- Email: [email protected]
Bi2WO6 with advanced visible-light photocatalytic activity for toluene degradation in air
Journal of Colloid and Interface Science 412 (2013) 31–38
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
One-step synthesis of Cl-doped Pt(IV)/Bi2WO6 with advanced visible-light photocatalytic activity for toluene degradation in air Zhuxing Sun a,b, Xiaofang Li a,b, Sen Guo a,b,c,⇑, Haiqiang Wang a,b, Zhongbiao Wu a,b,⇑ a
Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, PR China Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue Gas Pollution Control, Hangzhou 311202, PR China c Suzhou Industrial Technology Research Institute of Zhejiang University, Suzhou 215163, PR China b
a r t i c l e
i n f o
Article history: Received 17 May 2013 Accepted 6 September 2013 Available online 17 September 2013 Keywords: One-step synthesis Cl doped Pt(IV) Bi2WO6 Visible light Photo-catalysis Toluene degradation
a b s t r a c t In this study, Cl doped Pt(IV)/Bi2WO6 photocatalyst was successfully synthesized via a one-step hydrothermal route by adding H2PtCl6 immediately in the reaction system. X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption– desorption isotherms and photoluminescence spectra (PL) were utilized to analyze the morphology, chemical states, surface area and optical properties of as-prepared catalysts. The addition of H2PtCl6 to the reaction system inhibited the growth of Bi2WO6 square nanoplates but facilitated the generation of irregular Bi2WO6 nanosheets with enlarged surface area and extended visible light absorption. In the resulted sample, it was observed that Pt(IV) was uniformly dispersed on the surface of Bi2WO6 irregular nanosheets and Cl other than that from PtCl4 was doped into the sample by forming ionic bond with [Bi2O2]2+. Compared with pure Bi2WO6 nanosheets, Cl doped Pt(IV)/Bi2WO6 showed enhanced activity in photocatalytic decomposition of toluene under visible light irradiation (420 nm < k < 780 nm). The catalyst with theoretical 1.0 wt.% Pt showed the best performance, which degraded 25 ppm toluene thoroughly in 35 min. Finally, the forming mechanism of Cl doped Pt(IV)/Bi2WO6 in the one-step hydrothermal process was proposed. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Bi2WO6 is the simplest member of Aurivillius family with a general formula of Bi2An1BnO3n+3 (A = Ca, Sr, Ba, Pb, Bi, Na, K and B = Ti, Nb, Ta, Mo, W, Fe) [1,2]. The structure of Bi2WO6 crystal is 2nþ orthorhombic, constructed by alternating ðBi2 O2 Þn ) layers and 2n perovskite-like ðWO4 Þn layers [3]. The conduction band of Bi2WO6 is composed of W5d orbitals and the valence band is hybrid orbitals of O2p and Bi6s [4,5]. Bi2WO6 can absorb visible light and shows great promise as a visible-light photocatalyst [6,7]. Recently, it has been successfully used for photo-degradation of organic pollutants [8–10], photocatalytic splitting of water [11,12] and photo-reduction of CO2 to CH4 [13,14]. Lots of efforts have been paid to improve the photocatalytic activity of pure Bi2WO6, especially to prolong the life span of photo-generated electron– hole pairs on the catalyst. Common methods include morphology controlling [15,16], ion doping [8,10,17–19], metal deposition [20,21], semiconductors coupling [9,22–28] and embedding on large-surface-area materials [29].
It is known that photo-sensitizing with noble metal complex (e.g., PtCl4 and H2PtCl6) is an effective method for enhancing the visible light response of photocatalysts [24,30–33]. However, Pt or its related salts modified photocatalytic system were usually achieved by retreatment after the photocatalysts were synthesized, such as impregnating the prepared catalysts in H2PtCl6 solution following thermal treatment [24,32,33], through a sol–gel process [30] or girding with PtCl4 or H2PtCl6 [31]. Considering the former bulk catalysts preparation procedure, the traditional approaches always take two or more steps and thus are uneasy to scale up for practical application [34]. In the present study, we prepared Cl-doped Pt(IV)/Bi2WO6 with advanced photocatalytic performance through a one-step hydrothermal route. To the best of our knowledge, this was for the first time that the one-step hydrothermal approach was used to synthesize Pt related salt modified photocatalysts. Thanks to its convenience, the process can also be applied to photocatalytic system other than Bi2WO6. 2. Experimental 2.1. Preparation of photocatalysts
⇑ Corresponding authors. Address: Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, PR China. Fax: +86 571 87953088. E-mail addresses: [email protected] (S. Guo), [email protected] (Z. Wu). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.09.004
All the chemicals were of analytical purity and used without any further purification. Deionized water was used in all experi-
32
Z. Sun et al. / Journal of Colloid and Interface Science 412 (2013) 31–38
ments. Typically, 8 mmol Bi(NO3)35H2O and 4 mmol Na2WO4 were mixed with 30 ml water. Following 3 h vigorously stirring of the resulted white suspension, appropriate amount of H2PtCl6 solution were added and stirred for another hour. The amount of H2PtCl6 was controlled according to the designed mass ratio of Pt to Bi2WO6 (0%, 0.02%, 0.1%, 0.5%, 1.0% and 2.0%). Subsequently, the precursor was transferred into a 100 mL Teflon-lined autoclave with deionized water added to fill 70% of the total volume. The autoclave was then sealed, maintained at 180 °C for 24 h and cooled to room temperature naturally. The resulted precipitates were washed several times with deionized water and absolute ethanol, respectively and then dried in an air oven at 60 °C for 10 h. Finally, the catalysts were ground into fine powders, stored in dark and denoted by PtClBiWO-x, where x represents the content of theoretical mass fraction of Pt (0.02, 0.1, 0.5, 1.0, 2.0 wt.%), and the sample without H2PtCl6 in the precursor was named as pure Bi2WO6. 2.2. Photocatalytic activity experiments To test the photocatalytic activities of the catalysts, photocatalytic degradation of toluene, a typical indoor pollutant, was conducted at room temperature using a 1.8 L batch photocatalytic reactor with a quartz window. The light source was a 300 W Xe lamp (PLS-SXE300, Beijing Trust-tech Co. Ltd., China) with two optical glass filters to obtain visible light (420 nm < k < 780 nm). For each experiment, 0.2 g catalyst was coated onto a glass dish (d = 12.5 cm) with ethanol. The dish was then dried at 60 °C for 30 min and cooled to room temperature before it was put into the photo-reactor. Before illumination, appropriate amount of toluene vapor was injected into the sealed reactor, detected by GC-FID (FULI 9790, China) to make sure that the absorption equilibrium was reached and the initial concentration was about 25 ppm. During photocatalytic reaction, the concentration of toluene was measured every 5 min. The photocatalytic oxidation of toluene is a pseudo-first-order reaction and its kinetics can be expressed as follows [35]: ln(C0/ C) = kt, where k is apparent reaction rate constant, C0 and C are the initial concentration and the reaction concentration of toluene, respectively. The photocatalytic activities of the catalysts can be quantitatively assessed by comparing their apparent reaction rate constants (k).
with a fluorospectrophotometer (PL: RAMANLOG 6, USA) using 430 nm lasers as excitation source. 3. Results and discussion 3.1. Phase and structure Fig. 1 shows the XRD patterns of pure Bi2WO6 and PtClBiWO-x. The pattern of each prepared sample could be readily indexed to the XRD pattern of orthorhombic phase Bi2WO6 (ICDD PDF#390256), and no diffraction peak of any other crystalline phase was observed except for PtClBiWO-2.0. The XRD pattern of PtClBiWO2.0 catalyst showed another four recognizable diffraction peaks of BiOCl (ICDD PDF#06-0249), respectively standing for the crystal plane of (0 0 1) that was dominant, (0 0 2), (1 0 1) and (0 0 3) of BiOCl crystal (Fig. 1b). No apparent diffraction peaks of Pt related substances were observed for all the samples. From Fig. 1a, it was notable that the diffraction peaks of catalyst with more H2PtCl6 addition were lower and wider, suggesting poorer crystallinity and smaller average size. Consistent with the XRD analysis result, TEM images (Fig. 2a and b) of pure Bi2WO6 and PtClBiWO-1.0 showed that pure Bi2WO6 was consisted of 50 nm square nanoplates with clear edges while PtClBi2WO61.0 was smaller gathered irregular nanosheets without distinct brims. It has been reported that pH value in hydrothermal process played an important role in determining the properties of Bi2WO6 [36,37]. However, during the formation of BiONO3 and HNO3 from Bi(NO3)35H2O and water, the initial pH of the suspension was 1–2 (measured by pH test strips), which was little changed by the additional H+ from H2PtCl6. Thus, the pH could not be the key factor
2.3. Characterization of photocatalysts The crystal phase and composition of as-prepared catalysts were analyzed by X-ray powder diffraction with Cu Ka irradiation (XRD: model D/max RA, Japan). The accelerating voltage and applied current were 40 kV and 150 mA, respectively, the 2h ranging from 10° to 80°. The morphologies, microstructures and grain sizes of as-prepared catalysts were investigated by transmission electron microscopy (TEM: FEI TECNAI G2 F20 S-TWIN, USA). Energydispersive X-ray analysis (EDX) in TEM was also taken for chemical analysis of the catalyst. X-ray photoelectron spectroscopy with Al Ka X-rays (ht = 1486.6 eV) irradiation operated at 150 W (XPS: Thermo ESCALAB 250, USA) was used to examine the surface properties of the catalysts. The binding energies obtained in XPS analysis are corrected for specimen charging by reference to a C1s value of 284.78 eV. The nitrogen adsorption–desorption isotherms were determined by BET method (BET-BJH: ASSP 2020, USA), from which the surface area were calculated by BJH method. The UV– Vis diffuse reflection spectra were obtained with a scan UV–Vis spectrophotometer (UV–Vis DRS: TU-1901, China) equipped with an integrating sphere assembly (BaSO4 as reflectance sample). The spectra were recorded at room temperature in air ranging from 200 to 800 nm. The photoluminescence spectra were measured
Fig. 1. XRD patterns of pure Bi2WO6 and PtClBiWO-x (a) and XRD pattern of PtClBiWO-2.0 in close up (b).
