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Hydrothermal synthesis, characterization, and visible light-driven photocatalytic properties of [Bi.sub.2]W[O.sub.6] nanoplates.

1. Introduction

In the past three decades, semiconducting photocatalyst has attracted extensive attention due to its potential applications in solar energy conversion and environmental remedy [1, 2]. Because of the good chemical stability, high oxidation activity, nontoxicity, and low price, Ti[O.sub.2] has been the most popular photocatalyst for environmental purification. Among different semiconductors, titania has been investigated most frequently since the discovery of the photocatalytic splitting of water on titania electrodes in 1972 by Fujishima and Honda [1, 3, 4]. The main shortcoming of titania is the absorption of only ultraviolet with the wavelength of less than 387.5 nm, which accounts for only about 4% of sunlight. Ti[O.sub.2]-based photocatalysts have been the most popular and have been shown to be a lot of progress. Due to the fact that Ti[O.sub.2] can only be activated by the ultraviolet light, the use of Ti[O.sub.2] is thus limited by its negligible activity under visible light irradiation [4, 5]. It is very urgent to develop highly efficient visible light induced photocatalysts to meet the high requirement of future environmental remedy and energy crisis. Therefore, it is of great importance to develop visible light responsive photocatalysts.

Bismuth tungstate was reported to show photocatalytic activity under visible light irradiation [1, 5]. The Aurivillius family of bismuth oxide layered perovskite has attracted considerable attention due to their important applied properties such as ionic conductivity, photocatalytic activity in waste purification, and ferroelectricity [6, 7]. Their common formula [[Bi.sub.2][O.sub.2]][[A.sub.n-1][B.sub.n][O.sub.3n+1]] (A = Ca, Sr, Ba, Pb, Bi, Na, K, and B = Ti, Nb, Ta, Mo, W, Fe) suggests easy tuning of these properties by iso- or aliovalent substitution at the A and B perovskitic sites with an only empirical restriction that the B position should be occupied with ions with the radius of ca. 0.6 [Angstrom] and over 60% of them having [nd.sup.0] electron configuration. The n = 1 case differs significantly from n > 1 series in several aspects. First, there are no A positions in the perovskitic slabs, and the possibilities of isovalent substitution among ferroelectrics are very restricted [4-6]. [Bi.sub.2]W[O.sub.6] as an Aurivillius-phase perovskite belongs to the bismuth oxide family with a structure consisting of perovskite layers ([A.sub.m-1][B.sub.m][O.sub.3m+1]) between corner-sharing structure of WO6 octahedrons sandwiched between [([Bi.sub.2][O.sub.2]).sub.2+] layers [8-10]. Bismuth tungstate ([Bi.sub.2]W[O.sub.6]) is a typical n-type direct band gap semiconductor with a band gap of 2.75 eV and has potential applications in electrode materials, solar energy conversion, and visible-light-driven photocatalysis [9].

The [Bi.sub.2]W[O.sub.6] nanostructures with different morphologies such as nanoplates [4,11], nanocages [7], hierarchical flowers [8], snow-like [9], and nanolaminars [10] have been prepared by solution-phase methods such as hydrothermal [4, 8, 9,11], refluxing [7], sonochemical method [10], and citrate complex method [12]. However, the hydrothermal method is a widely used technique because of low cost, low temperature, high yield, scalable process, and large scale method [13, 14]. It is able to control the shape and dimension of nanomaterials by reaction time, temperature, pH value, and capping agent during hydrothermal process [13,15,16]. Shang et al. [4] prepared [Bi.sub.2]W[O.sub.6] photocatalytic nanoplates with the size of ca. 30 nm. The photocatalytic activity of the as-prepared [Bi.sub.2]W[O.sub.6] was evaluated by the degradation of rhodamine-B (RhB) solution under visible light irradiation within 60 min while photodegradation efficiency of RhB of [Bi.sub.2]W[O.sub.6] nanoparticles reaches 75 min [17]. It was about 8-10 times higher than that of the product prepared by solid-state reaction.

