High Photocatalytic Activity under Visible Light for a New Morphology of Bi2WO6 Microcrystals

In this work, a new morphology was obtained for bismuth tungstate (Bi2WO6-glyc) using a hydrothermal method with the addition of glycerol as a surfactant. In order to compare, the bismuth tungstate without glycerol as the surfactant, i.e., Bi2WO6, was synthesized. Structural characterization by XRD and Rietveld refinement confirmed the orthorhombic structure as a single phase for all samples with high crystallinity. All active modes in Raman spectroscopy for the orthorhombic phase of bismuth tungstate were confirmed in agreement with XRD analysis. N2 adsorption/desorption and size pore distribution confirmed the high surface area (85.7 m2/g) for Bi2WO6-glyc when compared with Bi2WO6 (8.5 m2/g). The optical band gap by diffuse reflectance was 2.78 eV and 2.88 eV for Bi2WO6-glyc and Bi2WO6, respectively. SEM images confirmed the different morphology for these materials, and microstructures with cheese crisp were observed for Bi2WO6-glyc (cheese crisp). On the other hand, flower-like microcrystals were confirmed for Bi2WO6 sample. The photocatalytic performance of Bi2WO6-glyc (94.2%) in the photodegradation of rhodamine B (RhB) dye solutions at 60 min was more expressive than Bi2WO6 (81.3%) and photolysis (8.2%) at 90 min.


