 1.37:0.64 (Fig. 2). Upon doping with La (0.5–5 mol%), the diffraction patterns of Ag1.4 K0.6 Ta4 O11 remains essentially unchanged, except that a very small amount of La0.33 TaO3 is present (PDF#42-0061), as evidenced by the appearance of its characteristic peaks at 2 22.49◦ and 22.665◦ . The intensity of these peaks increases with the La doping level. The average crystallite sizes of the samples, which were calculated from the Debye-Scherrer formula, are shown in Table 1. It can be seen that La doping does not cause a significant change in the average crystallite size of the products, which is consistent with the XRD analysis. The SEM photographs of Ag1.4 K0.6 Ta4 O11 (Fig. 3a and b) show that it consists of hexagonal nanoplates of rather uniform size (diameter, 200–650 nm) and thickness (ca. 70 nm). Remarkably, the addition of just 0.5–5 mol% of La2 O3 results in significant changes in the morphology of the samples, which consist of a mixture of nanoplates and nanowires that are 35–125 nm in diameter and several m in length (Fig. 3c). The relative amount of the nanowires increases with increasing La2 O3 content (Fig. S2). The transmission electron micrograph (TEM, Fig. 3d) clearly shows the presence of metallic Ag nanoparticles on the surface of the Ag1.4 K0.6 Ta4 O11 nanoplates, with the particle size ranging from 20–30 nm. The (1 1 1) planes of the single-crystalline Ag can be seen in the HRTEM (Fig. 3e). The X-ray photoelectron spectra of Ag1.4 K0.6 Ta4 O11 and Ladoped Ag1.4 K0.6 Ta4 O11 are shown in Fig. 4. No significant contamination, besides carbon, is found in the spectra. The binding energy was determined by reference to C 1s line at 284.6 eV. In the whole energy range spectrum shown in Fig. 4a, the elements K, Ag, Ta and O can be observed in Ag1.4 K0.6 Ta4 O11 and 1%La-Ag1.4 K0.6 Ta4 O11 . However, La is present only in the
I /Sn
n
I /Si i i
where Cn is the element concentration, In and Ii are the peak intensities of the element and other elements, respectively; Sn and Si are the relative sensitivity of the element and other elements, respectively. For Ag1.4 K0.6 Ta4 O11 nanoplates, the atom ratio of Ag:K:Ta:O was calculated to be 1.4:0.7:3.7:11.7, which agrees reasonably 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 binding energies, respectively (Fig. 4b), are observed [48–50]. Each band could be further deconvoluted into two peaks at 367.5, 368.6 and 373.6, 374.8 eV, respectively. The peaks at 367.5 and 373.5 eV are attributed to the Ag+ of Ag1.4 K0.6 Ta4 O11 , and those at 368.6 and 374.8 eV are ascribed to metallic Ag [35]. The calculated surface mole ratio of Ag+ to Ag0 is 12:1. In 1%La-Ag1.4 K0.6 Ta4 O11 , the La concentration is calculated to be 1.3 atom%. There are two peaks centered at 852.2 eV and 835.1 eV, which are attributed to La 3d5/2 and La 3d3/2, respectively (Fig. 4c) [51]. Apart from the presence of La peaks, the positions and relative intensities of the other peaks in 1%La-Ag1.4 K0.6 Ta4 O11 are similar to those in Ag1.4 K0.6 Ta4 O11 . The specific surface areas and pore volumes of the samples were measured using the nitrogen gas sorption technique, and typical isotherms are shown in Fig. 5. For the Ag1.4 K0.6 Ta4 O11 nanoplates (Fig. 5 0%), a type IV isotherm along with one small, but obvious hysteresis 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 Ag1.4 K0.6 Ta4 O11 nanoplates exhibits a sharp increase at P/P0 = 0.88–1.00, corresponding to capillary condensation within mesopores, indicating a narrow pore-size distribution. The pore-size is centered at 12.5 nm in the sample. Apparently La doping did not cause significant change in the mesoporous structure of Ag1.4 K0.6 Ta4 O11 (Fig. 5 0.5–5%). However, there are additional peaks on the pore-size distribution curves, which presumably arise from the pores in the nanowires formed after La2 O3 doping. Table 1 shows the physicochemical properties of the Ag1.4 K0.6 Ta4 O11 nanoplates and the x%La-Ag1.4 K0.6 Ta4 O11 nanocomposites. Upon La doping, the surface area of x%LaAg1.4 K0.6 Ta4 O11 first decreases slightly (0.5–1%), then gradually increases (2–5%). The decrease in surface area at low La doping could be due to the slight increase in size of the nanoplates upon La doping (see Figs. 3 and S2). However, as La doping increases, nanowires of much smaller sizes are formed, which contribute to the increase in surface area. The UV–visible diffuse reflectance spectra (DRS) of the Ag1.4 K0.6 Ta4 O11 nanoplates and the x%La-Ag1.4 K0.6 Ta4 O11 nanocomposites are shown in Fig. S3. All samples exhibit a