In the present study, we report a facile, low-cost, and green hydrothermal route to synthesize the [Bi.sub.2]W[O.sub.6] nanoplates. The effects of hydrothermal temperature and reaction time on formation of [Bi.sub.2]W[O.sub.6] nanoplates, as well as their photocatalytic activities, were investigated. It was found that the [Bi.sub.2]W[O.sub.6] nanoplates have high visible light photocatalytic activity in the degradation of RhB.

2. Experimental Procedures

Several solutions were formed by dissolving 1.6493 g of sodium tungstate ([Na.sub.2]W[O.sub.4]) in 100 mL reverse osmosis (RO) water each and continuously stirred at room temperature for 30 min. Subsequently, 4.8511 g bismuth nitrate (Bi[(N[O.sub.3]).sub.3]) was added to each of these solutions with 30 min continuous stirring at room temperature and followed by adding 3 M NaOH until achieving the desired pH of 10. The solutions were hydrothermally processed at 100-200[degrees]C for 0-20 h. In the end, the as-synthesized precipitates were separated by filtration, rinsed with distilled water and ethanol, and dried at 80[degrees]C for 24 h for further characterization.

X-ray powder diffraction (XRD) patterns of the products were recorded on a Japan Rigaku D/MAX-[gamma]A X-ray diffractometer with graphite monochromator and Cu K[alpha] radiation ([lambda] = 0.154178 nm) at a scanning rate of 0.02deg/s ranging from 20 to 60 deg. Raman spectra were recorded on a HORIBA JOBIN YVON T64000 Raman spectrometer with 50 mW and 514.5 nm wavelength Ar green laser and FTIR spectra on a BRUKER TENSOR 27 Fourier transform infrared (FTIR) spectrometer with KBr as a diluting agent and operated in the range of 400-4,000 [cm.sup.-1]. SEM images were taken on a JEOL JSM-6335F scanning electron microscope operating at 20 kV and TEM images on a JEOL JEM2010 JEOL transmission electron microscope (TEM) at an accelerating voltage of 200 kV. UV-visible absorption was carried out on a Perkin Elmer Lambda 25 UV-vis spectrophotometer.

The photocatalytic activities of as-synthesized samples were tested by measuring the degradation of rhodamine-B (RhB) in an aqueous solution under visible light irradiation as light source of Xe lamp. Each 150 mg photocatalyst was suspended in each of 150 mL [10.sup.-5] M RhB solution aqueous solutions. The solutions were magnetically stirred for 30 min in dark to establish an adsorption/desorption equilibrium of RhB on the surfaces of the photocatalyst. Then the light was turned on to initiate the photocatalytic reaction. The solution was analyzed by a Lambda 25 spectrophotometer, using 450 W of Xe lamp with wavelength of 553 nm. The decolorization efficiency (%) has been calculated as follows:

Decolorization efficiency (%) = [C.sub.o] - C/[C.sub.o] x 100, (1)

where [C.sub.0] is the initial concentration of RhB and C is the concentration of RhB after light irradiation.

3. Results and Discussion

To investigate the formation process of [Bi.sub.2]W[O.sub.6] phase, XRD patterns of the samples processed at different temperatures and lengths of time were characterized and shown in Figure 1. The temperature and time have a great influence on the formation of the crystals. Figure 1(a) shows the XRD patterns of the as-synthesized samples at 100-200[degrees]C for 20 h by hydrothermal method. It was clearly seen that the crystallization has occurred even at 100[degrees]C which can be indexed to [Bi.sub.2]W[O.sub.6] mixed with cubic [Bi.sub.2][O.sub.3] phase according to the JCPDS number 27-0052 [18]. When the temperature further goes up to 200[degrees]C, the XRD patterns of the sample reveal the narrower peaks and the higher intensity, suggesting the enhanced crystallite size. However, the intensities of all diffraction peaks of [Bi.sub.2]W[O.sub.6] synthesized at 180[degrees]C for 20 h are the same as the [Bi.sub.2]W[O.sub.6] at 200[degrees]C for 20 h. Therefore, the hydrothermally temperature of 180[degrees]C is high enough for synthesizing of [Bi.sub.2]W[O.sub.6]. At 180 and 200[degrees]C, all the diffraction peaks can be readily indexed as pure orthorhombic well-crystallized [Bi.sub.2]W[O.sub.6] structure with cell parameters of a = 5.46 [Angstrom], b = 5.44 [Angstrom], and c = 16.43 [Angstrom], in good agreement with those of the JCPDS number 73-1126 [18]. It was found that the standard intensity of the (113) peak is about five times higher than that of the (200) peak which could be expressed as [I.sub.(113)]/[I.sub.(200)] = 5. However, as for the sample prepared at 180[degrees]C for 20 h, the value of [I.sub.(113)]/[I.sub.(200)] was 2.63, suggesting that the crystal has special anisotropic growth in (200) direction [11]. Therefore, the temperature reaction appropriate for the synthesis of [Bi.sub.2]W[O.sub.6] is at 180[degrees] C. Figure 1(b) shows the XRD