Z. Sun et al. / Journal of Colloid and Interface Science 412 (2013) 31–38
33
Fig. 2. TEM image (a) and HRTEM image (c) of pure Bi2WO6; TEM photograph (b), HRTEM image (d), HRTEM HAADF-STEM image (e) and EDX spectrum (f) of PtClBiWO-1.0. Insert in (c): FFT analysis of the selected area of pure Bi2WO6; Insert in (d): FFT analysis of the selected area of PtClBiWO-1.0. Inset in (e): EDX elemental mapping result of Pt in PtClBiWO-1.0. Note: Cu and C in EDX result (Fig. 2f) is from the copper grid with carbon film which is used to support samples during TEM detecting process.
that affected the catalysts’ morphology. The change in crystallinity and size of Bi2WO6 particle should mainly be due to the growth 2 environment with PtCl5 ; PtCl6 and Cl as the forming process of Bi2WO6 nanoplates was very sensitive to impurity ions [15]. The existence of these ions in the synthesis system not only inhibited the formation of nanosheets but also brought more defects in the lattice. Fig. 2c was the lateral view of pure Bi2WO6 under high resolution transmission electron microscope (HRTEM). By making FFT image (Fig. 2c insert) of the selected section and measuring the distance, lattice spacing of facet (0 4 0) and (0 2 0) could be recognized. This suggested that pure Bi2WO6 preferred to grow along {0 1 0} plane into nanoplates. TEM photograph of PtClBiWO-1.0 (Fig. 2b) showed shaggy irregular laminar morphology without square. A typical HRTEM image (Fig. 2d) of PtClBiWO-1.0 showed lattice spacing of crystal plane (1 3 1), (1 1 1) and (1 3 3) of Bi2WO6. However, no lattice of BiOCl could be found in PtClBiWO-1.0 by HRTEM. The reason might be that limited amount of Cl in Bi2WO6 was unable to form integrate BiOCl crystal.
Also, no lattices of Pt related substances (such as Pt, PtCl2, PtCl4, or PtO2) were investigated in the HRTEM images as XRD analysis result showed. However, EDX spectrum of PtClBiWO-1.0 (Fig. 2f) showed both Cl and Pt element which was evenly distributed (Fig. 2e). Also considering the small amount of Pt species introduced in total, it was reasonable that the size of Pt-related salt was too slight to be detected by TEM and XRD [33,38]. 3.2. Analysis of chemical state To know more about the chemical states of the elements composing the as-synthesized samples, XPS measurements were performed on pure Bi2WO6 and PtClBiWO-1.0 typically. The corresponding XPS spectrum of Bi4f, W4f, and O1s are shown in Fig. 3a–c. For pure Bi2WO6, the characteristic peaks at 159.20 eV and 164.50 eV were ascribed to Bi4f5/2 and Bi4f7/2 from Bi3+ in the lattice [25] and the binding energy of W4f5/2 and W4f7/2 at 37.60 eV and 35.45 eV respectively were corresponded to W6+. In the XPS spectrum of PtClBiO-1.0, the binding energy of Bi4f5/2
34
Z. Sun et al. / Journal of Colloid and Interface Science 412 (2013) 31–38
and Bi4f7/2 increased by 0.05 eV while that of W4f5/2 and W4f7/2 decreased by 0.1 eV. That suggested that the chemical environment surrounding Bi and W had changed [19], which was possibly influenced by Cl in the lattice. Fig. 3c shows the O1s XPS spectrum of pure Bi2WO6 and PtClBiWO-1.0. By fitting the O1s spectra of both pure Bi2WO6 and PtClBiWO-1.0, three peaks could be obtained, corresponding to three forms of oxygen [34,39]: lattice oxygen, hydroxyl groups (OH) and chemisorbed water. The percentages (ri) of the three oxygen species were presented in Table 1. It could be seen that the ri (%) of hydroxyl groups and chemisorbed water in PtClBiWO-1.0 was obviously higher than that in pure Bi2WO6. Hydroxyl groups and chemisorbed water could act as hole scavengers in photocatalytic
process, and produce active species OH, which was believed to be the main oxidant in photocatalytic oxidation reactions [40,41]. Fig. 3d and e are XPS spectrum of Pt4f and Cl2p for PtClBiWO1.0. In the Pt4f spectra appeared two different types of doublet peaks (4f5/2 and 4f7/2): 78.22 and 74.92 eV for Pt(IV) and 75.9 and 72.52 eV for Pt(II). However, since no reducing agent existed in this hydrothermal process, Pt(II) was not supposed to form. Some reports have mentioned that the Pt(IV) would be reduced to Pt(II) species by X-ray during XPS measuring process [35,42]. Thus it was very likely that the Pt(II) shown here was derived from Pt(IV). As shown in Fig. 3e, the Cl2p spectra of PtClBiWO-1.0 possessed two feature peaks at 198.2 and 199.7 eV, which could be ascribed to Cl in the sample [24].
Fig. 3. XPS spectra forBi4f (a), W4f (b), O1s (c), Pt4f (d), Cl2p (e) and VB-XPS (f) for pure Bi2WO6 and PtClBiWO-1.0.
Z. Sun et al. / Journal of Colloid and Interface Science 412 (2013) 31–38 Table 1 Curve-fitting result of O1s XPS spectra. Sample
Pure Bi2WO6
PtClBiWO1.0 a b
O1s
a Eb (eV) b ri (%) Eb (eV) ri (%)
Lattice oxygen
Hydroxyl groups
Adsorbed water
530.11
531.13
532.18
83.26 530.11 71.43
11.64 531.24 21.31
5.10 532.39 7.26
Eb: Binding energy of corresponding O. ri: The atomic ratio of each oxygen species to the total amount of oxygen.
The atomic content of different elements in PtClBiWO-1.0 detected by XPS measurement and that analyzed by EDX mapping were listed in Table 2, excluding the content of C or Cu from extrinsic impurities. The atomic content result from EDX analysis showed the total atomic content of different elements while that from XPS only indicated the surface atomic content. EDX atomic ratio of Pt in PtClBiWO-1.0 was 0.2 at.% (equal to 0.4 wt.%), which was less than the theoretical content of Pt in the precursor (1.0 wt.%) because of the loss of Pt in the solution. By contrast, surface Pt atomic content of PtClBiWO-1.0 by XPS detection was 0.28 at.% (>0.2 at.%), suggesting that Pt species was mainly dispersed on Bi2WO6 surface. From the table, while the ratio of Bi to W in PtClBiWO-1.0 was equal to 2.37, a bit larger than the theoretical value in Bi2WO6, the atomic ratio of Cl to Pt was about 8.33, much larger than that of PtCl4 or H2PtCl6. This implied that Cl in the sample not only interacted with Pt(IV) but also with other ions such as [Bi2O2]2+ or Na+ if any. XPS and EDX analysis of Na1s was also conducted but no relevant peaks were found, showing that the extra Cl could not be in the form of NaCl salt. Thus the residual Cl should be contacted with [Bi2O2]2+ forming Cl doped Pt(IV)/Bi2WO6, which was confirmed by the appearance of BiOCl in sample PtClBiWO-2.0 (Fig. 1b). The VB spectra of pure Bi2WO6 and PtClBiWO-1.0 are given in Fig. 3f. The dispersed electronic states below 3.0 eV and above the valence band edge were observed for both samples. For pure Bi2WO6, these electronic states could be attributed to the inherent valence band structure of Bi2WO6, consisting of Bi6s + O2p hybrid orbitals [4,5]. Comparatively, the electronic states of PtClBiWO1.0 dispersed more widely above 1.0 eV, possibly due to the influence of Cl2p to the orbitals of Bi6s and O2p [35,43].
