Preparation of Bi3.64Mo0.36O6.55 by reflux method and its application in photodegradation of organic pollution

Bismuth molybdate (γ-Bi2MoO6) photocatalyst has garnered huge attention in the field of photocatalysis because of its band gap (2.5–2.8 eV) and good visible-light response (420 ≤ λ ≤ 500 nm). However, as a kind of bismuth molybdates, there are only few studies on Bi3.64Mo0.36O6.55 (BMO), thus further exploration is needed. Herein, a simple reflux method was developed to synthesize the cubic phase of BMO. This method is simple and easy to operate under atmospheric pressure, showing great potential for large-scale production. In contrast with the nanosheet structure of Bi2MoO6, the morphology of BMO is a mixture of nanorod and nanoparticle-like structure. The band structures showed that the band gap, conduction band position and the valence band position of BMO was 2.77 eV, − 0.33 eV and 2.44 eV, respectively. A new mixed phase of 3Bi2O3·2MoO3 appeared in BMO crystal, showing that the phase transition of BMO began at 400 °C. When BMO was calcined at 300 °C, photocatalytic degradation rate was up to maximum. The photocatalytic activity of visible-light range was tested and compared with γ-Bi2MoO6. BMO had better photodegradation activity than that of the Aurivillius phase γ-Bi2MoO6 due to its larger band gap and strong oxidation ability.

The microwave-assisted methods and electrochemical method needing complex equipment cannot satisfy the large demand of production. Therefore, design and developing a simple method to prepare BMO photocatalyst is needed for really applications.
Herein, a re uxed method was developed for preparation of the BMO photocatalyst. This method is simple and easy to operate under atmospheric pressure. It thus shows great potential for large-scale production. The effect calcination temperature on structures and properties was studies. The relationships between the morphologies and structures of BMO and their photocatalytic activity were discussed. BMO exhibited the excellent photodegradation activity, which is higher than that of Bi 2 MoO 6 .
When the sample was calcined at 300°C, the photodegradation activity of BMO is up to the maximum. The possible photodegradation mechanism of the BMO photocatalyst was nally discussed. into a 250mL three-necked ask with 120mL of deionized water. After stirring for 10 minutes, 2mol/L NaOH solution was added to adjust the pH value to 9, then the solution was heated to 100℃ and re uxed for 12h, after cooling down to room temperature, the product was washed with deionized water and centrifuged at 5000 rpm three times, then dried at 90°C for 8 hours in air. The as-prepared sample were placed in a mu e furnace and heated from ambient temperature to 200°C, 300°C, 400°C, 500°C, 600°C at 2°C/min and held for 1h at each respective temperature. For comparison, we use the same method to prepare Bi 2 MoO 6 under the condition of pH = 9 and calcinated at 300°C for further use.

Characterizations
The crystallinity of as-prepared sample was characterized by the Bruker D8 Advance X-ray diffractometer (XRD) of Germany at room temperature. The morphologies and structures were observed by 81W/AIS2100 scanning electron microscopy (SEM) of Western Chemical (Beijing) Technology Co., Ltd. and Hitachi HT770 transmission electron microscopy with 100kV of acceleration voltage. Fourier Transform Infrared (FTIR) spectra was carried out on the Spectrum 100 infrared spectrometer of PerlinElmer of USA in the frequency range of 4000cm − 1 -450cm − 1 with 16 scanning times and a resolution of 4 cm − 1 , the KBr was as the background during the test period. Raman spectra were measured on the OmniRS-532 model Raman spectrometer of Zhuolihan Optical Instrument Co., Ltd. of Beijing. Ultraviolet-visible diffuse re ectance spectra (UV-vis DRS) was tested via Hitachi U-3900 UV-vis spectrophotometer, using BaSO 4 as a reference, the slit width, scanning speed and measurement range is 2 nm, 600 nm/min, 200-600 nm respectively. The photoelectrochemical was measured by using the Zennium electrochemical workstation of Zahner Company of Germany, the Mott-Schottky curve was carried out, then the band positions can be calculated combined with UV-vis DRS, the testing, the ITO glass was boiling in a mixed solution consists of NH 3 •H 2 O, H 2 O 2 , H 2 O with the ratio of 1:1:6 for 30min, then clean it with ethanol, after irradiated the resistance side upward with UV for 1 hour, and 10 mg of the sample was dispersed in 1ml of ethanol by ultrasonication for 15min, then the samples were spread onto the ITO glass by dropping and dried in the air for 12h before drying at the oven.

