Hydrothermal Synthesis , Characterization , and Visible Light-Driven Photocatalytic Properties of Bi 2 WO 6 Nanoplates

1 Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand 2Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 3Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 4Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand


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, TiO 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.TiO 2 -based photocatalysts have been the most popular and have been shown to be a lot of progress.Due to the fact that TiO 2 can only be activated by the ultraviolet light, the use of TiO 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].
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 Å and over 60% of them having  0 electron configuration.The  = 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][5][6].Bi 2 WO 6 as an Aurivillius-phase perovskite belongs to the bismuth oxide family with a structure consisting of perovskite layers ( −1    3+1 ) between corner-sharing structure of WO 6 octahedrons sandwiched between (Bi 2 O 2 ) 2+ layers [8][9][10].Bismuth tungstate (Bi 2 WO 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 2 WO 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 2 WO 6 photocatalytic nanoplates with the size of ca. 30 nm.The photocatalytic activity of the as-prepared Bi 2 WO 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 2 WO 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 2 WO 6 nanoplates.The effects of hydrothermal temperature and reaction time on formation of Bi 2 WO 6 nanoplates, as well as their photocatalytic activities, were investigated.It was found that the Bi 2 WO 6 nanoplates have high visible light photocatalytic activity in the degradation of RhB.

Experimental Procedures
Several solutions were formed by dissolving 1.6493 g of sodium tungstate (Na 2 WO 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(NO 3 ) 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 ∘ 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 ∘ C for 24 h for further characterization.
X-ray powder diffraction (XRD) patterns of the products were recorded on a Japan Rigaku D/MAX-A X-ray diffractometer with graphite monochromator and Cu K  radiation ( = 0.154178 nm) at a scanning rate of 0.02 deg/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 −1 .SEM images were taken on a JEOL JSM-6335F scanning electron microscope operating at 20 kV and TEM images on a JEOL JEM-2010 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 −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: where C  is the initial concentration of RhB and C is the concentration of RhB after light irradiation.