35
little red shift in the region of 420–500 nm. But for PtClBiWO-0.5 and PtClBiWO-1.0, the red shift was obvious and the absorption curve slope edge moved towards the right. More intensive and broad absorption of visible light was observed for PtClBiWO-2.0 and the absorption edge moved to 800 nm. The enhancement of absorption could be ascribed to both doped Cl ion and surface Pt(IV). For ex-situ PtCl4 modified Bi2WO6 reported by Duan et al. [24], though the absorption curve tail of modified catalysts shifted upward compared to the unmodified, the slopes of catalysts with different concentration of PtCl4 showed no difference. Xu et al. [43] had reported that Cl-doped TiO2 would show a red movement of the absorption line compared with undoped TiO2. We supposed that the upward movement of the absorption curve tail was mainly due to the merged absorption of Pt(IV) salt and Bi2WO6 while the shift of the curve slope was primarily from the shortened band gap of Bi2WO6 effected by Cl doping [44], which was consistent with the VB XPS results. In addition, though BiOCl was formed in PtClBiWO-2.0, it could not be the reason for the shift of absorption range because the band gap of BiOCl was about 3.5 eV, much larger than that of Bi2WO6 (2.7 eV) and unable to absorb visible light [45–47]. Photoluminenscence (PL) spectroscopy is a common way to study the recombination rate of photo-generated electrons and holes on photocatalysts. A stronger PL spectrum intensity reflects a higher recombination rate [48]. The PL spectrum of pure Bi2WO6, PtClBiWO-0.1, PtClBiWO-0.5, PtClBiWO-1.0 and PtClBiWO-2.0 using 430 nm light as the excitation light are demonstrated in Fig. 5. It showed that all the tested catalysts could absorb 430 nm light and generate photo-excited electrons and holes, which would recombine and give out lights with lower energy, forming the PL singles. The tested four catalysts showed apparent difference in the PL peak position: 515 nm for PtClBiWO-2.0, 493 nm for PtClBiWO-1.0, 490 nm for PtClBiWO-0.5, 487 nm for PtClBiWO-0.1 and 480 nm for pure Bi2WO6. This trend met with the shift trend of absorption curve in UV–Vis DRS test, reflecting the gradually shorten band gaps affected chiefly by Cl doping. The intensity of the PL signals indicated that the sequence of the ability to stabilize photo-generated electrons and holes of the tested catalysts was: PtClBiWO-1.0 > PtClBiWO-0.5 > PtClBiWO2.0 > PtClBiWO-0.1 > pure Bi2WO6. Both PtCl4 and Bi2WO6 could be excited by 430 nm light. Local excitation of the Pt(IV)-Cl by visible light afforded charge transfer-ligand-to-metal, yielding Pt(III) and a chlorine atom [30]. The labile Pt (III) intermediate would rapidly transfer an electron to the CB of Bi2WO6, while the chlorine atom would abstract an electron from the Bi2WO6 lattice. Thus Pt
3.3. Optical property Fig. 4 displays UV–Vis diffuse reflection spectra of pure Bi2WO6 and PtClBiWO-x (x = 0.02, 0.1, 0.5, 1.0, 2.0). As expected, the photoabsorption region of pure Bi2WO6 ranged from UV light to visible light shorter than 450 nm. Compared with pure Bi2WO6, a tendency of red shifts were observed in the other samples, suggesting that the visible light response of Bi2WO6 was improved by the addition of H2PtCl6. PtClBiWO-0.05 and PtClBiWO-0.1 showed a Table 2 Atomic content of elements in PtClBiWO-1.0 analyzed by EDX and XPS. Element
Pt4f Bi4f W4f O1s Cl2p Na1s
PtClBiWO-1.0 EDX atomic %
XPS atomic %
0.2 26.9 16.2 53.7 2.9 0
0.2825 22.7283 9.5811 65.0541 2.3540 0
Fig. 4. UV–Vis diffuse reflection spectra of pure Bi2WO6 and PtClBiWO-x (x = 0.02, 0.1, 0.5, 1.0, 2.0).
36
Z. Sun et al. / Journal of Colloid and Interface Science 412 (2013) 31–38
Fig. 5. Photoluminescence spectra of pure Bi2WO6, PtClBiWO-x (x = 0.1, 0.5, 1.0 and 2.0) exited by wavelength of 430 nm visible light.
(IV)-Cl helped to stabilize the photo-excited electrons on Bi2WO6, which might be the primary reason why the PL peak intensity of pure Bi2WO6, PtClBiWO-0.1, PtClBiWO-0.5 and PtClBiWO-1.0 reduced in turn. However, PtClBiWO-2.0 with the largest amount of Pt(IV) on the catalyst surface did not show the lowest e-h+ recombination rate. This could be explained by that PtClBiWO2.0 was the sample with the most intrinsic lattice defects according to the poorest crystallization form XRD result and with excessive Pt(IV) (0.48 at.% by XPS analysis) on the surface of Bi2WO6 sample. Both intrinsic lattice defect and excessive Pt(IV) could serve as photo-generated e-h+ recombination center.
3.4. Photocatalytic activities and stability Visible-light (k > 420 nm) photocatalytic degradation of toluene was carried out to test the photocatalytic activities of the synthesized catalysts. Fig. 6 shows change of the relative concentration of toluene with irradiation time and the apparent reaction rate constants of photocatalytic decomposition of toluene over as-prepared samples. The blank experiment in the absence of photocatalyst indicated that the self-degradation of toluene in air under visible-light irradiation was negligible. Though pure Bi2WO6 had already shown a good toluene degradation rate (decomposing 25 ppm toluene in 85 min), all PtClBiWO-x samples exhibited better activities. PtClBiWO-1.0 with 1.0 wt.% Pt showed the best performance and its apparent reaction rate constant (k, 0.152 min1) was 2.82 times higher than that of pure Bi2WO6 (0.054 min1). BET surface areas of pure Bi2WO6 and PtClBiWO-x (x = 0.1, 1.0, 2.0) was listed in Table 3 which indicated that PtClBiWO-1.0 had the largest surface area. According to the former measurements of the catalysts prepared, PtClBiWO-1.0 also had the lowest e-h+ recombination possibility, shorter band gap by the doping of Cl and smaller particle size compared with pure Bi2WO6. All these contributed to the best performance of PtClBiWO-1.0. Subsequently, the photochemical stability of PtClBiWO-1.0 was tested by repeating photo-degradation of gaseous toluene experiment. The reactor with catalyst-covered dish was open to air after each run of photocatalytic reaction for 60 min, and then dried at 80 °C oven for 20 min to remove any volatile by-products absorbed on the surface of the photocatalysts. After that the reactor was cooled to room temperature naturally, appropriate amount of toluene was injected into it with a micro-springe for a new round of reaction. The initial concentration of toluene was also controlled at 25 ppm. The photocatalytic activity of PtClBiWO-1.0 in four
Fig. 6. Photocatalytic toluene degradation activity of prepared catalysts under visible light (k > 420 nm): Change of the relative concentration of toluene with irradiation time (a); the apparent reaction rate constant of photocatalytic decomposition of toluene over different samples (b).
Table 3 BET surface areas of pure Bi2WO6 and PtClBiWO-x (x = 0.1, 1.0, 2.0). Sample
Pure Bi2WO6
PtClBiWO0.1
PtClBiWO1.0
PtClBiWO2.0
Surface area (m2/g)
19.9
20.2
32.4
29.9
continuous tests is shown in Fig. 7. A little decrease in photo-catalytic performance was observed after each round which might due to the chemical change of Pt(IV) and the residual by-product on surface. Yet 85% of toluene could be degraded after four recycles in 60 min, exhibiting a good photochemical stability. 3.5. Brief discussion about the formation process The formation process of Cl doped Pt(IV)/Bi2WO6 photocatalyst was briefly discussed here to give some inspiration for facile one-pot hydrothermal synthesis of well-performed photocatalysts. TEM image of pure Bi2WO6 (Fig. 2a) showed that the Bi2WO6 square nanoplates were formed and (0 1 0) was the dominant facet exposed for the catalyst. It was pointed out that (2 0 0) and (0 0 2) facets had much higher chemical potential compared to other facets and were very sensitive to the surrounding growth conditions [15]. As long as the foreign force input was enough (180 °C hydrothermal condition in this system), they would grow every fast, leading to the formation of (0 1 0) dominant nanosheets. However,
37
Z. Sun et al. / Journal of Colloid and Interface Science 412 (2013) 31–38
0
½PtCl5 ðH2 OÞ þ H2 O $ ½PtCl4 ðH2 OÞ2 þ Cl
Fig. 7. Circled photocatalytic degradation of toluene experiments with PtClBiWO1.0.
when H2PtCl6 was added into the system, the particles failed to grow into square nanoplates. Instead, irregular-shape nanosheets with Pt (IV) and extra Cl were formed. Based on the above results, the forming process of pure Bi2WO6 nanoplates and Cl doped Pt(IV)/Bi2WO6 photocatalyst was schematically deduced (Fig. 8) and discussed briefly here.
BiðNO3 Þ3 þ H2 O $ BiONO3 þ Hþ þ NO3
ð1Þ
2BiONO3 þ WO2 4 $ Bi2 WO6 þ 2NO3
ð2Þ
BiONO3 þ Cl $ BiOCl þ NO3
ð3Þ
At first, Bi(NO3)35H2O was dropped into the deionized water. Though the salt was not adequately soluble without enough H+, it would gradually transfer into BiONO3 and HNO3 with Bi3+ and [Bi2O2]2+ as intermediates, as formula (1) shows [49]. In this stage, the resulted suspension was acidic (pH 2). When Na2WO4 solu2þ tion was added, the ðBi2 O2 Þ would be released from BiONO3 (formula 1) and interconnected with WO2 to form Bi2WO6 crystal 4 (formula 2) which would reform to nanosheets under hydrothermal conditions due to the intrinsic anisotropic growth habit of Bi2WO6 [50]. If there was Cl in the solution, BiOCl was tended to be formed through formula (3) [51]. [PtCl6]2 in aqueous solution could be hydrolyzed to form 2 PtðOHÞx Cl6x (x = 1–5), depending on the pH [30]. In an acidic solution, the hydrolyzing equilibrations were as follows:
½PtCl6
2
þ H2O $ ½PtCl5 ðH2 OÞ þ Cl
ð4Þ
ð5Þ
As H2PtCl6 was added into the precursor suspension with Bi2WO6 nuclei, it was highly possible for Cl to contact with Bi2 O2þ 2 2þ and tend to form Cl ½Bi2 O2 ionic bond. It was reported by Xie et al. [52] that BiOCl favored to keep crystallinity in acidic environment and the Bi2WO6 was the main crystal formed with higher pH value in hydrothermal process of solution with BiCl3 and (NH4)2WO4. Thus it was referred that Cl in our experimental condition 2þ was more intensively bonded with ½Bi2 O2 than WO2 4 at higher temperature and pressure, which was bound to influence, even interrupt, the crystal generating process of Bi2WO4 nanosheets depending on the concentration of Cl. Also, the attraction of Bi2 O2þ to Cl promoted the positive direction reaction route of 2 Eqs. (4) and (5), eventually leading to the formation of PtCl4(H2O)2 that was prone to absorb ions and move to the surface of WO2 4 layer, as depicted in Fig. 8. While the temperature and pressure went up during hydrothermal treatment, the foreign energy forced all of the reactions to speed up, resulting in poor-crystalized Bi2WO6 irregular nanosheets with intrinsic crystal defects, surface Pt(IV)-Cl and doped Cl. From the former characterizations, we can see that the addition of H2PtCl6 in the hydrothermal process not only effected the morphology of resulted Bi2WO6, reducing the particle size and decreasing the crystallinity, but also modified Bi2WO6 with doped Cl and Pt(IV) on the catalyst surface, which had great impact on the optical properties and photocatalytic activities of the catalysts. But the amount of H2PtCl6 should be controlled precisely to achieve the best-performed sample. Too little addition of H2PtCl6 could not improve the photocatalytic activity much, while the excessive loading of H2PtCl6 would lead the formation of layer-structure BiOCl. A recent DFT study concluded that an intrinsic interface between BiOCl and Bi2WO6 failed to enhance the charge-carrier separation due to the improper band alignment between these two materials [47]. Also, extensive loading of Pt(IV)-Cl on Bi2WO6 would increase the photo-generated electron–hole recombination possibility as the PL results showed.