photocatalytic performance test
The photocatalytic performance of the as-sample was carried out on a photoreactor with a 500W xenon lamp as the light source, and a 420nm cutoff lter was used to make sure the reaction at a visible-light condition. The photocatalytic activity of the sample was evaluated by the MB degradation experiment.
50mg of photocatalyst was dispersed into 50mL of MB (2×10 − 5 mol/L) solution in a 50mL quartz tube via ultrasonication for 10min. Before light irradiation, the suspension was kept stirring at dark to reach the adsorption desorption equilibrium then exposed to visible light for reaction under stirring. At each given time interval, 3 mL suspension was sampled and centrifuged to remove the solid photocatalyst.
Methylene blue (MB) concentration during the degradation was detected by colorimetry using a UV2550 ultraviolet-visible spectrophotometer of Shanghai Jingyan Instrument Company.  of Bi 2 MoO 6 is mainly sheet-like structure (Fig. 2a), which is consistent with the results of previous reports [2]. Figure 2b shows that BMO sample appears the mixture of granular and rod-like structure. As the temperature increases, the amount of rod-like BMO decreases while that of particle-like BMO increase, suggesting that parts of nanorods might be changed to nanoparticles. Furthermore, with the calcination temperature increasing from 200 to 500°C, the particle size increase from the several decade nanometers to several hundred nanometers, and the particle shape changes from irregular to uniform. Figure 3 shows the TEM and HTEM of Bi 2 MoO 6 and BMO samples. Figure 3a, b, c showed the morphology of BMO sample presents the mixture rod-like and particle-like structure, which was consistent with the SEM analysis. With the calcination temperature increasing from 200 to 400℃, most of nanorods were changed into nanoparticles, and the shape changes from irregular to uniform. The high-resolution TEM (HRTEM) pictures of Bi 2 MoO 6 and BMO calcined at 300°C were shown in the Fig. 3d, 3e, 3f. In  Fig. 3f, attribute to metal Bi, was found because the bismuth salt is unstable and bismuth oxide is easy to reduce into metal Bi when the TEM electron beam hits the sample [39,40].  And the peaks range between 270cm − 1 and 360cm − 1 with Eg characteristics, which was corresponding to the rocking vibration mode of octahedral of MoO 6 . The peak at 145cm − 1 indicated the lattice mode of Bi 3+ was perpendicular to the layered structure [41,42], This was basically consistent with the characteristics of γ-Bi 2 MoO 6 . Comparing to BMO, two bands at 796 cm − 1 and 845 cm − 1 , the characteristic band of the MoO 6 octahedron on the Bi 2 MoO 6 , shift to a higher wavenumber, and merged into one band at 878 cm − 1 . Meanwhile, the two bands from 270 cm − 1 to 360 cm − 1 exhibited lower wavenumber shift, and merged into one band at 305 cm − 1 . However, the characteristic band of Bi 3+ was still at 145 cm − 1 and without any shift. This suggested that the octahedral structure of the Mo-O bond in the Bi 2 MoO 6 phase disappeared in the BMO phase, which resulting in a formation of new structure. The peak of MoO 6 gradually became wider as the increase of calcinated temperature of BMO. When the temperature reached above 300℃, the intensity of the peak decreases, After the temperature reaches 500℃, the crystal phase changes to 3Bi 2 O 3 •2MoO 3 , and the peak suddenly became wider and sharper.