patterns of the samples synthesized by the 180[degrees]C and 0-20 h hydrothermal method. The cubic [Bi.sub.2][O.sub.3] phase was detected at 180[degrees]C for 0 h (before hydrothermal processing). It can be seen that the intensity of the (111) plane of cubic [Bi.sub.2][O.sub.3] phase at 2[theta] = 27.93[degrees] decreased while intensity of the (113) plane of orthorhombic [Bi.sub.2]W[O.sub.6] phase at 20 = 28.30[degrees] increased. It implied that the percentage of orthorhombic [Bi.sub.2]W[O.sub.6] phase was increased when the reaction time was prolonged from 0 to 15 h at 180[degrees]C. When the reaction time was longer than 15 h, all the diffraction peaks can be readily indexed as a pure orthorhombic well-crystallized [Bi.sub.2]W[O.sub.6] structure of the JCPDS number 73-1126 [18]. The XRD results indicate that nanosized [Bi.sub.2]W[O.sub.6] could be selectively synthesized by adjusting the hydrothermal temperature and reaction time, and well-crystallized [Bi.sub.2]W[O.sub.6] crystals can be synthesized at the lowest temperature of 180[degrees]C for the reaction time of longer than 15 h.

The crystal structure of [Bi.sub.2]W[O.sub.6] as Aurivillius-type structure is layers of perovskite-like [(W[O.sub.4]).sup.2-] and fluorite-like [([Bi.sub.2][O.sub.2]).sup.2+] layers lying normal to the c direction. The W[O.sub.6] octahedrons are joined by equatorial oxygen atoms, whereas the apical oxygen atoms join the W[O.sub.6] and Bi[O.sub.6] polyhedrons. The tungsten exhibits a regular octahedral environment and neighbouring octahedrons are connected to each other by corner sharing. The coordination polyhedrons of bismuth are more complex but each one is bonded to six oxygen atoms [12, 19, 20]. There are only six Raman active (2[A.sub.1g] + [B.sub.1g] + [3.sub.Eg]) and 9IR active (4[A.sub.2u] + 5[E.sub.u]) modes for the ideal I4/mmm structure of [Bi.sub.2]W[O.sub.6]. These modes can be grouped into symmetric ([A.sub.1g]) and asymmetric stretching vibrations of the W[O.sub.6] octahedrons ([A.sub.2u] + [E.sub.u]), bending vibrations of the W[O.sub.6] octahedrons (Eg + 2[E.sub.u] + [A.sub.2u] + [B.sub.2u]), stretching and bending vibrations of the [([Bi.sub.2][O.sub.2]).sup.2+] layers ([B.sub.1g] + [E.sub.g] + [A.sub.2u] + [E.sub.u]), translational motions of the [Bi.sup.3+] ions ([A.sub.1g] + [E.sub.g]), and vibrations involving translational motions of [Bi.sup.3+] and [W.sup.6+] ions ([A.sub.2u] + [E.sub.u]) [19, 20]. Raman spectra of the samples are shown in Figure 2(a). The peaks in the range 600-1000 [cm.sup.-1] were assigned to the stretching of the W-O bonds. In more detail, the vibrations at 789.65 and 820.41[cm.sup.-1] of [Bi.sub.2]W[O.sub.6] were associated with the antisymmetric and symmetric Ag modes of terminal O-W-O modes. The peak at 714.61 [cm.sup.-1] was interpreted as an antisymmetric bridging mode, associated with the tungstate chains [21, 22]. The vibration of 310 [cm.sup.-1] could be assigned to the translational mode involving simultaneous motion of [Bi.sup.3+] and W[O.sub.6.sup.6-]. The peak at about 300 [cm.sup.-1] was assigned to the mode of terminal W[O.sub.2] groups [21, 23].