Table 1 Physicochemical properties of Ag1.4 K0.6 Ta4 O11 and La-Ag1.4 K0.6 Ta4 O11 samples. La-doped Ag1.4 K0.6 Ta4 O11
Average crystallite size (nm)a
BET surface area (m2 /g)b
Pore volume (cm3 /g)c
RhB k (h−1 )d
PCP k (h−1 )d
H2 rate (mol/h/g)e
0% 0.5% 1% 2% 4% 5% TiO2
48.2 48.5 47.9 48.5 49.1 49.1
2.59 2.38 2.47 2.86 3.53 3.68
0.007 0.006 0.007 0.01 0.01 0.012
0.08 0.11 0.19 0.14 0.12 0.13 0.09
0.46 0.33 0.97 0.54 0.35 0.32 0.11
0.7 1.0 1.4 4.0 5.4 8.6 6.0
a b c d e
The average crystallite size was calculated by using the Scherer equation. Obtained from N2 adsorption data in the P/P0 range from 0.07 to 0.22. Single-point pore volume calculated from the adsorption isotherm at P/P0 = 0.97. k is the rate constant for the photodegradation of RhB or PCP at > 420 nm. H2 evolution from 20% methanol in water at > 390 nm.
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Fig. 3. (a) Low-magnification SEM image of Ag1.4 K0.6 Ta4 O11 , (b) high-magnification FESEM image of Ag1.4 K0.6 Ta4 O11 , (c) low-magnification SEM image of the 1%LaAg1.4 K0.6 Ta4 O11 , (d) TEM image of Ag1.4 K0.6 Ta4 O11 nanoparticles, and (e) HRTEM image of Ag nanoparticle.
band gap absorption threshold below 400 nm. In addition, they also show absorptions in the visible region (400–800 nm) that are centered at around 510 nm, which is attributed to plasmonic resonance from Ag nanoparticles present on the surface of the samples [35,41,42], as observed by XPS. The absorptions of the La-doped samples are slightly red-shifted relative to the undoped sample, suggesting electronic interaction between La3+ and tantalate.
Generally, the surface plasmon resonance (SPR) wavelength depends strongly on the size and shape of the nanoparticles, the interparticle distance, and the dielectric property of the surrounding medium [53]. The relationship between the highest plasmon absorption wavelength of silver and its particle size was shown in Table S1. In the current study, the Ag particle size was determined to be 20–30 nm by TEM (as shown in Fig. 3d).
Fig. 4. XPS spectra of (a) survey spectrum; (b) Ag 3d; (c) La 3d for Ag1.4 K0.6 Ta4 O11 and 1%La-Ag1.4 K0.6 Ta4 O11 .
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Fig. 5. Nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curve calculated from adsorption branch of the nitrogen isotherm (inset) of x%La-Ag1.4 K0.6 Ta4 O11 .
Thus the large red-shift of the plasmon resonance to 510 nm was expected. 3.2. Photocatalytic activity 3.2.1. Photocatalytic degradation of organic pollutants in water The photocatalytic activities of the Ag1.4 K0.6 Ta4 O11 nanoplates and the x%La-Ag1.4 K0.6 Ta4 O11 nanocomposites were evaluated by degradation of two common organic pollutants, rhodamine B (RhB) and pentachlorophenol (PCP), in water under visible light irradiation ( > 420 nm). Before light irradiation, the mixture of organic pollutant and photocatalyst in water was stirred for 60 min in the dark to achieve sorption equilibrium. RhB and PCP were readily degraded upon visible light irradiation in the presence of Ag1.4 K0.6 Ta4 O11 and La-Ag1.4 K0.6 Ta4 O11 (Fig. 6). Control experiments showed that both light and photocatalyst are required for the
degradation of the organic substrates. The rates of the photodegradation of the organic substrates could be approximately described by the first-order equation, ln(C/C0 ) = −kt, where k is the rate constant, C and C0 are the concentration of organic pollutant at time t and 0, respectively. Kinetic data are summarized in Table 1. The rate of photodegradation of RhB by Ag1.4 K0.6 Ta4 O11 (k = 0.08 h−1 ) is slightly slower than that of P25 TiO2 (k = 0.09 h−1 ). On the other hand, Ag1.4 K0.6 Ta4 O11 is four times faster than TiO2 in the photodegradation of PCP (0.46 vs. 0.11 h−1 ). Doping with La enhances the photocatalytic activity of Ag1.4 K0.6 Ta4 O11 (Table 1 and Fig. 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, the photocatalytic activity of metal oxide semiconductors depends on the surface area, optical absorption capability and diffusion rates
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Fig. 6. Plot of C/C0 (C = concentration at time t, C0 = initial concentration) vs. time for the photodegradation of RhB (left panel) and PCP (right panel) by Ag1.4 K0.6 Ta4 O11 and x%La-Ag1.4 K0.6 Ta4 O11 under visible light irradiation ( > 420 nm).