4. Conclusion In summary, a facile one-pot hydrothermal method was successfully applied in preparing Cl doped Pt(IV)/Bi2WO6 photocatalysts. By adjusting the content of H2PtCl6 in the precursor, catalysts with different content of Pt(IV) and Cl were achieved. All the modified catalysts had improved visible-light photocatalytic activity compared to pure Bi2WO6. PtClBiWO-1.0 with theoretical 1.0 wt.% Pt exhibited the best photocatalytic performance as it possessed the largest surface area (32.4 m2/g); appropriate amount
Fig. 8. Schematic forming process of pure Bi2WO6 nanoplates and Cl doped Pt(IV)/Bi2WO6.
38
Z. Sun et al. / Journal of Colloid and Interface Science 412 (2013) 31–38
of uniformly dispersed Pt(IV) on the surface and doped Cl, leading to superior capacity to absorb reactant, enhanced absorption in visible-light range and advanced stability of photo-excited electrons and holes. Synthesis of good-preformed Cl-doped Pt(IV)/Bi2WO6 by such a facile approach provided new insight for further high-efficiency photocatalyst preparation in an easier way. And the resulted catalysts showed promise for practical environmental pollution remediation. Acknowledgments We acknowledge the financial supports of the National High Technology Research and Development Program (863 Program) of China (No. 2010AA064905), Changjiang Scholar Incentive Program (Ministry of Education, China, 2009), Natural Science Foundation of Zhejiang Province (No. Z5100116), National Natural Science Foundation of China (No. 50808156), Foundation of Zhejiang Educational Committee (No. Y201018656) and China Postdoctoral Science Foundation (No. 20100480088). References [1] P. Boullay, G. Trolliard, D. Mercurio, J.M. Perez-Mato, L. Elcoro, J. Solid State Chem. 164 (2002) 252. [2] W. Sugimoto, M. Shirata, Y. Sugahara, K. Kuroda, J. Am. Chem. Soc. 121 (1999) 11601. [3] N. Kim, R.N. Vannier, C.P. Grey, Chem. Mater. 17 (2005) 1952. [4] J.W. Tang, Z.G. Zou, J.H. Ye, Catal. Lett. 92 (2004) 53. [5] H.B. Fu, C.S. Pan, W.Q. Yao, Y.F. Zhu, J. Phys. Chem. B 109 (2005) 22432. [6] J.G. Yu, J.F. Xiong, B. Cheng, Y. Yu, J.B. Wang, J. Solid State Chem. 178 (2005) 1968. [7] A. Kudo, S. Hijii, Chem. Lett. (1999) 1103. [8] H.B. Fu, S.C. Zhang, T.G. Xu, Y.F. Zhu, J.M. Chen, Environ. Sci. Technol. 42 (2008) 2085. [9] L. Ge, J. Liu, Appl. Catal. B-Environ. 105 (2011) 289. [10] S. Guo, X. Li, H. Wang, F. Dong, Z. Wu, J. Colloid Interface Sci. 369 (2012) 373. [11] A.P. Finlayson, V.N. Tsaneva, L. Lyons, M. Clark, B.A. Glowacki, Phys. Status Solidi A 203 (2006) 327. [12] L. Zhang, C. Baumanis, L. Robben, T. Kandiel, D. Bahnemann, Small 7 (2011) 2714. [13] Y. Zhou, Z. Tian, Z. Zhao, Q. Liu, J. Kou, X. Chen, J. Gao, S. Yan, Z. Zou, ACS Appl. Mater. Interfaces 3 (2011) 3594. [14] H. Cheng, B. Huang, Y. Liu, Z. Wang, X. Qin, X. Zhang, Y. Dai, Chem. Commun. 48 (2012) 9729. [15] C. Zhang, Y. Zhu, Chem. Mater. 17 (2005) 3537.
[16] H. Huang, H. Chen, Y. Xia, X. Tao, Y. Gan, X. Weng, W. Zhang, J. Colloid Interface Sci. 370 (2012) 132. [17] X.C. Song, Y.F. Zheng, R. Ma, Y.Y. Zhang, H.Y. Yin, J. Hazard. Mater. 192 (2011) 186. [18] Y. Liu, W.M. Wang, Z.Y. Fu, H. Wang, Y.C. Wang, J.Y. Zhang, Mater. Sci. Eng. BAdv. Funct. Solid-State Mater. 176 (2011) 1264. [19] R. Shi, G.L. Huang, J. Lin, Y.F. Zhu, J. Phys. Chem. C 113 (2009) 19633. [20] L.S. Zhang, K.H. Wong, Z.G. Chen, J.C. Yu, J.C. Zhao, C. Hu, C.Y. Chan, P.K. Wong, Appl. Catal. A-Gen. 363 (2009) 221. [21] J.H. Xu, W.Z. Wang, E.P. Gao, J. Ren, L. Wang, Catal. Commun. 12 (2011) 834. [22] S.B. Zhu, T.G. Xu, H.B. Fu, J.C. Zhao, Y.F. Zhu, Environ. Sci. Technol. 41 (2007) 6234. [23] M. Ge, Y.F. Li, L. Liu, Z. Zhou, W. Chen, J. Phys. Chem. C 115 (2011) 5220. [24] F. Duan, Y. Zheng, M.Q. Chen, Appl. Surf. Sci. 257 (2011) 1972. [25] Q. Xiao, J. Zhang, C. Xiao, X.K. Tan, Catal. Commun. 9 (2008) 1247. [26] Y. Zheng, K.L. Lv, X.F. Li, K.J. Deng, J. Sun, L.Q. Chen, L.Z. Cui, D.Y. Du, Chem. Eng. Technol. 34 (2011) 1630. [27] Q.C. Xu, D.V. Wellia, Y.H. Ng, R. Amal, T.T.Y. Tan, J. Phys. Chem. C 115 (2011) 7419. [28] Z.-Q. Li, X.-T. Chen, Z.-L. Xue, J. Colloid Interface Sci. 394 (2013) 69. [29] Y. Guo, G. Zhang, H. Gan, J. Colloid Interface Sci. 369 (2012) 323. [30] L. Zang, C. Lange, I. Abraham, S. Storck, W.F. Maier, H. Kisch, J. Phys. Chem. B 102 (1998) 10765. [31] W. Macyk, H. Kisch, Chem. – A Eur. J. 7 (2001) 1862. [32] G.Q. Li, D.F. Wang, Z.G. Zou, J.H. Ye, J. Phys. Chem. C 112 (2008) 20329. [33] L. Ge, J. Mol. Catal. A: Chem. 282 (2008) 62. [34] F. Dong, S. Guo, H. Wang, X. Li, Z. Wu, J. Phys. Chem. C 115 (2011) 13285. [35] F. Dong, H.Q. Wang, G. Sen, Z.B. Wu, S.C. Lee, J. Hazard. Mater. 187 (2011) 509. [36] C. Wang, H. Zhang, F. Li, L. Zhu, Environ. Sci. Technol. 44 (2010) 6843. [37] P. Dumrongrojthanath, T. Thongtem, A. Phuruangrat, S. Thongtem, Superlattice Microstruct. 54 (2013) 71. [38] F. Shi, M.K. Tse, M.-M. Pohl, A. Brückner, S. Zhang, M. Beller, Angew. Chem. Int. Ed. 46 (2007) 8866. [39] J.G. Yu, G.H. Wang, B. Cheng, M.H. Zhou, Appl. Catal. B-Environ. 69 (2007) 171. [40] T. Hirakawa, K. Yawata, Y. Nosaka, Appl. Catal. A 325 (2007) 105. [41] A.L. Linsebigler, G.Q. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735. [42] J.S. Lee, W.Y. Choi, J. Phys. Chem. B 109 (2005) 7399. [43] H. Xu, Z. Zheng, L. Zhang, H. Zhang, F. Deng, J. Solid State Chem. 181 (2008) 2516. [44] G. Liu, C. Sun, L. Wang, S.C. Smith, G.Q. Lu, H.-M. Cheng, J. Mater. Chem. 21 (2011) 14672. [45] J. Jiang, K. Zhao, X. Xiao, L. Zhang, J. Am. Chem. Soc. 134 (2012) 4473. [46] S. Shamaila, A.K.L. Sajjad, F. Chen, J. Zhang, J. Colloid Interface Sci. 356 (2011) 465. [47] W. Wang, W. Yang, R. Chen, X. Duan, Y. Tian, D. Zeng, B. Shan, Phys. Chem. Chem. Phys. (2011). [48] J.C. Yu, J.G. Yu, W.K. Ho, Z.T. Jiang, L.Z. Zhang, Chem. Mater. 14 (2002) 3808. [49] H. Gnayem, Y. Sasson, ACS Catal. 3 (2013) 186. [50] D. Ma, S. Huang, W. Chen, S. Hu, F. Shi, K. Fan, J. Phys. Chem. C 113 (2009) 4369. [51] F. Dong, Y. Sun, M. Fu, Z. Wu, S.C. Lee, J. Hazard. Mater. 219–220 (2012) 26. [52] H. Xie, D. Shen, X. Wang, G. Shen, Mater. Chem. Phys. 103 (2007) 334.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
One-step synthesis of Cl-doped Pt(IV)/Bi2WO6 with advanced visible-light photocatalytic activity for toluene degradation in air Zhuxing Sun a,b, Xiaofang Li a,b, Sen Guo a,b,c,⇑, Haiqiang Wang a,b, Zhongbiao Wu a,b,⇑ a
Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, PR China Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue Gas Pollution Control, Hangzhou 311202, PR China c Suzhou Industrial Technology Research Institute of Zhejiang University, Suzhou 215163, PR China b
a r t i c l e
i n f o
Article history: Received 17 May 2013 Accepted 6 September 2013 Available online 17 September 2013 Keywords: One-step synthesis Cl doped Pt(IV) Bi2WO6 Visible light Photo-catalysis Toluene degradation
a b s t r a c t In this study, Cl doped Pt(IV)/Bi2WO6 photocatalyst was successfully synthesized via a one-step hydrothermal route by adding H2PtCl6 immediately in the reaction system. X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption– desorption isotherms and photoluminescence spectra (PL) were utilized to analyze the morphology, chemical states, surface area and optical properties of as-prepared catalysts. The addition of H2PtCl6 to the reaction system inhibited the growth of Bi2WO6 square nanoplates but facilitated the generation of irregular Bi2WO6 nanosheets with enlarged surface area and extended visible light absorption. In the resulted sample, it was observed that Pt(IV) was uniformly dispersed on the surface of Bi2WO6 irregular nanosheets and Cl other than that from PtCl4 was doped into the sample by forming ionic bond with [Bi2O2]2+. Compared with pure Bi2WO6 nanosheets, Cl doped Pt(IV)/Bi2WO6 showed enhanced activity in photocatalytic decomposition of toluene under visible light irradiation (420 nm < k < 780 nm). The catalyst with theoretical 1.0 wt.% Pt showed the best performance, which degraded 25 ppm toluene thoroughly in 35 min. Finally, the forming mechanism of Cl doped Pt(IV)/Bi2WO6 in the one-step hydrothermal process was proposed. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Bi2WO6 is the simplest member of Aurivillius family with a general formula of Bi2An1BnO3n+3 (A = Ca, Sr, Ba, Pb, Bi, Na, K and B = Ti, Nb, Ta, Mo, W, Fe) [1,2]. The structure of Bi2WO6 crystal is 2nþ orthorhombic, constructed by alternating ðBi2 O2 Þn ) layers and 2n perovskite-like ðWO4 Þn layers [3]. The conduction band of Bi2WO6 is composed of W5d orbitals and the valence band is hybrid orbitals of O2p and Bi6s [4,5]. Bi2WO6 can absorb visible light and shows great promise as a visible-light photocatalyst [6,7]. Recently, it has been successfully used for photo-degradation of organic pollutants [8–10], photocatalytic splitting of water [11,12] and photo-reduction of CO2 to CH4 [13,14]. Lots of efforts have been paid to improve the photocatalytic activity of pure Bi2WO6, especially to prolong the life span of photo-generated electron– hole pairs on the catalyst. Common methods include morphology controlling [15,16], ion doping [8,10,17–19], metal deposition [20,21], semiconductors coupling [9,22–28] and embedding on large-surface-area materials [29].
