The degradation of methylene blue (MB) dye by tungsten oxide (WO3) photocatalyst synthesized by the 200°C conventional-hydrothermal (C-H) and 270 W microwave-hydrothermal (M-H) methods and commercial WO3 was studied under UV light irradiation for 360 min. 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 33.56 m2/g for the C-H WO3 nanoplates, M-H WO3 nanoplates, and commercial WO3 nanorods, respectively. In this research, the M-H WO3 nanoplates have the highest photocatalytic efficiency of 90.07% within 360 min, comparing to the C-H WO3 nanoplates and even commercial WO3 nanorods.

1. Introduction

In the past decade, nanostructured materials with zero-dimensional 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 [35], humidity sensors [6], electrochromic devices [7], and photocatalysts [1, 5, 8, 9]. During the last several decades, nanostructured WO3 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 WO3 nanomaterials such as conventional-hydrothermal and microwave-hydrothermal methods have several advantages. Komarneni [14] reported that these methods consumed less energy and were cost-effective 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. WO3 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], WO3 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 WO3 nanoplates synthesized by the conventional-hydrothermal (C-H) method at 200°C for 12 h, WO3 nanoplates synthesized by the 270 W microwave-hydrothermal (M-H) method for 180 min [13], and commercial (com) WO3 nanorods was investigated by determining the photodegradation of methylene blue (MB) dye under UV light irradiation.

2. Experimental Procedure

2.1. Synthesis

To synthesize WO3 nanoplates, 1 mmol sodium tungstate (Na2WO42H2O, 99.0%) and 0.2 g citric acid (C6H8O7·H2O) 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.

2.2. Characterization

The final products were characterized by Rigaku MiniFlex X-ray diffractometer with Cu-Kα radiation (λ = 1.54178 Å) 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.

2.3. Photocatalysis

Photocatalytic activities of the WO3 nanoplates synthesized by the M-H and C-H methods and the commercial WO3 (Merck) nanorods were tested in a methylene blue (C16H18N3SCI, MB) aqueous media under UV radiation. The initial concentration of 100 mL aqueous MB solution was set as  mol/L and loaded with 0.1 g WO3. The suspension was irradiated by a 18 W UV lamp with the average light intensity of 50.7 W/m2. 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.

3. Results and Discussion

3.1. XRD

Comparing XRD spectra (Figure 1) of the products synthesized by the C-H method at 120, 160, and 200°C for 12 h with the JCPDS database (WO3, No. 72-1465) [19], they corresponded with pure monoclinic crystal system and P21/n 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 WO3 and orthorhombic WO3H2O (JCPDS No. 18-1418) [19]. Upon increasing the operation temperature to 160 and 200°C, pure monoclinic phase of WO3 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 WO3 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 WO3, the XRD pattern confirmed that the powder was composed of orthorhombic phase of WO3 (JCPDS No. 20-1324) [19].

3.2. 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 WO3 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 WO3 nanoplates of the M-H. These imply that stability of the pressure, temperature, and prolonged time can play a role in the crystal growth. The growth mechanism can be explained as follows: tiny nuclei formed by the reaction of Na2WO42H2O and C6H8O7H2O in the solution with the pH of 1 and grew as orthorhombic WO3H2O irregular nanoplates at 120°C. By increasing the temperature to 160 and 200°C, monoclinic WO3 rectangular nanoplates with anisotropic growth rate were detected, especially, at 200°C. It should be noted that the growth of WO3 nanoparticles could be changed during phase transformation and was controlled by citric acid [13]. The SEM image of commercial WO3 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.

3.3. TEM

The morphology and phase of the product synthesized by the C-H at 200°C for 12 h were characterized by TEM, HRTEM, and SAED and are shown in Figure 3. In this research, the WO3 product was shaped like rectangular nanoplates with the width and length of several 10 nm. SAED pattern of the product was indexed [19] to correspond with the (0 2 2), (0 0 2), (0 −2 2), (0 −2 0), (0 −2 −2), (0 0 −2), (0 2 −2), and (0 2 0) planes, specified as single crystalline WO3 [19]. The electron beam was in the direction. The HRTEM image of the product revealed the presence of lattice plane separation of 0.3850 nm and 0.3634 nm corresponding to the interlayer stacking of the (0 0 2) and (2 0 0) crystallographic planes of WO3 (JCPDS No. 72-1465) [19], respectively.

