Photocatalysis of WO 3 Nanoplates Synthesized by Conventional-Hydrothermal and Microwave-Hydrothermal Methods and of Commercial WO 3 Nanorods

The degradation of methylene blue (MB) dye by tungsten oxide (WO 3 ) photocatalyst synthesized by the 200C conventionalhydrothermal (C-H) and 270W microwave-hydrothermal (M-H) methods and commercial WO 3 was studied under UV light irradiation for 360min. The photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, Raman spectrophotometry, and UV visible spectroscopy to determine phase, morphology, vibration mode, and optical property. The BET analysis revealed the specific surface area of 29.74, 37.25, and 33.56m/g for the C-H WO 3 nanoplates, M-H WO 3 nanoplates, and commercial WO 3 nanorods, respectively. In this research, the M-H WO 3 nanoplates have the highest photocatalytic efficiency of 90.07% within 360min, comparing to the C-HWO 3 nanoplates and even commercial WO 3 nanorods.


Introduction
In the past decade, nanostructured materials with zerodimensional quantum dots, one-dimensional nanofibers, nanotubes, and nanorods, and two-dimensional nanoplates and nanodisks have been widely synthesized and studied of their novel properties which are different from their counterparts [1].Particularly, transition metal oxide based nanostructured semiconductors have become a rapid expansion in modern materials science, physics, and chemistry [2,3].Tungsten oxide is an -type semiconductor having attractive properties, especially as photochromic and electrochromic materials, and novel potential applications for using as gas sensors [3][4][5], humidity sensors [6], electrochromic devices [7], and photocatalysts [1,5,8,9].During the last several decades, nanostructured WO 3 has been synthesized by different processes: hydrothermal method [5,9,10], microwave radiation [11], microwave plasma [12], microwave-assisted hydrothermal synthesis [8,13], and sol-gel [6,7].Most previous approaches to the synthesis of WO 3 nanomaterials such as conventional-hydrothermal and microwave-hydrothermal methods have several advantages.Komarneni [14] reported that these methods consumed less energy and were costeffective and environmental friendly, and that the products are high purified single crystal.Although the hydrothermal process is slow kinetics at any given temperature, a combination of microwave and hydrothermal systems has been used to increase the kinetics of crystallization [14].Due to the environmental remedy and energy-saving, photocatalytic semiconductors have been carried out.Sun et al. [15] described two main limitations for the wide use of the semiconductors: the low solar energy conversion efficiency due to their wide band gap and the high recombination rate of photo-induced electron-hole pairs.WO 3 as one of the photocatalytic materials has been extensively investigated, mainly due to its high activities for hydrogen evolution from water and degradation of pollutants for water treatment [9,16,17].The physical and chemical properties of the solid semiconductor surfaces are controlled by the synthesis method [18].In the previous research [13], WO 3 nanoplates were successfully synthesized by a 270 W microwave-hydrothermal reaction for 180 min and the formation mechanism was also proposed according to the experimental results.At present, photocatalysis of WO 3 nanoplates synthesized by the conventional-hydrothermal (C-H) method at 200 ∘ C for 12 h, WO 3 nanoplates synthesized by the 270 W microwave-hydrothermal (M-H) method for 180 min [13], and commercial (com) WO 3 nanorods was investigated by determining the photodegradation of methylene blue (MB) dye under UV light irradiation.O) were dissolved in 40 mL deionized water with 10 min vigorous magnetic stirring till complete dissolution.Subsequently, 37% HCl was dropped to the solution until pH reaching 1.Then the solution was vigorously stirred by a magnetic stirrer for 30 min until the turbid yellow solution was obtained.Each of the turbid yellow solutions was transferred into Teflon lined stainless steel autoclaves, which were heated at 120, 160, and 200 ∘ C for 12 h.In the end, the autoclaves were naturally cooled to room temperature.The as-synthesized yellow precipitates were washed with ethanol and distilled water three times and dried at 80 ∘ C for 24 h for further studies.The system was also processed by the 270 W microwave-hydrothermal reaction for 180 min.