It is known that photo-sensitizing with noble metal complex (e.g., PtCl4 and H2PtCl6) is an effective method for enhancing the visible light response of photocatalysts [24,30–33]. However, Pt or its related salts modified photocatalytic system were usually achieved by retreatment after the photocatalysts were synthesized, such as impregnating the prepared catalysts in H2PtCl6 solution following thermal treatment [24,32,33], through a sol–gel process [30] or girding with PtCl4 or H2PtCl6 [31]. Considering the former bulk catalysts preparation procedure, the traditional approaches always take two or more steps and thus are uneasy to scale up for practical application [34]. In the present study, we prepared Cl-doped Pt(IV)/Bi2WO6 with advanced photocatalytic performance through a one-step hydrothermal route. To the best of our knowledge, this was for the first time that the one-step hydrothermal approach was used to synthesize Pt related salt modified photocatalysts. Thanks to its convenience, the process can also be applied to photocatalytic system other than Bi2WO6. 2. Experimental 2.1. Preparation of photocatalysts
⇑ Corresponding authors. Address: Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, PR China. Fax: +86 571 87953088. E-mail addresses: [email protected] (S. Guo), [email protected] (Z. Wu). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.09.004
All the chemicals were of analytical purity and used without any further purification. Deionized water was used in all experi-
32
Z. Sun et al. / Journal of Colloid and Interface Science 412 (2013) 31–38
ments. Typically, 8 mmol Bi(NO3)35H2O and 4 mmol Na2WO4 were mixed with 30 ml water. Following 3 h vigorously stirring of the resulted white suspension, appropriate amount of H2PtCl6 solution were added and stirred for another hour. The amount of H2PtCl6 was controlled according to the designed mass ratio of Pt to Bi2WO6 (0%, 0.02%, 0.1%, 0.5%, 1.0% and 2.0%). Subsequently, the precursor was transferred into a 100 mL Teflon-lined autoclave with deionized water added to fill 70% of the total volume. The autoclave was then sealed, maintained at 180 °C for 24 h and cooled to room temperature naturally. The resulted precipitates were washed several times with deionized water and absolute ethanol, respectively and then dried in an air oven at 60 °C for 10 h. Finally, the catalysts were ground into fine powders, stored in dark and denoted by PtClBiWO-x, where x represents the content of theoretical mass fraction of Pt (0.02, 0.1, 0.5, 1.0, 2.0 wt.%), and the sample without H2PtCl6 in the precursor was named as pure Bi2WO6. 2.2. Photocatalytic activity experiments To test the photocatalytic activities of the catalysts, photocatalytic degradation of toluene, a typical indoor pollutant, was conducted at room temperature using a 1.8 L batch photocatalytic reactor with a quartz window. The light source was a 300 W Xe lamp (PLS-SXE300, Beijing Trust-tech Co. Ltd., China) with two optical glass filters to obtain visible light (420 nm < k < 780 nm). For each experiment, 0.2 g catalyst was coated onto a glass dish (d = 12.5 cm) with ethanol. The dish was then dried at 60 °C for 30 min and cooled to room temperature before it was put into the photo-reactor. Before illumination, appropriate amount of toluene vapor was injected into the sealed reactor, detected by GC-FID (FULI 9790, China) to make sure that the absorption equilibrium was reached and the initial concentration was about 25 ppm. During photocatalytic reaction, the concentration of toluene was measured every 5 min. The photocatalytic oxidation of toluene is a pseudo-first-order reaction and its kinetics can be expressed as follows [35]: ln(C0/ C) = kt, where k is apparent reaction rate constant, C0 and C are the initial concentration and the reaction concentration of toluene, respectively. The photocatalytic activities of the catalysts can be quantitatively assessed by comparing their apparent reaction rate constants (k).
with a fluorospectrophotometer (PL: RAMANLOG 6, USA) using 430 nm lasers as excitation source. 3. Results and discussion 3.1. Phase and structure Fig. 1 shows the XRD patterns of pure Bi2WO6 and PtClBiWO-x. The pattern of each prepared sample could be readily indexed to the XRD pattern of orthorhombic phase Bi2WO6 (ICDD PDF#390256), and no diffraction peak of any other crystalline phase was observed except for PtClBiWO-2.0. The XRD pattern of PtClBiWO2.0 catalyst showed another four recognizable diffraction peaks of BiOCl (ICDD PDF#06-0249), respectively standing for the crystal plane of (0 0 1) that was dominant, (0 0 2), (1 0 1) and (0 0 3) of BiOCl crystal (Fig. 1b). No apparent diffraction peaks of Pt related substances were observed for all the samples. From Fig. 1a, it was notable that the diffraction peaks of catalyst with more H2PtCl6 addition were lower and wider, suggesting poorer crystallinity and smaller average size. Consistent with the XRD analysis result, TEM images (Fig. 2a and b) of pure Bi2WO6 and PtClBiWO-1.0 showed that pure Bi2WO6 was consisted of 50 nm square nanoplates with clear edges while PtClBi2WO61.0 was smaller gathered irregular nanosheets without distinct brims. It has been reported that pH value in hydrothermal process played an important role in determining the properties of Bi2WO6 [36,37]. However, during the formation of BiONO3 and HNO3 from Bi(NO3)35H2O and water, the initial pH of the suspension was 1–2 (measured by pH test strips), which was little changed by the additional H+ from H2PtCl6. Thus, the pH could not be the key factor
2.3. Characterization of photocatalysts The crystal phase and composition of as-prepared catalysts were analyzed by X-ray powder diffraction with Cu Ka irradiation (XRD: model D/max RA, Japan). The accelerating voltage and applied current were 40 kV and 150 mA, respectively, the 2h ranging from 10° to 80°. The morphologies, microstructures and grain sizes of as-prepared catalysts were investigated by transmission electron microscopy (TEM: FEI TECNAI G2 F20 S-TWIN, USA). Energydispersive X-ray analysis (EDX) in TEM was also taken for chemical analysis of the catalyst. X-ray photoelectron spectroscopy with Al Ka X-rays (ht = 1486.6 eV) irradiation operated at 150 W (XPS: Thermo ESCALAB 250, USA) was used to examine the surface properties of the catalysts. The binding energies obtained in XPS analysis are corrected for specimen charging by reference to a C1s value of 284.78 eV. The nitrogen adsorption–desorption isotherms were determined by BET method (BET-BJH: ASSP 2020, USA), from which the surface area were calculated by BJH method. The UV– Vis diffuse reflection spectra were obtained with a scan UV–Vis spectrophotometer (UV–Vis DRS: TU-1901, China) equipped with an integrating sphere assembly (BaSO4 as reflectance sample). The spectra were recorded at room temperature in air ranging from 200 to 800 nm. The photoluminescence spectra were measured
Fig. 1. XRD patterns of pure Bi2WO6 and PtClBiWO-x (a) and XRD pattern of PtClBiWO-2.0 in close up (b).