Figure 2(b) shows FTIR spectra of the as-synthesized [Bi.sub.2]W[O.sub.6] samples. The very weak band located at 3420 [cm.sup.-1] can be attributed to the symmetric stretching vibration of -OH groups of adsorbed water on surface of [Bi.sub.2]W[O.sub.6] samples. The spectra present the main absorption bands at 400-1000 [cm.sup.-1] which are assigned to Bi-O stretching, W-O stretching, and W-O-W bridging stretching modes, corresponding to the Bi-O band located at 844.8 [cm.sup.-1] and the W-O band at 821.6 [cm.sup.-1] [24-26].

The morphologies of [Bi.sub.2]W[O.sub.6] samples were revealed by field-emission scanning electron microscopy (FE-SEM) as shown in Figure 3. As seen from a low magnification FE-SEM image, the samples exhibited mainly plate-like rectangular shape with a lateral size of average length of 400-600 nm for 180[degrees]C 15 h and 300-500 nm for 180[degrees]C 20 h. Their magnified images are shown in Figures 3(b) and 3(d), which show many nanoplates with thickness of 50 nm for 180[degrees]C 15 h and 20 nm for 180[degrees]C 20 h with their smooth surfaces.

Figure 4 shows TEM images of [Bi.sub.2]W[O.sub.6] samples. The as-synthesized [Bi.sub.2]W[O.sub.6] at 180[degrees]C for 15 h shows uniform square nanoplates grown larger and thinner. When the reaction time was further increased to 20 h, the sample was still to be square plate-like morphology with shorter edges. The length of the edges could reach 100-150 nm. The SAED pattern confirms that the nanoplates are single crystalline [Bi.sub.2]W[O.sub.6] since the observed spot pattern corresponded to the most intense diffraction peaks of [Bi.sub.2]W[O.sub.6]. The SAED pattern reveals that the crystalline structure can be indexed to the expected orthorhombic crystal lattice of [Bi.sub.2]W[O.sub.6] with characteristic (200), (220), and (020) reflection with [001] as zone axis, in agreement with the above XRD analysis. In this research, the nanoplates preferentially grew along the (001) plane, which is parallel to the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] direction.

The UV-visible absorption spectrum of [Bi.sub.2]W[O.sub.6] is shown in Figure 5, which shows the absorption wavelength of less than 460 nm assigned to the intrinsic energy band gap of [Bi.sub.2]W[O.sub.6] [8].

To study the photocatalytic activities of the hydrothermally synthesized samples, tetraethylated rhodamine (RhB) with a major absorption band at 553 nm was chosen as a model organic pollutant. Figure 6 shows the temporal evolution of the spectra during the photodegradation of RhB photocatalyzed by typical [Bi.sub.2]Mo[O.sub.6] nanoplates under visible light illumination. The absorption intensity decreased gradually and the major absorption band shifted from 553 nm to 498 nm in photodegradation process during 180 min, confirming that RhB (553 nm) was deethylated to rhodamine (498 nm) under visible light radiation [27, 28]. The deethylated products of RhB are N,N,N;-triethylated rhodamine (539 nm), N,N'-diethylated rhodamine (522 nm), Nethylated rhodamine (510 nm), and rhodamine (498 nm) [29, 30]. The absorption peaks corresponding to RhB completely disappeared after about 100 min. It suggests that [Bi.sub.2]W[O.sub.6] nanoplates are the excellent photocatalytic activity.