Introduction
In recent decades there has been reported an increase in wastewater containing organic pollutants and other compounds derived from industrial production and populations, which represent a risk due to their exhibiting high stability, carcinogenic effects and causing a decrease of oxygen gas in aquatic ecosystems [1]. Among these, organic textile dyes are commonly mentioned as the main compounds present in wastewater and are produced after industrial activity related to dyeing textile fibers, natural fibers, and paper; they are also stable to biodegradation processes [2]. Hence, the development of some methodologies for wastewater treatment have attracted significant attention around the world [3,4]. In this context, heterogeneous photocatalysis, in which it is possible to use solar energy as a source for oxidation/reduction reactions promoted after absorption of photons by photocatalysts in an aqueous medium, is considered an important option for wastewater treatment [1]. TiO 2 is well-known as an efficient photocatalyst for application in the photodegradation of organic compounds and pathogens by redox processes. However, due to the large optical band gap of TiO 2 (3.2 eV), the excitation/recombination process for electrons is more effective when using ultraviolet radiation, which represents only 4% of solar light. Thus, the development of visible-light-driven photocatalysts have been studied for applications using solar energy [5,6].
Bismuth tungstate (Bi 2 WO 6 ) is a member of the Aurivillius family (n = 1), which has the general formula Bi 2 A n-1 B n O 3n+3 (where A = Ca, Sr, Ba, Pb, Bi, Na, or K and B = Ti, Nb, Ta, Mo, W, or Fe), and has also been described as a semiconductor prepared by a combination of layers alternately containing (Bi 2 O 2 ) 2+ perovskite-like type and (WO 4 ) 2ions. Due to its exhibiting excellent physical and chemical properties, including effective absorption of photons from the visible-light region, Bi 2 WO 6 has attracted attention in photocatalytic applications [5][6][7].
The size and morphology of catalysts are important factors to be considered in photocatalytic performance [8]. Thus, the investigation of morphological aspects is frequently reported, mainly as a result of the effects promoted by surfactants [9]. Commonly, bismuth tungstate (Bi 2 WO 6 ) prepared by the hydrothermal method is associated with the formation of nanoplates linked to anisotropic growth along the (001) plane [9,10].
In recent years, an impressive number of publications reporting the preparation of Bi 2 WO 6 photocatalysts using different methods has arisen [11][12][13]. It is obvious that the strong electrostatic attraction of the WO 4 2+ and Bi 3+ ions in aqueous solution may result in different morphologies of Bi 2 WO 6 crystals according to the adjustment of the experimental conditions adopted [8]. Microspheres with a flower-like morphology have therefore been commonly reported using a template-free hydrothermal method [9]. Using the same method, Zhang et al. [14] have reported obtaining flower-like microspheres using the hydrothermal method in the absence of surfactants [14]. Moreover, Dai et al. [15] have acquired Bi 2 WO 6 hollow sphere microcrystals using the hydrothermal route with addition of poly(vinylpyrrolidone) as a surfactant. Shang et al. [12] have obtained nanocage-like microcrystals for Bi 2 WO 6 as a single morphology using carbon spheres.
In this work, we report an easy and fast way to obtain Bi 2 WO 6 microcrystals with a new morphology using glycerol as a surfactant which has not until now been reported in the literature. Moreover, we evaluated the photocatalytic efficiency under simulated visible light using the photodegradation of Rhodamine B dye, as an application of water treatment. Figure 1a,c show the XRD patterns and Rietveld refinement plots of Bi 2 WO 6 and Bi 2 WO 6 -glyc crystals synthesized by the hydrothermal method.
Structural analysis by Rietveld refinement method for Bi 2 WO 6 ( Figure 1b) and Bi 2 WO 6 -glyc ( Figure 1c) was performed using Fullprof software [21], which confirmed the orthorhombic structure as the only phase present. It is interesting to note that these crystals exhibit a high degree of crystallinity and purity without diffraction peaks associated with precursors or secondary phases. The excellent concordance of the experimental data with the crystallographic information contained in ICSD card number 67647 was confirmed by checking the residual line (Y obs -Y cal ) and R parameters (R p , R wp , R exp , and χ 2 ), which suggest that these refinements are very available [22,23].  Figure 1. (a) XRD patterns of bismuth tungstate synthesized without (Bi2WO6) and with the addition of (Bi2WO6-glyc) glycerol, and Rietveld refinement plots for (b) Bi2WO6 and (c) Bi2WO6-glyc.
Structural analysis by Rietveld refinement method for Bi2WO6 ( Figure 1b) and Bi2WO6-glyc ( Figure 1c) was performed using Fullprof software [21], which confirmed the orthorhombic structure as the only phase present. It is interesting to note that these crystals exhibit a high degree of crystallinity and purity without diffraction peaks associated with precursors or secondary phases. The excellent concordance of the experimental data with the crystallographic information contained in ICSD card number 67647 was confirmed by checking the residual line (Yobs -Ycal) and R parameters (Rp, Rwp, Rexp, and χ 2 ), which suggest that these refinements are very available [22,23].
Table S1 (see supplementary electronic information) summarizes the Rietveld refinement results for the atomic position (x, y, and z) of bismuth, tungsten, and oxygen atoms, as well as the occupation (Occ) and anisotropic thermal factors for Bi2WO6 and Bi2WO6-glyc. In this The average crystallite size (Dhkl) for Bi2WO6 and Bi2WO6-10 was calculated using Debye-Scherrer's equation [16], Dhkl = Kλ/βcos(θ), where, K is the shape factor (K = 0.9, spherical shape), λ is the wavelength of the CuKα radiation, β is the full width at half maximum (FWHM), and θ is the angle for each diffraction peak. In this study we used all the diffractions peaks contained in the XRD patterns for the Bi2WO6 and Bi2WO6-glyc crystals. Table 1 presents the lattice parameters (a, b, and c), direct cell volume (V), crystallite size (Dhkl), and for Bi2WO6 and Bi2WO6-glyc the Rietveld refinement results and values contained in ICSD card number 67647 and reported in the literature [12].
The results show that all crystallographic parameters are in good agreement with those reported in the literature [12] and contained in ICSD card number 67647. Moreover, it was observed that the lattice parameters decreased with the addition of glycerol as the surfactant in the synthesis of Bi2WO6glyc (a = 5.437(1) Å, b = 16.433(4) Å, and c = 5.457(9) Å) when compared to the absence of this, which was represented by Bi2WO6 (a = 5.443(3) Å, b = 16.428(1) Å, and c = 5.451(2) Å). In addition, there was a decrease in the direct cell volume and crystallite size from 487.66(4) Å 3 to 487.45(8) Å 3 and 20.153(80) nm to 24.761(78) nm, respectively.
Mishra et al. [24] have reported on the contribution of the addition of organic surfactant in the production of inorganic compounds, mainly in relation to the control of morphology, size, and velocity of the formation of nanocrystals. Therefore, the addition of glycerol in the synthesis of  The average crystallite size (D hkl ) for Bi 2 WO 6 and Bi 2 WO 6 -10 was calculated using Debye-Scherrer's equation [16], D hkl = Kλ/βcos(θ), where, K is the shape factor (K = 0.9, spherical shape), λ is the wavelength of the CuKα radiation, β is the full width at half maximum (FWHM), and θ is the angle for each diffraction peak. In this study we used all the diffractions peaks contained in the XRD patterns for the Bi 2 WO 6 and Bi 2 WO 6 -glyc crystals. Table 1 presents the lattice parameters (a, b, and c), direct cell volume (V), crystallite size (D hkl ), and for Bi 2 WO 6 and Bi 2 WO 6 -glyc the Rietveld refinement results and values contained in ICSD card number 67647 and reported in the literature [12]. Table 1. Lattice parameters (a, b, and c), direct unit cell volume, and crystallite size (D hkl ) of Bi 2 WO 6 and Bi 2 WO 6 -10 crystals and reported values by references.