Characterization.
The final products were characterized by Rigaku MiniFlex X-ray diffractometer with Cu-K  radiation ( = 1.54178Å) ranging from 10 ∘ to 80 ∘ , in combination with database of the Joint Committee on Powder Diffraction Standards (JCPDS) [19].The scanning electron microscopic (SEM) images were carried out by field emission scanning electron microscopy (JEOL JSM-6335F).Transmission electron microscopic (TEM) and high-resolution transmission electron microscopic (HRTEM) images and selected area electron diffraction (SAED) pattern were taken on a transmission electron microscope (JEOL JEM-2010) accelerating at a voltage of 200 kV.Fourier transform infrared (FTIR) spectra were recorded on a Bruker Tensor 27 spectrometer with KBr as a diluting agent and operated in the range of 400-4000 cm −1 .Raman spectra were recorded on a T64000 HORIBA Jobin Yvon spectrometer using a 50 mW and 514.5 nm wavelength Ar green laser.UV-visible absorption spectra were recorded on a Lambda 25 Perkin Elmer spectrometer using a UV lamp with 1 nm resolution ranging from 300 to 1000 nm.The Brunauer-Emmett-Teller (BET) surface area was determined by a Quantachrome Autosorb 1-MP.a methylene blue (C 16 H 18 N 3 SCI, MB) aqueous media under UV radiation.The initial concentration of 100 mL aqueous MB solution was set as 1 × 10 −5 mol/L and loaded with 0.1 g WO 3 .The suspension was irradiated by a 18 W UV lamp with the average light intensity of 50.7 W/m 2 .The concentration of MB was traced by UV-visible spectroscopy (Lambda 25 Perkin Elmer) in the range of 450-900 nm and the absorbance at the characteristic band of 664 nm was used to determine MB concentration.space group at the synthesis temperatures of 160 and 200 ∘ C for 12 h.At 120 ∘ C, the product was composed of mixed phases of monoclinic WO 3 and orthorhombic WO 3 ⋅H 2 O (JCPDS No. 18-1418) [19].Upon increasing the operation temperature to 160 and 200 ∘ C, pure monoclinic phase of WO 3 was detected.Their XRD intensity peaks were strengthened in sequence with the increase in the test temperature from 160 ∘ C to 200 ∘ C. In addition, the diffraction peaks of WO 3 synthesized by 270 W microwave-hydrothermal processed for 180 min [13] are also identical to the monoclinic phase of the JCPDS No. 72-1465 [19].For commercial WO 3 , the XRD pattern confirmed that the powder was composed of orthorhombic phase of WO 3 (JCPDS No. 20-1324) [19].

SEM.
Morphologies of the products synthesized at 120, 160, and 200 ∘ C for 12 h were characterized by SEM as shown in Figures 2(a)-2(c).It can be clearly seen that irregular nanoplates of the products gradually transformed into completely rectangular shape by increasing the synthesis temperature.The particles were randomly oriented in different directions as those of the previous report [13].The C-H synthesis surface of WO 3 was smoother than the M-H synthesis one (Figure 2(d)).Some traces of cracks were also detected on the M-H rectangular nanoplates.These cracks can play the role in promoting the product porosity.In this research, the C-H nanoplates are 50-100 nm thick, thicker than WO 3 nanoplates of the M-H.These imply that stability It should be noted that the growth of WO 3 nanoparticles could be changed during phase transformation and was controlled by citric acid [13].The SEM image of commercial WO 3 was also shown in Figure 2(e).It was composed of nanoparticles clustered together in the shape of nanorods with <100 nm diameter and ∼200 nm length.

UV-Visible Absorption.
UV-visible absorption of the products synthesized by the C-H at different temperatures for 12 h and commercial WO 3 is shown in Figure 5.For crystalline semiconductors, the UV absorption near band edge follows the following Wood and Tauc equation [28]: where  is the absorbance, ℎ is the Planck constant, ] is the photon frequency,   is the energy gap, and  is a pure number associated with the different types of charged transition.The transitions are directly allowed, indirectly allowed, directly forbidden, and indirectly forbidden for  equals 1/2, 2, 3/2, and 3, respectively.The absorption was controlled by two photon energy ranges relative to energy gap.For ℎ] >   , absorption is linearly increased with the increasing of photon energy caused by the transition of electrons from the topmost occupied state of valence band to the bottommost unoccupied state of conduction band.For ℎ] <   , the absorption curves are different from linearity, caused by charged transition relating to defects.In the present research, direct energy gaps of the products synthesized by the C-H at 120, 160, and 200 ∘ C for 12 h were determined to be 2.60, 2.96, and 3.22 eV, respectively [2,29,30].In addition, the direct band gap of commercial WO 3 was determined to be 2.63 eV [8].