Z. Sun et al. / Journal of Colloid and Interface Science 412 (2013) 31–38
33
Fig. 2. TEM image (a) and HRTEM image (c) of pure Bi2WO6; TEM photograph (b), HRTEM image (d), HRTEM HAADF-STEM image (e) and EDX spectrum (f) of PtClBiWO-1.0. Insert in (c): FFT analysis of the selected area of pure Bi2WO6; Insert in (d): FFT analysis of the selected area of PtClBiWO-1.0. Inset in (e): EDX elemental mapping result of Pt in PtClBiWO-1.0. Note: Cu and C in EDX result (Fig. 2f) is from the copper grid with carbon film which is used to support samples during TEM detecting process.
that affected the catalysts’ morphology. The change in crystallinity and size of Bi2WO6 particle should mainly be due to the growth 2 environment with PtCl5 ; PtCl6 and Cl as the forming process of Bi2WO6 nanoplates was very sensitive to impurity ions [15]. The existence of these ions in the synthesis system not only inhibited the formation of nanosheets but also brought more defects in the lattice. Fig. 2c was the lateral view of pure Bi2WO6 under high resolution transmission electron microscope (HRTEM). By making FFT image (Fig. 2c insert) of the selected section and measuring the distance, lattice spacing of facet (0 4 0) and (0 2 0) could be recognized. This suggested that pure Bi2WO6 preferred to grow along {0 1 0} plane into nanoplates. TEM photograph of PtClBiWO-1.0 (Fig. 2b) showed shaggy irregular laminar morphology without square. A typical HRTEM image (Fig. 2d) of PtClBiWO-1.0 showed lattice spacing of crystal plane (1 3 1), (1 1 1) and (1 3 3) of Bi2WO6. However, no lattice of BiOCl could be found in PtClBiWO-1.0 by HRTEM. The reason might be that limited amount of Cl in Bi2WO6 was unable to form integrate BiOCl crystal.
Also, no lattices of Pt related substances (such as Pt, PtCl2, PtCl4, or PtO2) were investigated in the HRTEM images as XRD analysis result showed. However, EDX spectrum of PtClBiWO-1.0 (Fig. 2f) showed both Cl and Pt element which was evenly distributed (Fig. 2e). Also considering the small amount of Pt species introduced in total, it was reasonable that the size of Pt-related salt was too slight to be detected by TEM and XRD [33,38]. 3.2. Analysis of chemical state To know more about the chemical states of the elements composing the as-synthesized samples, XPS measurements were performed on pure Bi2WO6 and PtClBiWO-1.0 typically. The corresponding XPS spectrum of Bi4f, W4f, and O1s are shown in Fig. 3a–c. For pure Bi2WO6, the characteristic peaks at 159.20 eV and 164.50 eV were ascribed to Bi4f5/2 and Bi4f7/2 from Bi3+ in the lattice [25] and the binding energy of W4f5/2 and W4f7/2 at 37.60 eV and 35.45 eV respectively were corresponded to W6+. In the XPS spectrum of PtClBiO-1.0, the binding energy of Bi4f5/2
34
Z. Sun et al. / Journal of Colloid and Interface Science 412 (2013) 31–38
and Bi4f7/2 increased by 0.05 eV while that of W4f5/2 and W4f7/2 decreased by 0.1 eV. That suggested that the chemical environment surrounding Bi and W had changed [19], which was possibly influenced by Cl in the lattice. Fig. 3c shows the O1s XPS spectrum of pure Bi2WO6 and PtClBiWO-1.0. By fitting the O1s spectra of both pure Bi2WO6 and PtClBiWO-1.0, three peaks could be obtained, corresponding to three forms of oxygen [34,39]: lattice oxygen, hydroxyl groups (OH) and chemisorbed water. The percentages (ri) of the three oxygen species were presented in Table 1. It could be seen that the ri (%) of hydroxyl groups and chemisorbed water in PtClBiWO-1.0 was obviously higher than that in pure Bi2WO6. Hydroxyl groups and chemisorbed water could act as hole scavengers in photocatalytic
process, and produce active species OH, which was believed to be the main oxidant in photocatalytic oxidation reactions [40,41]. Fig. 3d and e are XPS spectrum of Pt4f and Cl2p for PtClBiWO1.0. In the Pt4f spectra appeared two different types of doublet peaks (4f5/2 and 4f7/2): 78.22 and 74.92 eV for Pt(IV) and 75.9 and 72.52 eV for Pt(II). However, since no reducing agent existed in this hydrothermal process, Pt(II) was not supposed to form. Some reports have mentioned that the Pt(IV) would be reduced to Pt(II) species by X-ray during XPS measuring process [35,42]. Thus it was very likely that the Pt(II) shown here was derived from Pt(IV). As shown in Fig. 3e, the Cl2p spectra of PtClBiWO-1.0 possessed two feature peaks at 198.2 and 199.7 eV, which could be ascribed to Cl in the sample [24].
Fig. 3. XPS spectra forBi4f (a), W4f (b), O1s (c), Pt4f (d), Cl2p (e) and VB-XPS (f) for pure Bi2WO6 and PtClBiWO-1.0.
Z. Sun et al. / Journal of Colloid and Interface Science 412 (2013) 31–38 Table 1 Curve-fitting result of O1s XPS spectra. Sample
Pure Bi2WO6
PtClBiWO1.0 a b
O1s
a Eb (eV) b ri (%) Eb (eV) ri (%)
Lattice oxygen
Hydroxyl groups
Adsorbed water
530.11
531.13
532.18
83.26 530.11 71.43
11.64 531.24 21.31
5.10 532.39 7.26
Eb: Binding energy of corresponding O. ri: The atomic ratio of each oxygen species to the total amount of oxygen.
The atomic content of different elements in PtClBiWO-1.0 detected by XPS measurement and that analyzed by EDX mapping were listed in Table 2, excluding the content of C or Cu from extrinsic impurities. The atomic content result from EDX analysis showed the total atomic content of different elements while that from XPS only indicated the surface atomic content. EDX atomic ratio of Pt in PtClBiWO-1.0 was 0.2 at.% (equal to 0.4 wt.%), which was less than the theoretical content of Pt in the precursor (1.0 wt.%) because of the loss of Pt in the solution. By contrast, surface Pt atomic content of PtClBiWO-1.0 by XPS detection was 0.28 at.% (>0.2 at.%), suggesting that Pt species was mainly dispersed on Bi2WO6 surface. From the table, while the ratio of Bi to W in PtClBiWO-1.0 was equal to 2.37, a bit larger than the theoretical value in Bi2WO6, the atomic ratio of Cl to Pt was about 8.33, much larger than that of PtCl4 or H2PtCl6. This implied that Cl in the sample not only interacted with Pt(IV) but also with other ions such as [Bi2O2]2+ or Na+ if any. XPS and EDX analysis of Na1s was also conducted but no relevant peaks were found, showing that the extra Cl could not be in the form of NaCl salt. Thus the residual Cl should be contacted with [Bi2O2]2+ forming Cl doped Pt(IV)/Bi2WO6, which was confirmed by the appearance of BiOCl in sample PtClBiWO-2.0 (Fig. 1b). The VB spectra of pure Bi2WO6 and PtClBiWO-1.0 are given in Fig. 3f. The dispersed electronic states below 3.0 eV and above the valence band edge were observed for both samples. For pure Bi2WO6, these electronic states could be attributed to the inherent valence band structure of Bi2WO6, consisting of Bi6s + O2p hybrid orbitals [4,5]. Comparatively, the electronic states of PtClBiWO1.0 dispersed more widely above 1.0 eV, possibly due to the influence of Cl2p to the orbitals of Bi6s and O2p [35,43].