Results and Discussion
To investigate the formation process of Bi 2 WO 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 ∘ C for 20 h by hydrothermal method.It was clearly seen that the crystallization has occurred even at 100 ∘ C which can be indexed to Bi 2 WO 6 mixed with cubic Bi 2 O 3 phase according to the JCPDS number 27-0052 [18].When the temperature further goes up to 200 ∘ 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 2 WO 6 synthesized at 180 ∘ C for 20 h are the same as the Bi 2 WO 6 at 200 ∘ C for 20 h.Therefore, the hydrothermally temperature of 180 ∘ C is high enough for synthesizing of Bi 2 WO 6 .At 180 and 200 ∘ C, all the diffraction peaks can be readily indexed as pure orthorhombic wellcrystallized Bi 2 WO 6 structure with cell parameters of a = 5.46 Å, b = 5.44 Å, and c = 16.43Å, 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 (113) /I (200) = 5.However, as for the sample prepared at 180 ∘ C for 20 h, the value of I (113) /I (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 2 WO 6 is at 180 ∘ C. patterns of the samples synthesized by the 180 ∘ C and 0-20 h hydrothermal method.The cubic Bi 2 O 3 phase was detected at 180 ∘ C for 0 h (before hydrothermal processing).It can be seen that the intensity of the (111) plane of cubic Bi 2 O 3 phase at 2 = 27.93 ∘ decreased while intensity of the (113) plane of orthorhombic Bi 2 WO 6 phase at 2 = 28.30∘ increased.It implied that the percentage of orthorhombic Bi 2 WO 6 phase was increased when the reaction time was prolonged from 0 to 15 h at 180 ∘ 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 2 WO 6 structure of the JCPDS number 73-1126 [18].The XRD results indicate that nanosized Bi 2 WO 6 could be selectively synthesized by adjusting the hydrothermal temperature and reaction time, and well-crystallized Bi 2 WO 6 crystals can be synthesized at the lowest temperature of 180 ∘ C for the reaction time of longer than 15 h.The crystal structure of Bi 2 WO 6 as Aurivillius-type structure is layers of perovskite-like (WO 4 ) 2− and fluorite-like (Bi 2 O 2 ) 2+ layers lying normal to the c direction.The WO 6 octahedrons are joined by equatorial oxygen atoms, whereas the apical oxygen atoms join the WO 6 and BiO 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 (2A 1g + B 1g + 3E g ) and 9 IR active (4A 2u + 5E u ) modes for the ideal I4/mmm structure of Bi 2 WO 6 .These modes can be grouped into symmetric (A 1g ) and asymmetric stretching vibrations of the WO 6 octahedrons (A 2u + E u ), bending vibrations of the WO 6 octahedrons (E g + 2E u + A 2u + B 2u ), stretching and bending vibrations of the (Bi 2 O 2 ) 2+ layers (B 1g + E g + A 2u + E u ), translational motions of the Bi 3+ ions (A 1g + E g ), and vibrations involving translational motions of Bi 3+ and W 6+ ions (A 2u + E u ) [19,20].Raman spectra of the samples are shown in Figure 2(a).The peaks in the range 600-1000 cm −1 were assigned to the stretching of the W-O bonds.In more detail, the vibrations at 789.65 and 820.41 cm −1 of Bi 2 WO 6 were associated with the antisymmetric and symmetric A g modes of terminal O-W-O modes.The peak at 714.61 cm −1 was interpreted as an antisymmetric bridging mode, associated with the tungstate chains [21,22].The vibration of 310 cm −1 could be assigned to the translational mode involving simultaneous motion of Bi 3+ and WO 6  6− .The peak at about 300 cm −1 was assigned to the mode of terminal WO 2 groups [21,23].Figure 2(b) shows FTIR spectra of the as-synthesized Bi 2 WO 6 samples.The very weak band located at 3420 cm −1 can be attributed to the symmetric stretching vibration of −OH groups of adsorbed water on surface of Bi 2 WO 6 samples.The spectra present the main absorption bands at 400-1000 cm −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 −1 and the W−O band at 821.6 cm −1 [24][25][26].
The morphologies of Bi 2 WO 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 ∘ C 15 h and 300-500 nm for 180 ∘ 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 ∘ C 15 h and 20 nm for 180 ∘ C 20 h with their smooth surfaces.
Figure 4 shows TEM images of Bi 2 WO 6 samples.The assynthesized Bi 2 WO 6 at 180 ∘ 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 2 WO 6 since the  observed spot pattern corresponded to the most intense diffraction peaks of Bi 2 WO 6 .The SAED pattern reveals that the crystalline structure can be indexed to the expected orthorhombic crystal lattice of Bi 2 WO 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 ⃗  × ⃗  direction.The UV-visible absorption spectrum of Bi 2 WO 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 2 WO 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  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 2 WO 6 nanoplates are the excellent photocatalytic activity.Figure 7 shows degradation efficiency of the photocatalytic Bi 2 WO 6 samples against RhB.It indicates the obvious difference in percent degradation of RhB by both Bi 2 WO 6 samples.The control experiment in the absence of Bi 2 WO 6 catalyst under identical visible irradiation was also carried out.The results show that photodegradation of RhB by Bi 2 WO 6 nanoplates synthesized at 180 ∘ C for 20 h is higher than that by Bi 2 WO 6 nanoplates synthesized at 180 ∘ C for 15 h, due to its higher degree of crystallinity.In case of both Bi 2 WO 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 2 WO 6 nanoplates synthesized at 180 ∘ C for 20 h (surface area of 53.1 m 2 g −1 ) is 98.24% for RhB photodegradation.Comparing to the previous report, the photocatalytic activity of P25 TiO 2 is ∼18% after 120 min under visible radiation with  > 420 nm, reported by Cheng et al. [31].Lu et al. [32] synthesized octahedron-like hierarchical structure of Bi 2 WO 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 2 WO 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 2 WO 6 can be explained by e − /h + recombination [33].Generally, the e − /h + recombination in the materials is divided into two categories: volume and surface recombination.The unique 2D layers of Bi 2 WO 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 2 WO 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 − /h + recombination on the semiconductor surface is able to control with ease, because the nanosheets have large space to enhance the photochemical activity.

Figure 1 Figure 1 :
Figure 1: XRD patterns over the 2 range of 20-60 deg. of the Bi 2 WO 6 samples synthesized by the hydrothermal process (a) at 100-200 ∘ C for 20 h and (b) 180 ∘ C for 0-20 h.

Figure 2 :
Figure 2: (a) Raman and (b) FTIR spectra of Bi 2 WO 6 synthesized by hydrothermal process at 180 ∘ C for 15 h and 20 h.

Figure 3 :
Figure 3: SEM images with low and high magnifications of Bi 2 WO 6 synthesized by hydrothermal process at 180 ∘ C for (a, b) 15 h and (c, d) 20 h.

Figure 4 :
Figure 4: TEM images and SAED pattern of Bi 2 WO 6 synthesized by hydrothermal process at 180 ∘ C for (a) 15 h and (b-d) 20 h.