Results And Discussion
The light absorption of Bi 2 MoO 6 and BMO calcinated at different temperature was investigated by DRS in Fig. 7. Bi 2 MoO 6 is obviously a visible-light photocatalyst, which has the absorption edge of 480 nm. After Bi 2 MoO 6 is calcinated at 300°C, the light absorption has a little blue shift. Compared with Bi 2 MoO 6 , the absorption edges of all the BMO samples shift to the lower wavelength, suggesting that BMO samples have a larger bang gap. It can be seen form Fig. 7 that the calcination temperature has great effect on the light absorption for BMO samples. Before 300°C, the light absorption exhibits a red shift compared with untreated BMO, after that, it shifts to the lower wavelength. Figure 7b compared the UV-vis DRS of Bi 2 MoO 6 calcinated at 300℃ and BMO calcinated at 300℃. The band gap of semiconductor can be obtained from the formula Eg(eV) = 1240/λg (nm) [43] ( λg: the abscissa wave number of the point of intersection). Therefore, the band gap of BMO and Bi 2 MoO 6 is calculated to be 2.77 eV and 2.64 eV, respectively. This result shows that BMO has the larger bang gap than Bi 2 MoO 6 . In generally, the high bang gap means the larger redox ability. Figure 8 was Mott-Schottky curve of BMO calcinated at 300℃. The speci c algorithm of the conduction band position of the sample can be roughly obtained by the tangent method [44], two tangent lines are drawn at the two in ection points of the curve to intersect at a point, and the abscissa of this point is the conduction position (CB) position, which is around -0.55ev. According to the formula of VB= E g + CB, the position of the valence band (VB) of the BMO sample is 2.22 eV (CB=-0.33eV, VB=2.44eV vs NHE). According to the references, the positions of CB and VB are -0.37eV, 2.27eV, respectively [45]. Compared with Bi 2 MoO 6 , BMO sample has the lower VB position, indicating it has the stronger oxidation ability ( Figure 9).
The photodegradation activities and their corresponding degradation kinetic rate constants of Bi 2 MoO 6 , Bi 2 MoO 6 calcinated at 300℃, BMO and BMO calcinated at different temperatures were shown in Fig. 10 (a) and10 (b), respectively. Figure 10(a) shows that Bi 2 MoO 6 calcinated at 300℃ has the higher photodegradation activity than that of Bi 2 MoO 6 due to the improved crystallinity at high calcination temperature. The same result can also be observed in BMO samples. With the increase of calcination temperature, the photodegradation activity increase, and it is up to the maximum at 300℃. After 300℃, the photocatalytic activity is gradually decreased due to the formation of new phase in BMO sample. The histogram of the K value of MB degradation of BMO and Bi 2 MoO 6 calcinated at different temperatures were illustrated in Fig. 10(b). The Bi 2 MoO 6 calcinated at 300℃ has the largest kinetic rate constant among the BMO samples. In addition, the kinetic rate constant of BMO calcined at 300℃ is about 1.27 times higher than that of Bi 2 MoO 6 calcinated at 300℃. The enhanced photocatalytic activity of BMO calcined at 300℃ is that BMO has the larger band gap and stronger oxidation ability than that of Bi 2 MoO 6 .

Conclusion
In conclusion, BMO is successfully synthesized by a simple re ux method. The BMO sample consists of the nanorod and nanoparticle-like structure. With the increase of calcinated temperature, the nanoparticle size of BMO gradually grows larger and the amount of rod-like BMO decreases. The phase transition of BMO begins at 400℃, and a new mixed phase of 3Bi 2 O 3 •2MoO 3 was appeared. With increase the calcination temperature from 200 to 300°C, the photocatalytic activity increases due to the enhanced visible absorption and crystallinity. After that, the photocatalytic activity largely decreases because of the appearance of new mixed phase in BMO sample. BMO sample calcinated at 300°C exhibited the best activity, which is 1.5 times higher than uncalcinated BMO. Compared to Bi 2 MoO 6 the photodegradation activity of BMO is much higher because of the larger band gap and strong oxidation ability. Declarations