ID
Lattice The results show that all crystallographic parameters are in good agreement with those reported in the literature [12] and contained in ICSD card number 67647. Moreover, it was observed that the lattice parameters decreased with the addition of glycerol as the surfactant in the synthesis of Bi 2 WO 6 -glyc Mishra et al. [24] have reported on the contribution of the addition of organic surfactant in the production of inorganic compounds, mainly in relation to the control of morphology, size, and velocity of the formation of nanocrystals. Therefore, the addition of glycerol in the synthesis of Bi 2 WO 6 -glyc promotes the increase of stability in the process of the creation of crystalline structures and the decrease of particle size.

Spectroscopic Properties
Group theory for bismuth tungstate with an orthorhombic structure and point group of symmetry C 2v confirm 105 vibrational modes in infrared and Raman spectroscopy which are represented by the irreducible representation Г (IR+Raman) = 26A 1 + 27A 2 + 26B 1 + 26B 2 [9]. However, the A 2 vibrational modes can be identified only in Raman spectroscopy, while A 1 , B 1 , and B 2 can be identified with both vibrational spectroscopies [16,20].
The experimental Raman spectra in the range from 85 to 1000 cm −1 and [F(R)hν] 2 against a photon energy plot by uv-vis using the diffuse reflectance of Bi 2 WO 6 and Bi 2 WO 6 -glyc crystals and the experimental Raman spectrum of glycerol are shown in Figure 2a