of charge carriers [54–57]. In the present case, increasing the surface area apparently has no advantageous effect on RhB and PCP photodegradation; the 2–5% La-Ag1.4 K0.6 Ta4 O11 samples, which possess larger surface areas than the 1%La-Ag1.4 K0.6 Ta4 O11 , show lower photocatalytic activity than that of 1% La sample (Table 1). Also it can be seen that all the La-doped Ag1.4 K0.6 Ta4 O11 samples display very similar absorption both in the UV and visible regions (Fig. S3), so the band gap effect is not expected to be large. Hence the main effects of La3+ could be due to its ability to inhibit recombination of photogenerated electrons–holes pairs by trapping excited electrons [58,59]. Moreover, the addition of La3+ leads to the formation of nanowires (Figs. 3c and S2), which should be more efficient in light scattering and absorption, as well as in charge separation [60–62]. The optimal La content is 1%, presumably because its hetero-nanostructure is most efficient for separation of the photoinduced electron–hole pairs at > 420 nm [63]. 3.2.2. Photocatalytic H2 evolution The photocatalytic H2 production efficiency of Ag1.4 K0.6 Ta4 O11 nanoplates, x%La-Ag1.4 K0.6 Ta4 O11 nanocomposites and P25 TiO2 were investigated at > 390 nm with methanol (20 vol%) as the sacrificial donor (Fig. 7, Table 1). Control experiments showed that light, catalyst and sacrificial agent are all required for H2 generation. Ag1.4 K0.6 Ta4 O11 itself is a rather inefficient photocatalyst. However, the H2 evolution rate is significantly increased with La doping. The highest H2 evolution rate was obtained from 5%La-Ag1.4 K0.6 Ta4 O11 , which is approximately 10 times that of the undoped sample and 1.6 times that of P25 TiO2 . This is in contrast to photodegradation
of organic substrates, in which 1%La-Ag1.4 K0.6 Ta4 O11 is the most active photocatalyst. This could be due to the difference in the wavelengths of light used; in water reduction > 390 nm light was used, while in photodegradation > 420 nm light was used. Apparently 5%La-Ag1.4 K0.6 Ta4 O11 is a more efficient photocatalyst at lower wavelengths. In addition, the increased surface area may also contribute to this enhancement (Table 1). 4. Conclusions A new silver tantalate photocatalyst, Ag1.4 K0.6 Ta4 O11 , has been prepared. This material makes use of Ag nanoparticles on the surface to enhance absorption in the visible region. The photocatalytic activity of this material is significantly enhanced by doping with a few % of La3+ . The rates of photocatalytic degradation of organic pollutants and H2 generation from water by La-Ag1.4 K0.6 Ta4 O11 are higher than that of P25 TiO2 . The beneficial effects of La doping are attributed to trapping of excited electrons by La3+ as well as to change in morphology from nanoplates to nanoplates/nanowire composites, which would enhance the separation of photo-induced electron/hole pairs. Acknowledgements The work described in this paper was supported by the University Grants Committee (UGC) of Hong Kong Special Administrative Region (AoE/P-04/04) and the State Key Laboratory in Marine Pollution (SKLMP). The photochemical equipment used in this work was supported by a Special Equipment Grant from UGC (SEG CityU02). The XRD and XPS used in this work were provided by the Institute of Advanced Materials of the Hong Kong Baptist University. Funding for the XRD and XPS were from the Special Equipment Grant from the University Grants Committee of the Hong Kong Special Administrative Region, China (SEG HKBU06).” Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata. 2013.07.041. References
Fig. 7. Plot of amount of H2 evolved vs. time for photocatalytic water reduction by Ag1.4 K0.6 Ta4 O11 and x%La-Ag1.4 K0.6 Ta4 O11 in aqueous methanol (20 vol%) at > 390 nm.
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