3.4. FTIR

The FTIR spectra (Figure 4(a)) provided further insight into the structure of the products synthesized by the C-H at different temperatures for 12 h and commercial WO3. 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 [2023]. 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 WO3, the observed peak was assigned to W–O bonding.

3.5. Raman Analysis

A definite existence of the products synthesized by the C-H at different temperatures for 12 h and commercial WO3 was revealed by Raman analysis (Figure 4(b)). For WO3 synthesized at 160 and 200°C and commercial WO3, two main peaks are typical O–W–O stretching modes of crystalline WO3 at 802 cm−1 (symmetric) for the shorter bonds, and 712 cm−1 (asymmetric) for the longer ones. Weak peaks at 610 cm−1 are assigned as the O–W–O stretching modes of WO3. Those at 325, 273, and 241 cm−1 are specified as W–O–W bending modes of the bridging oxygen. The peaks at 186 cm−1 are attributed to the lattice vibration. Additional mode belonging to W=O stretching of the product processed at 120°C for 12 h was also detected at 940 cm−1 [20, 2327].

3.6. UV-Visible Absorption

UV-visible absorption of the products synthesized by the C-H at different temperatures for 12 h and commercial WO3 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, 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 , 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 , 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 WO3 was determined to be 2.63 eV [8].

3.7. BET Analysis

The BET analysis was used to determine surface area of the products (Figure 6(a)). It was found that pure WO3 of the M-H (37.25 m2/g) has larger surface area than the product of the C-H and commercial WO3 (33.56 m2/g). Surface area of the product synthesized by the C-H at 120°C for 12 h (WO3H2O) was determined to be 30.65 m2/g. Pure WO3 products at 160 and 200°C for 12 h were determined to be 27.87 and 29.74 m2/g, respectively. The M-H processed WO3 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.

3.8. Photocatalysis

The photocatalytic activities of WO3 synthesized by the 200°C C-H and 270 W M-H and of the commercial WO3 photocatalyst were evaluated by identifying the degradation of methylene blue (C16H18N3SCI, MB) under UV radiation. Before UV irradiation, 100 mL of  mol/L MB aqueous solution containing 0.1 g of photocatalyst was magnetically stirred in the dark for 30 min. For comparison, WO3 synthesized by the 200°C of the C-H for 12 h, WO3 of the 270 W M-H for 180 min, and commercial WO3 were used as photocatalytic materials for degrading MB under UV radiation. Figures 6(b)6(d) show the UV-visible absorption spectra of MB in aqueous solution containing WO3 synthesized by the C-H and M-H methods and commercial WO3 for different lengths of irradiation time. The characteristic absorption peaks of MB at 664 nm gradually decrease with the prolonging irradiation time. For WO3 photocatalyst of the M-H, the characteristic absorption peak decreases almost disappear within 360 min. The MB degradation under a series of the experimental conditions is shown in Figure 6(e), where and are the initial concentration after the equilibrium absorption and residual concentration of MB within the length of time (), respectively. The MB concentration of WO3 photocatalyst of the C-H decreases slower than WO3 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 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 WO3 (monoclinic, nanoplates) are difference from those of commercial WO3 (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 pseudo-first-order rate equation was given by [18] where is the equilibrium concentration after absorption, is the concentration of MB at time , and represents the apparent pseudo-first-order rate constant of initial degradation. The pseudo-first-order rate constant () 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,  min−1, and 0.00138 min−1 in the solutions containing the M-H WO3, C-H WO3, and commercial WO3, respectively.

4. Conclusions

In summary, WO3 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 WO3 was evaluated by identifying the degradation of MB dye under UV irradiation and compared with the M-H WO3 and commercial WO3. The results indicated that the M-H WO3 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.

Conflict of Interests

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


The authors wish to thank the Thailand Research Fund (TRF) for providing financial support through the Royal Golden Jubilee Ph.D. Program and the TRF Research Grant BRG5380020; the National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA), Thailand, through the project P-10-11345 for Research, Development and Engineering (RD & E); and the Thailand’s Office of the Higher Education Commission through the National Research University (NRU) Project for Chiang Mai University (CMU), including the Graduate School of CMU through a general support.