35
little red shift in the region of 420–500 nm. But for PtClBiWO-0.5 and PtClBiWO-1.0, the red shift was obvious and the absorption curve slope edge moved towards the right. More intensive and broad absorption of visible light was observed for PtClBiWO-2.0 and the absorption edge moved to 800 nm. The enhancement of absorption could be ascribed to both doped Cl ion and surface Pt(IV). For ex-situ PtCl4 modified Bi2WO6 reported by Duan et al. [24], though the absorption curve tail of modified catalysts shifted upward compared to the unmodified, the slopes of catalysts with different concentration of PtCl4 showed no difference. Xu et al. [43] had reported that Cl-doped TiO2 would show a red movement of the absorption line compared with undoped TiO2. We supposed that the upward movement of the absorption curve tail was mainly due to the merged absorption of Pt(IV) salt and Bi2WO6 while the shift of the curve slope was primarily from the shortened band gap of Bi2WO6 effected by Cl doping [44], which was consistent with the VB XPS results. In addition, though BiOCl was formed in PtClBiWO-2.0, it could not be the reason for the shift of absorption range because the band gap of BiOCl was about 3.5 eV, much larger than that of Bi2WO6 (2.7 eV) and unable to absorb visible light [45–47]. Photoluminenscence (PL) spectroscopy is a common way to study the recombination rate of photo-generated electrons and holes on photocatalysts. A stronger PL spectrum intensity reflects a higher recombination rate [48]. The PL spectrum of pure Bi2WO6, PtClBiWO-0.1, PtClBiWO-0.5, PtClBiWO-1.0 and PtClBiWO-2.0 using 430 nm light as the excitation light are demonstrated in Fig. 5. It showed that all the tested catalysts could absorb 430 nm light and generate photo-excited electrons and holes, which would recombine and give out lights with lower energy, forming the PL singles. The tested four catalysts showed apparent difference in the PL peak position: 515 nm for PtClBiWO-2.0, 493 nm for PtClBiWO-1.0, 490 nm for PtClBiWO-0.5, 487 nm for PtClBiWO-0.1 and 480 nm for pure Bi2WO6. This trend met with the shift trend of absorption curve in UV–Vis DRS test, reflecting the gradually shorten band gaps affected chiefly by Cl doping. The intensity of the PL signals indicated that the sequence of the ability to stabilize photo-generated electrons and holes of the tested catalysts was: PtClBiWO-1.0 > PtClBiWO-0.5 > PtClBiWO2.0 > PtClBiWO-0.1 > pure Bi2WO6. Both PtCl4 and Bi2WO6 could be excited by 430 nm light. Local excitation of the Pt(IV)-Cl by visible light afforded charge transfer-ligand-to-metal, yielding Pt(III) and a chlorine atom [30]. The labile Pt (III) intermediate would rapidly transfer an electron to the CB of Bi2WO6, while the chlorine atom would abstract an electron from the Bi2WO6 lattice. Thus Pt
3.3. Optical property Fig. 4 displays UV–Vis diffuse reflection spectra of pure Bi2WO6 and PtClBiWO-x (x = 0.02, 0.1, 0.5, 1.0, 2.0). As expected, the photoabsorption region of pure Bi2WO6 ranged from UV light to visible light shorter than 450 nm. Compared with pure Bi2WO6, a tendency of red shifts were observed in the other samples, suggesting that the visible light response of Bi2WO6 was improved by the addition of H2PtCl6. PtClBiWO-0.05 and PtClBiWO-0.1 showed a Table 2 Atomic content of elements in PtClBiWO-1.0 analyzed by EDX and XPS. Element
Pt4f Bi4f W4f O1s Cl2p Na1s
PtClBiWO-1.0 EDX atomic %
XPS atomic %
0.2 26.9 16.2 53.7 2.9 0
0.2825 22.7283 9.5811 65.0541 2.3540 0
Fig. 4. UV–Vis diffuse reflection spectra of pure Bi2WO6 and PtClBiWO-x (x = 0.02, 0.1, 0.5, 1.0, 2.0).
36
Z. Sun et al. / Journal of Colloid and Interface Science 412 (2013) 31–38
Fig. 5. Photoluminescence spectra of pure Bi2WO6, PtClBiWO-x (x = 0.1, 0.5, 1.0 and 2.0) exited by wavelength of 430 nm visible light.
(IV)-Cl helped to stabilize the photo-excited electrons on Bi2WO6, which might be the primary reason why the PL peak intensity of pure Bi2WO6, PtClBiWO-0.1, PtClBiWO-0.5 and PtClBiWO-1.0 reduced in turn. However, PtClBiWO-2.0 with the largest amount of Pt(IV) on the catalyst surface did not show the lowest e-h+ recombination rate. This could be explained by that PtClBiWO2.0 was the sample with the most intrinsic lattice defects according to the poorest crystallization form XRD result and with excessive Pt(IV) (0.48 at.% by XPS analysis) on the surface of Bi2WO6 sample. Both intrinsic lattice defect and excessive Pt(IV) could serve as photo-generated e-h+ recombination center.
3.4. Photocatalytic activities and stability Visible-light (k > 420 nm) photocatalytic degradation of toluene was carried out to test the photocatalytic activities of the synthesized catalysts. Fig. 6 shows change of the relative concentration of toluene with irradiation time and the apparent reaction rate constants of photocatalytic decomposition of toluene over as-prepared samples. The blank experiment in the absence of photocatalyst indicated that the self-degradation of toluene in air under visible-light irradiation was negligible. Though pure Bi2WO6 had already shown a good toluene degradation rate (decomposing 25 ppm toluene in 85 min), all PtClBiWO-x samples exhibited better activities. PtClBiWO-1.0 with 1.0 wt.% Pt showed the best performance and its apparent reaction rate constant (k, 0.152 min1) was 2.82 times higher than that of pure Bi2WO6 (0.054 min1). BET surface areas of pure Bi2WO6 and PtClBiWO-x (x = 0.1, 1.0, 2.0) was listed in Table 3 which indicated that PtClBiWO-1.0 had the largest surface area. According to the former measurements of the catalysts prepared, PtClBiWO-1.0 also had the lowest e-h+ recombination possibility, shorter band gap by the doping of Cl and smaller particle size compared with pure Bi2WO6. All these contributed to the best performance of PtClBiWO-1.0. Subsequently, the photochemical stability of PtClBiWO-1.0 was tested by repeating photo-degradation of gaseous toluene experiment. The reactor with catalyst-covered dish was open to air after each run of photocatalytic reaction for 60 min, and then dried at 80 °C oven for 20 min to remove any volatile by-products absorbed on the surface of the photocatalysts. After that the reactor was cooled to room temperature naturally, appropriate amount of toluene was injected into it with a micro-springe for a new round of reaction. The initial concentration of toluene was also controlled at 25 ppm. The photocatalytic activity of PtClBiWO-1.0 in four
Fig. 6. Photocatalytic toluene degradation activity of prepared catalysts under visible light (k > 420 nm): Change of the relative concentration of toluene with irradiation time (a); the apparent reaction rate constant of photocatalytic decomposition of toluene over different samples (b).
Table 3 BET surface areas of pure Bi2WO6 and PtClBiWO-x (x = 0.1, 1.0, 2.0). Sample
Pure Bi2WO6
PtClBiWO0.1
PtClBiWO1.0
PtClBiWO2.0
Surface area (m2/g)
19.9
20.2
32.4
29.9
continuous tests is shown in Fig. 7. A little decrease in photo-catalytic performance was observed after each round which might due to the chemical change of Pt(IV) and the residual by-product on surface. Yet 85% of toluene could be degraded after four recycles in 60 min, exhibiting a good photochemical stability. 3.5. Brief discussion about the formation process The formation process of Cl doped Pt(IV)/Bi2WO6 photocatalyst was briefly discussed here to give some inspiration for facile one-pot hydrothermal synthesis of well-performed photocatalysts. TEM image of pure Bi2WO6 (Fig. 2a) showed that the Bi2WO6 square nanoplates were formed and (0 1 0) was the dominant facet exposed for the catalyst. It was pointed out that (2 0 0) and (0 0 2) facets had much higher chemical potential compared to other facets and were very sensitive to the surrounding growth conditions [15]. As long as the foreign force input was enough (180 °C hydrothermal condition in this system), they would grow every fast, leading to the formation of (0 1 0) dominant nanosheets. However,
37
Z. Sun et al. / Journal of Colloid and Interface Science 412 (2013) 31–38
0
½PtCl5 ðH2 OÞ þ H2 O $ ½PtCl4 ðH2 OÞ2 þ Cl
Fig. 7. Circled photocatalytic degradation of toluene experiments with PtClBiWO1.0.
when H2PtCl6 was added into the system, the particles failed to grow into square nanoplates. Instead, irregular-shape nanosheets with Pt (IV) and extra Cl were formed. Based on the above results, the forming process of pure Bi2WO6 nanoplates and Cl doped Pt(IV)/Bi2WO6 photocatalyst was schematically deduced (Fig. 8) and discussed briefly here.
BiðNO3 Þ3 þ H2 O $ BiONO3 þ Hþ þ NO3
ð1Þ
2BiONO3 þ WO2 4 $ Bi2 WO6 þ 2NO3
ð2Þ
BiONO3 þ Cl $ BiOCl þ NO3
ð3Þ
At first, Bi(NO3)35H2O was dropped into the deionized water. Though the salt was not adequately soluble without enough H+, it would gradually transfer into BiONO3 and HNO3 with Bi3+ and [Bi2O2]2+ as intermediates, as formula (1) shows [49]. In this stage, the resulted suspension was acidic (pH 2). When Na2WO4 solu2þ tion was added, the ðBi2 O2 Þ would be released from BiONO3 (formula 1) and interconnected with WO2 to form Bi2WO6 crystal 4 (formula 2) which would reform to nanosheets under hydrothermal conditions due to the intrinsic anisotropic growth habit of Bi2WO6 [50]. If there was Cl in the solution, BiOCl was tended to be formed through formula (3) [51]. [PtCl6]2 in aqueous solution could be hydrolyzed to form 2 PtðOHÞx Cl6x (x = 1–5), depending on the pH [30]. In an acidic solution, the hydrolyzing equilibrations were as follows:
½PtCl6
2
þ H2O $ ½PtCl5 ðH2 OÞ þ Cl
ð4Þ
ð5Þ
As H2PtCl6 was added into the precursor suspension with Bi2WO6 nuclei, it was highly possible for Cl to contact with Bi2 O2þ 2 2þ and tend to form Cl ½Bi2 O2 ionic bond. It was reported by Xie et al. [52] that BiOCl favored to keep crystallinity in acidic environment and the Bi2WO6 was the main crystal formed with higher pH value in hydrothermal process of solution with BiCl3 and (NH4)2WO4. Thus it was referred that Cl in our experimental condition 2þ was more intensively bonded with ½Bi2 O2 than WO2 4 at higher temperature and pressure, which was bound to influence, even interrupt, the crystal generating process of Bi2WO4 nanosheets depending on the concentration of Cl. Also, the attraction of Bi2 O2þ to Cl promoted the positive direction reaction route of 2 Eqs. (4) and (5), eventually leading to the formation of PtCl4(H2O)2 that was prone to absorb ions and move to the surface of WO2 4 layer, as depicted in Fig. 8. While the temperature and pressure went up during hydrothermal treatment, the foreign energy forced all of the reactions to speed up, resulting in poor-crystalized Bi2WO6 irregular nanosheets with intrinsic crystal defects, surface Pt(IV)-Cl and doped Cl. From the former characterizations, we can see that the addition of H2PtCl6 in the hydrothermal process not only effected the morphology of resulted Bi2WO6, reducing the particle size and decreasing the crystallinity, but also modified Bi2WO6 with doped Cl and Pt(IV) on the catalyst surface, which had great impact on the optical properties and photocatalytic activities of the catalysts. But the amount of H2PtCl6 should be controlled precisely to achieve the best-performed sample. Too little addition of H2PtCl6 could not improve the photocatalytic activity much, while the excessive loading of H2PtCl6 would lead the formation of layer-structure BiOCl. A recent DFT study concluded that an intrinsic interface between BiOCl and Bi2WO6 failed to enhance the charge-carrier separation due to the improper band alignment between these two materials [47]. Also, extensive loading of Pt(IV)-Cl on Bi2WO6 would increase the photo-generated electron–hole recombination possibility as the PL results showed.