Spectroscopic Properties
Group theory for bismuth tungstate with an orthorhombic structure and point group of symmetry C2v confirm 105 vibrational modes in infrared and Raman spectroscopy which are represented by the irreducible representation Г(IR+Raman) = 26A1 + 27A2 + 26B1 + 26B2 [9]. However, the A2 vibrational modes can be identified only in Raman spectroscopy, while A1, B1, and B2 can be identified with both vibrational spectroscopies [16,20].
The experimental Raman spectra in the range from 85 to 1000 cm -1 and [F(R)hν] 2 against a photon energy plot by uv-vis using the diffuse reflectance of Bi2WO6 and Bi2WO6-glyc crystals and the experimental Raman spectrum of glycerol are shown in Figure 2a,b.
In Figure 2a, the experimental Raman spectra of Bi2WO6 and Bi2WO6-glyc exhibit a similar profile, suggesting that there were no active modes associated with the presence of the remaining glycerol, as can be seen when compared with the experimental Raman spectra of glycerol (Figure 2a) from the synthesis. Thus, all characteristic peaks of Bi2WO6 were identified as being in the range 85 to 1000 cm -1 [24]. The active modes at 792 cm -1 and 827 cm -1 are signatures of bismuth tungstate and may be attributed to the antisymmetric modes and symmetric modes of the O-W-O bonds present in the octahedral clusters [WO6], respectively [16]. In 180 cm -1 , 230 cm -1 , and 500 cm -1 , it is possible to identify couplings of the active modes associated with distortions of the bonds present in the octahedra composed of tungsten and oxygen (WO6) and in 332 cm -1 deltahedral clusters of bismuth and oxygen (Bi-O) [20,24,25]. The peak at 306 cm -1 is associated with the simultaneous translation movements of Bi 3+ and WO6 6-along the crystal lattice [16,24]. In Figure 2a, the experimental Raman spectra of Bi 2 WO 6 and Bi 2 WO 6 -glyc exhibit a similar profile, suggesting that there were no active modes associated with the presence of the remaining glycerol, as can be seen when compared with the experimental Raman spectra of glycerol (Figure 2a) from the synthesis. Thus, all characteristic peaks of Bi 2 WO 6 were identified as being in the range 85 to 1000 cm −1 [24]. The active modes at 792 cm −1 and 827 cm −1 are signatures of bismuth tungstate and may be attributed to the antisymmetric modes and symmetric modes of the O-W-O bonds present in the octahedral clusters [WO 6 ], respectively [16]. In 180 cm −1 , 230 cm −1 , and 500 cm −1 , it is possible to identify couplings of the active modes associated with distortions of the bonds present in the octahedra composed of tungsten and oxygen (WO 6 ) and in 332 cm −1 deltahedral clusters of bismuth and oxygen (Bi-O) [20,24,25]. The peak at 306 cm −1 is associated with the simultaneous translation movements of Bi 3+ and WO 6 6− along the crystal lattice [16,24].
The optical band gap of Bi 2 WO 6 and Bi 2 WO 6 -glyc was performed by UV-Vis using the diffuse reflectance spectra (Figure 2b). In this study, we adopted the Kubelka-Munk model [26,27], as represented by the equation [F(R)hν] 1/n = C 1 (E gap -hν), where F(R) is the Kubelka-Munk function, hν is the energy of a photon, and C 1 is the proportionality constant.
While n is the constant associated with different types of electronic transition, in this study direct allowed transitions (n = 0.5), which are associated with the electron transitions from hybrid Bi 6s-O 2p in the valence band (VB) to the W 5d orbitals in the conduction band (CB), were considered [28]. The E gap values 2.88 eV (Bi 2 WO 6 ) and 2.78 eV (Bi 2 WO 6 -glyc) were obtained via the plot of [F(R)hν] 2 against photon energy from the intercept of the tangent to the paraboloid curve.
The   [30] confirm that a significant percentage of microcrystals exhibited size diameter and size width in the ranges 1.1 to 1.5 µm and 0.35 to 0.6 µm, respectively. It is interesting to note that this morphology has not been reported until now [17,19,30,31]. On the other hand, the Bi 2 WO 6 microcrystals exhibit flower-like morphology (Figure 3d) [17,30] with the majority percentage of microcrystals in the range 3 to 5 µm for size diameter and 0 to 200 nm for size width. The optical band gap of Bi2WO6 and Bi2WO6-glyc was performed by UV-Vis using the diffuse reflectance spectra (Figure 2b). In this study, we adopted the Kubelka-Munk model [26,27]

, as represented by the equation [F(R)hν] 1/n = C1(Egap -hν), where F(R) is the Kubelka-Munk function, hν is the energy of a photon, and C1 is the proportionality constant.
While n is the constant associated with different types of electronic transition, in this study direct allowed transitions (n = 0.5), which are associated with the electron transitions from hybrid Bi 6s-O 2p in the valence band (VB) to the W 5d orbitals in the conduction band (CB), were considered [28]. The Egap values 2.88 eV (Bi2WO6) and 2.78 eV (Bi2WO6-glyc) were obtained via the plot of [F(R)hν] 2 against photon energy from the intercept of the tangent to the paraboloid curve.
The lesser value for the Egap obtained for Bi2WO6-glyc indicates the presence of intermediate levels between the VB and CB associated with deformation of the W-O and Bi-O bonds in the [BiO6] and [WO4] clusters and oxygen vacancies, which was previously observed in the Rietveld refinement results. However, these values are in good agreement with the values reported in [29] and [28].  [30] confirm that a significant percentage of microcrystals exhibited size diameter and size width in the ranges 1.1 to 1.5 μm and 0.35 to 0.6 μm, respectively. It is interesting to note that this morphology has not been reported until now [17,19,30,31]. On the other hand, the Bi2WO6 microcrystals exhibit flower-like morphology ( Figure 3d) [17,30] with the majority percentage of microcrystals in the range 3 to 5 μm for size diameter and 0 to 200 nm for size width. The behavior associated with the formation of Bi2WO6 microcrystals suggests that in the absence of surfactant, the obtention of nanoparticles was initiated by the strong electrostatic attraction between the Bi +3 and WO4 -2 ions. In this case, nanoplates with size thickness of 146.55(13) nm ( Figure   Figure 3. SEM images of (a) Bi 2 WO 6 -glyc and (d) Bi 2 WO 6 and histograms for size diameter and size width for Bi 2 WO 6 -glyc (b,c) and Bi 2 WO 6 (e,f).