BET Analysis.
The BET analysis was used to determine surface area of the products (Figure 6 in the photocatalytic activity but the crystalline composition of the photocatalyst also shows an important effect [31].The crystalline structure and morphology of the M-H and C-H WO 3 (monoclinic, nanoplates) are difference from those of commercial WO 3 (orthorhombic, nanorods).They also have the influence to control the photocatalytic rate.The Langmuir-Hinshelwood (L-H) kinetics model was used to investigate the degradation of MB solution and the pseudofirst-order rate equation was given by [18] ln (     ) =  app , where   is the equilibrium concentration after absorption,   is the concentration of MB at time , and  app represents the apparent pseudo-first-order rate constant of initial degradation.The pseudo-first-order rate constant ( app ) was calculated from the slope of the ln (  /  ) versus irradiation time () shown in Figure 6(f).The rate constant of MB degradation was 0.00599 min −1 , 8.38072 × 10 −4 min −1 , and 0.00138 min −1 in the solutions containing the M-H WO 3 , C-H WO 3 , and commercial WO 3 , respectively.

Conclusions
In summary, WO 3 nanoplates were successfully synthesized by the citric acid-assisted conventional hydrothermal reaction at 200 ∘ C for 12 h.Phase, morphology, and optical properties of the products were investigated.The photocatalytic property of the C-H WO 3 was evaluated by identifying the degradation of MB dye under UV irradiation and compared with the M-H WO 3 and commercial WO 3 .The results indicated that the M-H WO 3 nanoplates showed the highest efficiency for the degradation of MB dye at the rate of 90.07%under UV illumination within 360 min, corresponding to the BET surface area analysis.
WO 3   nanoplates synthesized by the M-H and C-H methods and the commercial WO 3 (Merck) nanorods were tested in

Figure 1 :
Figure 1: XRD patterns of the products synthesized by the C-H at 120, 160, and 200 ∘ C for 12 h and M-H at 270 W for 180 min and commercial (com) WO 3 .

Figure 2 :
Figure 2: SEM images of the products, respectively, synthesized by the (a)-(c) C-H at 120, 160, and 200 ∘ C for 12 h and (d) M-H at 270 W for 180 min and (e) commercial WO 3 .

Figure 3 :
Figure 3: (a) TEM image, (b) SAED pattern, and (c) HRTEM image of WO 3 synthesized by the C-H at 200 ∘ C for 12 h.

( 2 0
0) crystallographic planes of WO 3 (JCPDS No. 72-1465)[19], respectively.3.4.FTIR.The FTIR spectra (Figure4(a)) provided further insight into the structure of the products synthesized by the C-H at different temperatures for 12 h and commercial WO 3 .At 120 ∘ C synthesis, the major vibration modes associated with O-H stretching of residual water was detected at 3655-3122 cm −1 , C=O stretching modes at 1626 cm −1 , C-O stretching modes of carboxyl at 948 cm −1 , O-W-O stretching modes at 814 and 746 cm −1 , and W-O-W stretching modes at 669 cm −1[20][21][22][23].Upon increasing the temperature from 120 ∘ C to 160 ∘ C and 200 ∘ C, the O-H and C=O stretching modes were no longer detected.For commercial WO 3 , the observed peak was assigned to W-O bonding.

Figure 6 :
Figure 6: (a) BET surface areas of the products synthesized by the C-H at 120, 160, and 200 ∘ C for 12 h and M-H at 270 W for 180 min and commercial (com) WO 3 .(b)-(d) UV-visible absorption spectra and (e), (f) photocatalytic degradation kinetics of MB dye by the WO 3 photocatalysts of the 200 ∘ C C-H and 270 W M-H and the commercial WO 3 irradiated with UV light for different lengths of time.
2.1.Synthesis.To synthesize WO 3 nanoplates, 1 mmol sodium tungstate (Na 2 WO 4 ⋅2H 2 O, 99.0%) and 0.2 g citric acid (C 6 H 8 O 7 ⋅H 2 (a)).It was found that pure WO 3 of the M-H (37.25 m 2 /g) has larger surface area than the product of the C-H and commercial WO 3 (33.56m 2 /g).Surface area of the product synthesized by the C-H at 120 ∘ C for 12 h (WO 3 ⋅H 2 O) was determined to be 30.65 m 2 /g.Pure WO 3 products at 160 and 200 ∘ C for 12 h were determined to be 27.87 and 29.74 m 2 /g, respectively.The M-H processed WO 3 nanostructure has different size distributions due to thermal stress during rapid heating, and the M-H product is also shown as rough surface.The traces of cracks on the M-H nanoplates and small debris separation have the influence to increase surface area.
3photocatalyst of the C-H decreases slower than WO 3 of the M-H and commercial one under UV radiation.The degradation efficiencies for the M-H, commercial, and C-H photocatalysts were 90.07%, 42.53%, and 25.80% within 360 min irradiation, respectively.These results corresponded with the BET analysis.Not only surface area plays the role