Figure 7 shows degradation efficiency of the photocatalytic [Bi.sub.2]W[O.sub.6] samples against RhB. It indicates the obvious difference in percent degradation of RhB by both [Bi.sub.2]W[O.sub.6] samples. The control experiment in the absence of [Bi.sub.2]W[O.sub.6] catalyst under identical visible irradiation was also carried out. The results show that photodegradation of RhB by [Bi.sub.2]W[O.sub.6] nanoplates synthesized at 180[degrees]C for 20h is higher than that by [Bi.sub.2]W[O.sub.6] nanoplates synthesized at 180[degrees]C for 15 h, due to its higher degree of crystallinity. In case of both [Bi.sub.2]W[O.sub.6] samples, the test for the degradation of RhB was very rapid until reaching 60 min. During photocatalysis, the visible light induced photocatalytic efficiency of the [Bi.sub.2]W[O.sub.6] nanoplates synthesized at 180[degrees]C for 20 h (surface area of 53.1 [m.sup.2][g.sup.-1]) is 98.24% for RhB photodegradation. Comparing to the previous report, the photocatalytic activity of P25 Ti[O.sub.2] is ~18% after 120 min under visible radiation with A > 420 nm, reported by Cheng et al. [31]. Lu et al. [32] synthesized octahedron-like hierarchical structure of [Bi.sub.2]W[O.sub.6] by a facile hydrothermal method. It shows a photodegradation of the RhB solution of 56% after the visible irradiation for 6 h. Thus, [Bi.sub.2]W[O.sub.6] nanoplates show higher activities for degradation of RhB under visible light and are new candidate for visible-light-driven photocatalytic degradation of organic dyes.

The photodegradation phenomena of RhB by [Bi.sub.2]W[O.sub.6] can be explained by [e.sup.-]/[h.sup.+] recombination [33]. Generally, the [e.sup.-]/[h.sup.+] recombination in the materials is divided into two categories: volume and surface recombination. The unique 2D layers of [Bi.sub.2]W[O.sub.6] nanoplates supported the e-/h+ surface separation, caused by the abundant surface trapping sites and insufficient driving force for e-/h+ recombination. Thus, the highly photocatalytic efficiency of [Bi.sub.2]W[O.sub.6] nanoplates was accelerated by the interfacial charged transfer process, which can play the role in the content and diffusion of adsorbed RhB. Their surfaces can be modified by the adsorption of some metallic atoms. In general, the [e.sup.-]/[h.sup.+] recombination on the semiconductor surface is able to control with ease, because the nanosheets have large space to enhance the photochemical activity.

4. Conclusions

In this research, orthorhombic [Bi.sub.2]W[O.sub.6] nanoplates as a photocatalyst were successfully synthesized by hydrothermal reaction at 180[degrees]C for 20 h in the solution with the pH of 10. The photocatalytic activity of [Bi.sub.2]W[O.sub.6] nanoplates was determined to be 98.24% degradation of RhB for 100 min irradiated by Xe light.

http://dx.doi.org/10.1155/2014/138561

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

The authors are extremely grateful to the Prince of Songkla University under Contact no. SCI560002S, Thailand.

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Anukorn Phuruangrat, (1) Phattranit Dumrongrojthanath, (2) Nuengruethai Ekthammathat, (2) Somchai Thongtem, (3,4) and Titipun Thongtem (2,4)

(1) Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand

(2) Department of Chemistry, Faculty of Science, ChiangMai University, Chiang Mai 50200, Thailand

(3) Department of Physics and Materials Science, Faculty of Science, ChiangMai University, Chiang Mai 50200, Thailand

(4) Materials Science Research Center, Faculty of Science, ChiangMai University, ChiangMai 50200, Thailand

Correspondence should be addressed to Anukorn Phuruangrat; phuruangrat@hotmail.com and Titipun Thongtem; ttpthongtem@yahoo.com

Received 8 January 2014; Revised 9 April 2014; Accepted 9 April 2014; Published 7 May 2014

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Author:Phuruangrat, Anukorn; Dumrongrojthanath, Phattranit; Ekthammathat, Nuengruethai; Thongtem, Somchai;
Publication:Journal of Nanomaterials
Date:Jan 1, 2014
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