Morphology of Bi2WO6 and Bi2WO6-glyc Microcrystals
The behavior associated with the formation of Bi 2 WO 6 microcrystals suggests that in the absence of surfactant, the obtention of nanoparticles was initiated by the strong electrostatic attraction between the Bi +3 and WO 4 −2 ions. In this case, nanoplates with size thickness of 146.55(13) nm (Figure 3f) were formed. In the second step, stable flower-like mesostructures (Figure 3e) were conducted by self-assembled nanoplates conducted by high temperature (180 • C) and pressure of the system. Conversely, the reaction processed using the glycerol as a surfactant was carried out by the slow attraction of the Bi +3 and WO 4 −2 ions. There was therefore a slow and gradual formation of nanoparticles due to the increase in the viscosity of the aqueous medium by the addition of glycerol.
The experimental results suggest that the obtention of crispy cheese chip morphology was initiated by the self-assembling effect of the nanoparticles, followed by compaction of these nanoparticles to disordered agglomerates with a high density of voids under pressure and temperature.

Catalytic Performance
The catalytic performance for the Bi 2 WO 6 and Bi 2 WO 6 -glyc microcrystals under visible light in the photodegradation of rhodamine B (RhB) dye and photolysis is shown in Figure 4a-d. In order to evaluate the contribution of photolysis, within this process an experiment was performed with RhB dye solution under visible light (Figure 4a) without a catalyst for 90 min. Therefore, a significant decrease in the maximum absorbance (λ max = 554 nm) characteristic of chromophore groups of RhB dye was not observed.
The experimental results suggest that the obtention of crispy cheese chip morphology was initiated by the self-assembling effect of the nanoparticles, followed by compaction of these nanoparticles to disordered agglomerates with a high density of voids under pressure and temperature.

Catalytic Performance
The catalytic performance for the Bi2WO6 and Bi2WO6-glyc microcrystals under visible light in the photodegradation of rhodamine B (RhB) dye and photolysis is shown in Figure 4a-d. In order to evaluate the contribution of photolysis, within this process an experiment was performed with RhB dye solution under visible light (Figure 4a) without a catalyst for 90 min. Therefore, a significant decrease in the maximum absorbance (λmax = 554 nm) characteristic of chromophore groups of RhB dye was not observed.
Moreover, when experiments were carried out with the addition of Bi2WO6 (Figure 4b) or Bi2WO6-glyc microcrystals (Figure 4c) in solution, a significant decrease was observed as a result of oxidation of the RhB molecules by oxidant species. In general, the hydroxyl radicals (HO ⦁ ), holes (h + ), and superoxide radicals (O2 ⦁− ), formed in the excitation/recombination process after absorption of a visible light photon by bismuth tungstate. However, the catalytic performance for Bi2WO6-glyc was more significant than compared with Bi2WO6 microcrystals, which in this case were associated with the lowest optical band gap (2.75 ± 01 eV), surface area, and morphology exhibited for the Bi2WO6 microcrystals.