4. Conclusion In summary, a facile one-pot hydrothermal method was successfully applied in preparing Cl doped Pt(IV)/Bi2WO6 photocatalysts. By adjusting the content of H2PtCl6 in the precursor, catalysts with different content of Pt(IV) and Cl were achieved. All the modified catalysts had improved visible-light photocatalytic activity compared to pure Bi2WO6. PtClBiWO-1.0 with theoretical 1.0 wt.% Pt exhibited the best photocatalytic performance as it possessed the largest surface area (32.4 m2/g); appropriate amount
Fig. 8. Schematic forming process of pure Bi2WO6 nanoplates and Cl doped Pt(IV)/Bi2WO6.
38
Z. Sun et al. / Journal of Colloid and Interface Science 412 (2013) 31–38
of uniformly dispersed Pt(IV) on the surface and doped Cl, leading to superior capacity to absorb reactant, enhanced absorption in visible-light range and advanced stability of photo-excited electrons and holes. Synthesis of good-preformed Cl-doped Pt(IV)/Bi2WO6 by such a facile approach provided new insight for further high-efficiency photocatalyst preparation in an easier way. And the resulted catalysts showed promise for practical environmental pollution remediation. Acknowledgments We acknowledge the financial supports of the National High Technology Research and Development Program (863 Program) of China (No. 2010AA064905), Changjiang Scholar Incentive Program (Ministry of Education, China, 2009), Natural Science Foundation of Zhejiang Province (No. Z5100116), National Natural Science Foundation of China (No. 50808156), Foundation of Zhejiang Educational Committee (No. Y201018656) and China Postdoctoral Science Foundation (No. 20100480088). References [1] P. Boullay, G. Trolliard, D. Mercurio, J.M. Perez-Mato, L. Elcoro, J. Solid State Chem. 164 (2002) 252. [2] W. Sugimoto, M. Shirata, Y. Sugahara, K. Kuroda, J. Am. Chem. Soc. 121 (1999) 11601. [3] N. Kim, R.N. Vannier, C.P. Grey, Chem. Mater. 17 (2005) 1952. [4] J.W. Tang, Z.G. Zou, J.H. Ye, Catal. Lett. 92 (2004) 53. [5] H.B. Fu, C.S. Pan, W.Q. Yao, Y.F. Zhu, J. Phys. Chem. B 109 (2005) 22432. [6] J.G. Yu, J.F. Xiong, B. Cheng, Y. Yu, J.B. Wang, J. Solid State Chem. 178 (2005) 1968. [7] A. Kudo, S. Hijii, Chem. Lett. (1999) 1103. [8] H.B. Fu, S.C. Zhang, T.G. Xu, Y.F. Zhu, J.M. Chen, Environ. Sci. Technol. 42 (2008) 2085. [9] L. Ge, J. Liu, Appl. Catal. B-Environ. 105 (2011) 289. [10] S. Guo, X. Li, H. Wang, F. Dong, Z. Wu, J. Colloid Interface Sci. 369 (2012) 373. [11] A.P. Finlayson, V.N. Tsaneva, L. Lyons, M. Clark, B.A. Glowacki, Phys. Status Solidi A 203 (2006) 327. [12] L. Zhang, C. Baumanis, L. Robben, T. Kandiel, D. Bahnemann, Small 7 (2011) 2714. [13] Y. Zhou, Z. Tian, Z. Zhao, Q. Liu, J. Kou, X. Chen, J. Gao, S. Yan, Z. Zou, ACS Appl. Mater. Interfaces 3 (2011) 3594. [14] H. Cheng, B. Huang, Y. Liu, Z. Wang, X. Qin, X. Zhang, Y. Dai, Chem. Commun. 48 (2012) 9729. [15] C. Zhang, Y. Zhu, Chem. Mater. 17 (2005) 3537.
[16] H. Huang, H. Chen, Y. Xia, X. Tao, Y. Gan, X. Weng, W. Zhang, J. Colloid Interface Sci. 370 (2012) 132. [17] X.C. Song, Y.F. Zheng, R. Ma, Y.Y. Zhang, H.Y. Yin, J. Hazard. Mater. 192 (2011) 186. [18] Y. Liu, W.M. Wang, Z.Y. Fu, H. Wang, Y.C. Wang, J.Y. Zhang, Mater. Sci. Eng. BAdv. Funct. Solid-State Mater. 176 (2011) 1264. [19] R. Shi, G.L. Huang, J. Lin, Y.F. Zhu, J. Phys. Chem. C 113 (2009) 19633. [20] L.S. Zhang, K.H. Wong, Z.G. Chen, J.C. Yu, J.C. Zhao, C. Hu, C.Y. Chan, P.K. Wong, Appl. Catal. A-Gen. 363 (2009) 221. [21] J.H. Xu, W.Z. Wang, E.P. Gao, J. Ren, L. Wang, Catal. Commun. 12 (2011) 834. [22] S.B. Zhu, T.G. Xu, H.B. Fu, J.C. Zhao, Y.F. Zhu, Environ. Sci. Technol. 41 (2007) 6234. [23] M. Ge, Y.F. Li, L. Liu, Z. Zhou, W. Chen, J. Phys. Chem. C 115 (2011) 5220. [24] F. Duan, Y. Zheng, M.Q. Chen, Appl. Surf. Sci. 257 (2011) 1972. [25] Q. Xiao, J. Zhang, C. Xiao, X.K. Tan, Catal. Commun. 9 (2008) 1247. [26] Y. Zheng, K.L. Lv, X.F. Li, K.J. Deng, J. Sun, L.Q. Chen, L.Z. Cui, D.Y. Du, Chem. Eng. Technol. 34 (2011) 1630. [27] Q.C. Xu, D.V. Wellia, Y.H. Ng, R. Amal, T.T.Y. Tan, J. Phys. Chem. C 115 (2011) 7419. [28] Z.-Q. Li, X.-T. Chen, Z.-L. Xue, J. Colloid Interface Sci. 394 (2013) 69. [29] Y. Guo, G. Zhang, H. Gan, J. Colloid Interface Sci. 369 (2012) 323. [30] L. Zang, C. Lange, I. Abraham, S. Storck, W.F. Maier, H. Kisch, J. Phys. Chem. B 102 (1998) 10765. [31] W. Macyk, H. Kisch, Chem. – A Eur. J. 7 (2001) 1862. [32] G.Q. Li, D.F. Wang, Z.G. Zou, J.H. Ye, J. Phys. Chem. C 112 (2008) 20329. [33] L. Ge, J. Mol. Catal. A: Chem. 282 (2008) 62. [34] F. Dong, S. Guo, H. Wang, X. Li, Z. Wu, J. Phys. Chem. C 115 (2011) 13285. [35] F. Dong, H.Q. Wang, G. Sen, Z.B. Wu, S.C. Lee, J. Hazard. Mater. 187 (2011) 509. [36] C. Wang, H. Zhang, F. Li, L. Zhu, Environ. Sci. Technol. 44 (2010) 6843. [37] P. Dumrongrojthanath, T. Thongtem, A. Phuruangrat, S. Thongtem, Superlattice Microstruct. 54 (2013) 71. [38] F. Shi, M.K. Tse, M.-M. Pohl, A. Brückner, S. Zhang, M. Beller, Angew. Chem. Int. Ed. 46 (2007) 8866. [39] J.G. Yu, G.H. Wang, B. Cheng, M.H. Zhou, Appl. Catal. B-Environ. 69 (2007) 171. [40] T. Hirakawa, K. Yawata, Y. Nosaka, Appl. Catal. A 325 (2007) 105. [41] A.L. Linsebigler, G.Q. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735. [42] J.S. Lee, W.Y. Choi, J. Phys. Chem. B 109 (2005) 7399. [43] H. Xu, Z. Zheng, L. Zhang, H. Zhang, F. Deng, J. Solid State Chem. 181 (2008) 2516. [44] G. Liu, C. Sun, L. Wang, S.C. Smith, G.Q. Lu, H.-M. Cheng, J. Mater. Chem. 21 (2011) 14672. [45] J. Jiang, K. Zhao, X. Xiao, L. Zhang, J. Am. Chem. Soc. 134 (2012) 4473. [46] S. Shamaila, A.K.L. Sajjad, F. Chen, J. Zhang, J. Colloid Interface Sci. 356 (2011) 465. [47] W. Wang, W. Yang, R. Chen, X. Duan, Y. Tian, D. Zeng, B. Shan, Phys. Chem. Chem. Phys. (2011). [48] J.C. Yu, J.G. Yu, W.K. Ho, Z.T. Jiang, L.Z. Zhang, Chem. Mater. 14 (2002) 3808. [49] H. Gnayem, Y. Sasson, ACS Catal. 3 (2013) 186. [50] D. Ma, S. Huang, W. Chen, S. Hu, F. Shi, K. Fan, J. Phys. Chem. C 113 (2009) 4369. [51] F. Dong, Y. Sun, M. Fu, Z. Wu, S.C. Lee, J. Hazard. Mater. 219–220 (2012) 26. [52] H. Xie, D. Shen, X. Wang, G. Shen, Mater. Chem. Phys. 103 (2007) 334.