Photolysis
Bi 2 WO 6 (cheese crisp) Bi 2 WO 6 (flower)  Moreover, when experiments were carried out with the addition of Bi 2 WO 6 ( Figure 4b) or Bi 2 WO 6 -glyc microcrystals (Figure 4c) in solution, a significant decrease was observed as a result of oxidation of the RhB molecules by oxidant species. In general, the hydroxyl radicals (HO ), holes (h + ), and superoxide radicals (O 2 − ), formed in the excitation/recombination process after absorption of a visible light photon by bismuth tungstate. However, the catalytic performance for Bi 2 WO 6 -glyc was more significant than compared with Bi 2 WO 6 microcrystals, which in this case were associated with the lowest optical band gap (2.75 ± 01 eV), surface area, and morphology exhibited for the Bi 2 WO 6 microcrystals. Figure 4d shows the relative degradation (C/C 0 ) of RhB dye for all photocatalytic assays performed [30]. C 0 and C are the initial concentration and the concentration at the desired reaction time, respectively. Thus, the first 30 min was carried out without visible light (only magnetic stirring) Catalysts 2019, 9, 667 7 of 11 to obtain the adsorption equilibrium between the microcrystals and RhB dye molecules. It is important to note that the adsorption rates for Bi 2 WO 6 and Bi 2 WO 6 -glyc were 25.2% and 29.4%, respectively. Table 2 summarizes the results for all catalytic assays performed with and without bismuth tungstate as the catalyst and the results reported in the literature for both materials. In this study, the pseudo first-order equation ln(C/C 0 ) = -k obs t, which is used in the Langmuir-Hinshelwood model [24] was applied for adjustment of experimental values and comprehension of catalytic behavior for each catalyst performed in the degradation of the RhB dye solution. In this equation, -k obs is the observed reaction rate constant and t is the time. Legend: ID = identification; E gap = optical band gap; observed reaction rate constant (k obs ); t 1/2 = half-life time.
The degradation rates for RhB dye by photolysis and Bi 2 WO 6 were 8.2% and 81.3%, respectively, after 90 min under exposure to visible light. Ninety-four point two percent of the degradation rate of RhB dye solution was confirmed for Bi 2 WO 6 -glyc at 60 min under visible light. The high catalytic performance noted for the Bi 2 WO 6 -10 microcrystals was associated with a lower optical band gap (E gap = 2.78 eV), superficial area (85.7 m 2 /g), the morphology of microcrystals, and the deformation of Bi-O and W-O bonds and oxygen vacancies in the structure, which was confirmed using structural analysis by XRD and Raman spectroscopy.
The mechanism for the oxidation process of organic molecules by oxidant species from the excitation/recombination processes in Bi 2 WO 6 after the absorption of a photon has been reported in the literature [13,15]. In this study, the proposed mechanism was obtained by the initial absorption of a photon in visible light by the electrons present in the valence band associated with the small band gap value (E gap = 2.78 eV) and high surface area (85.7 m 2 /g). The water molecules adsorbed on the surface of Bi 2 WO 6 -glyc were oxidized by holes (h + ) to hydroxyl radicals (HO ) and cations (H + ). On the other hand, the electrons present in the conduction band were captured by the adsorbed oxygen molecules on the surface of Bi 2 WO 6 -glyc, which were reduced to superoxide radicals (O2 − ) [36]. These processes are demonstrated in the following equations: The high oxidative potential of these species promoted the attack of the RhB dye strands that underwent deacetylation of the original molecule by breaking the bonds into lower molecular weight (CCO) colorless byproducts and the subsequent mineralization of these molecules into water molecules and carbon dioxide gases, oxygen, and carbon monoxide. Figure 4a it is possible to see that the degradations of RhB dye molecules also occurred under the action of visible light only; however, this occurred under a low conversion rate which explains the 8.2(5)% degradation over 90 min exposure to visible light result. In addition, there was interaction of dye chains on the surface of the microcrystals, which underwent the degradation by the action of holes, a process which has been reported in the literature [37].
The reusability of Bi 2 WO 6 -glyc microcrystals ( Figure S1, see supplementary electronic information) in the photodegradation of RhB dye solution was performed over five consecutive photocatalytic runs. Thus, it was noted that the RhB dye solution was degraded at 90 min for all five runs and that these microcrystals were demonstrated to be stable and exhibiting high photocatalytic performance after all cycles.

Synthesis of Bi 2 WO 6
Using a typical hydrothermal method [36], 0.9701g of Bi(NO 3 ) 3 x5H 2 O (Sigma-Aldrich, San Luis, MO, USA) was dispersed in 20 mL of distilled water under magnetic stirring for 15 min. Then, 20 mL of an aqueous solution of Na 2 WO 4 x2H 2 O (0.3218g) (Sigma-Aldrich, San Luis, MO, USA) was added. The mixture was stirred for another 15 min and transferred to a Teflon-lined autoclave which was kept heated at 170 • C for 24 h. The product was collected by filtration, washed with distilled water and absolute ethanol several times, and then dried at 80 • C in air and calcined at 550 • C for 4 h.

Obtention of Bi 2 WO 6 -glyc Microcrystals
The synthesis was typically performed by dissolving 0.9708 g of Bi(NO 3 ) 3 .xH 2 O (Sigma-Aldrich, San Luis, MO, USA) into 20 mL of HNO 3 (Synth) aqueous solution (50 mM) under magnetic stirring for 15 min. Then, 0.3334g Na 2 WO 4 ·xH 2 O (Sigma-Aldrich, San Luis, MO, USA) aqueous solution (20 mL) was added dropwise. After 15 min of stirring, 10 mL glycerol P.A. (Impex) was added to the suspension. The solution was transferred into a Teflon-lined autoclave and subjected to hydrothermal synthesis at 150 • C for 20 h. The product was collected by filtration, washed with distilled water and ethanol several times, and dried at 80 • C in air and calcined at 550 • C for 4 h.

Characterization
The morphology was analyzed by scanning electron microscopy using a TESCAN model VEJA 3 SBU microscope with a voltage of 20 kV. X-ray diffraction was performed using a PANalytical PERT PRO MPD (PW 3040/60) diffractometer operating with CuKα (λ = 1.5418 Å) using the powder method. Structural refinement was performed using the free software Fullprof (June 2018 version) using the Pseudo-Voigt function and a six polynomial degree in the profile adjustment of the diffraction and background peaks, respectively.
UV-Vis spectra for each sample were recorded in the range of 200 nm to 900 nm using a Shimadzu spectrometer (model UV 2600 UV-Vis) under diffuse reflectance with BaSO 4 as a reference. The surface area was determined using the Brunauer-Emmett-Teller (BET) method using a Micromeritics Instrument Corporation TriStar II model 3020 machine. Raman spectroscopy was performed with a Horiba (model T6400) spectrometer with line excitation at 514 nm with an Ar + laser and power of 20 mW.

Photocatalytic Tests
The photocatalytic activities of bismuth tungstate samples were evaluated by the degradation of RhB under simulated sunlight radiation using a 400 W metal vapor lamp with flow of 32,000 lm and efficiency of 80 lm/W with intensity of irradiation, in w/m 2 , through an ICEL SP-2000 solar intensity meter. A glass plate was used as an ultraviolet radiation filter.
In each experiment, 0.1 g of Bi 2 WO 6 or Bi 2 WO 6 -glyc was added to 100 mL of RhB solution (10 mg/L). The suspensions were stirred in the dark for 30 min to obtain an adsorption-desorption equilibrium of the photocatalyst sample with RhB molecules. Subsequently, the solution was exposed to visible light irradiation under magnetic stirring with constant air flow. At regular time intervals, 3 mL aliquots were collected, centrifuged, and analyzed by maximal absorption (553 nm) in the UV-vis spectra of RhB using a thermo evolution array spectrophotometer. The degradation rate (D%) was measured using the equation where C 0 is the initial concentration and C is the concentration at a given time.

Conclusions
In summary, bismuth tungstate microcrystals (Bi 2 WO 6 -glyc) photocatalysts were synthesized using the hydrothermal method with the addition of glycerol as a surfactant. XRD and Raman analysis confirmed the obtention of an orthorhombic phase with a high crystallinity degree. The morphology obtained for microcrystals was provided by compaction of nanocrystals initially directed by the glycerol effect, which created materials with a high surface area. The photocatalytic activity of catalysts was evaluated in photodegradation of rhodamine B dye, obtaining 94.2(8)% degradation of RhB dye solution at 60 min under visible light. Thus, such photocatalysts possess great potential for application in water treatment.